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15.27: Jawed Fishes - Biology

15.27: Jawed Fishes - Biology


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Gnathostomes or “jaw-mouths” are vertebrates that possess jaws. The evolution of jaws allowed early gnathostomes to exploit food resources that were unavailable to jawless fishes.

Early gnathostomes also possessed two sets of paired fins, allowing the fishes to maneuver accurately. Pectoral fins are typically located on the anterior body, and pelvic fins on the posterior. Evolution of the jaw and paired fins permitted gnathostomes to expand from the sedentary suspension feeding of jawless fishes to become mobile predators. The ability of gnathostomes to exploit new nutrient sources likely is one reason that they replaced most jawless fishes during the Devonian period. Two early groups of gnathostomes were the acanthodians and placoderms (Figure 1), which arose in the late Silurian period and are now extinct. Most modern fishes are gnathostomes that belong to the clades Chondrichthyes and Osteichthyes.

Chondrichthyes: Cartilaginous Fishes

The clade Chondrichthyes is diverse, consisting of sharks (Figure 2), rays, and skates, together with sawfishes and a few dozen species of fishes called chimaeras, or “ghost” sharks.” Chondrichthyes are jawed fishes that possess paired fins and a skeleton made of cartilage. This clade arose approximately 370 million years ago in the early or middle Devonian. They are thought to be descended from the placoderms, which had skeletons made of bone; thus, the cartilaginous skeleton of Chondrichthyes is a later development. Parts of shark skeleton are strengthened by granules of calcium carbonate, but this is not the same as bone.

Most cartilaginous fishes live in marine habitats, with a few species living in fresh water for a part or all of their lives. Most sharks are carnivores that feed on live prey, either swallowing it whole or using their jaws and teeth to tear it into smaller pieces. Shark teeth likely evolved from the jagged scales that cover their skin, called placoid scales. Some species of sharks and rays are suspension feeders that feed on plankton.

Sharks have well-developed sense organs that aid them in locating prey, including a keen sense of smell and electroreception, with the latter perhaps the most sensitive of any animal. Organs called ampullae of Lorenzini allow sharks to detect the electromagnetic fields that are produced by all living things, including their prey. Electroreception has only been observed in aquatic or amphibious animals. Sharks, together with most fishes and aquatic and larval amphibians, also have a sense organ called the lateral line, which is used to detect movement and vibration in the surrounding water, and is often considered homologous to “hearing” in terrestrial vertebrates. The lateral line is visible as a darker stripe that runs along the length of a fish’s body.

Sharks reproduce sexually, and eggs are fertilized internally. Most species are ovoviviparous: The fertilized egg is retained in the oviduct of the mother’s body and the embryo is nourished by the egg yolk. The eggs hatch in the uterus, and young are born alive and fully functional. Some species of sharks are oviparous: They lay eggs that hatch outside of the mother’s body.

Embryos are protected by a shark egg case or “mermaid’s purse” (Figure 3) that has the consistency of leather. The shark egg case has tentacles that snag in seaweed and give the newborn shark cover. A few species of sharks are viviparous: The young develop within the mother’s body and she gives live birth.

Rays and skates comprise more than 500 species and are closely related to sharks. They can be distinguished from sharks by their flattened bodies, pectoral fins that are enlarged and fused to the head, and gill slits on their ventral surface (Figure 4). Like sharks, rays and skates have a cartilaginous skeleton. Most species are marine and live on the sea floor, with nearly a worldwide distribution.

Osteichthyes: Bony Fishes

Members of the clade Osteichthyes, also called bony fishes, are characterized by a bony skeleton. The vast majority of present-day fishes belong to this group, which consists of approximately 30,000 species, making it the largest class of vertebrates in existence today.

Nearly all bony fishes have an ossified skeleton with specialized bone cells (osteocytes) that produce and maintain a calcium phosphate matrix. This characteristic has only reversed in a few groups of Osteichthyes, such as sturgeons and paddlefish, which have primarily cartilaginous skeletons. The skin of bony fishes is often covered by overlapping scales, and glands in the skin secrete mucus that reduces drag when swimming and aids the fish in osmoregulation. Like sharks, bony fishes have a lateral line system that detects vibrations in water.

All bony fishes use gills to breathe. Water is drawn over gills that are located in chambers covered and ventilated by a protective, muscular flap called the operculum. Many bony fishes also have a swim bladder, a gas-filled organ that helps to control the buoyancy of the fish. Bony fishes are further divided into two extant clades: Actinopterygii (ray-finned fishes) and Sarcopterygii (lobe-finned fishes).

Actinopterygii, the ray-finned fishes, include many familiar fishes—tuna, bass, trout, and salmon (Figure 5a), among others. Ray-finned fishes are named for their fins that are webs of skin supported by bony spines called rays. In contrast, the fins of Sarcopterygii are fleshy and lobed, supported by bone (Figure 5b). Living members of this clade include the less-familiar lungfishes and coelacanths.


15.27: Jawed Fishes - Biology

Gnathostomes , or “jaw-mouths,” are vertebrates that possess true jaws—a milestone in the evolution of the vertebrates. In fact, one of the most significant developments in early vertebrate evolution was the development of the jaw: a hinged structure attached to the cranium that allows an animal to grasp and tear its food. Jaws were probably derived from the first pair of gill arches supporting the gills of jawless fishes.

Figure 1. Dunkleosteous was an enormous placoderm from the Devonian period, 380–360 million years ago. It measured up to 10 meters in length and weighed up to 3.6 tons. Its head and neck were armored with heavy bony plates. Although Dunkleosteus had no true teeth, the edge of the jaw was armed with sharp bony blades. (credit: Nobu Tamura)

Early gnathostomes also possessed two sets of paired fins, allowing the fishes to maneuver accurately. Pectoral fins are typically located on the anterior body, and pelvic fins on the posterior. Evolution of the jaw and paired fins permitted gnathostomes to expand their food options from the scavenging and suspension feeding of jawless fishes to active predation. The ability of gnathostomes to exploit new nutrient sources probably contributed to their replacing most jawless fishes during the Devonian period. Two early groups of gnathostomes were the acanthodians and placoderms (Figure 1), which arose in the late Silurian period and are now extinct. Most modern fishes are gnathostomes that belong to the clades Chondrichthyes and Osteichthyes (which include the class Actinoptertygii and class Sarcopterygii).


Gnathostomes: Jawed Fishes

Gnathostomes, jawed vertebrates, can be divided into two types of fish: Chondrichthyes (cartilaginous fish) or Osteichthyes (bony fish).

Learning Objectives

Differentiate among the types of jawed fishes

Key Takeaways

Key Points

  • Early jawed fish (gnathostomes) were able to exploit new nutrient sources because of their jaws and paired fins.
  • Chondrichthyes includes all jawed fish with cartilagenous skeletons, such as sharks, rays, skates, and chimaeras.
  • Osteichthyes includes all jawed fish with ossified (bony) skeletons this includes the majority of modern fish.
  • Osteichthyes can be further separated into Actinopterygii (the ray-finned fishes) and Sarcopterygii (lobe-finned fishes).
  • The majority of modern fish species are actinopterygii, from trout to clownfish.
  • Early Sarcopterygii (lobe-finned fishes) evolved into modern tetrapods, including reptiles, amphibians, birds, and mammals.

Key Terms

  • ossified: composed of bone, which is a calcium phosphate matrix created by special cells called osteoblasts
  • operculum: a covering flap or lidlike structure in plants and animals, such as a gill cover
  • Chondrichthyes: a taxonomic class within the subphylum Vertebrata: the cartilaginous fish
  • Osteichthyes: a taxonomic class within the subphylum vertebrata: the bony fish

Gnathostomes: Jawed Fishes

Gnathostomes or “jaw-mouths” are vertebrates that possess jaws. One of the most significant developments in early vertebrate evolution was the development of the jaw, which is a hinged structure attached to the cranium that allows an animal to grasp and tear its food. The evolution of jaws allowed early gnathostomes to exploit food resources that were unavailable to the jawless animals. In early evolutionary history, there were gnathostomes (jawed fishes) and agnathans (jawless fishes). Gnathostomes later evolved into all tetrapods (animals with four limbs) including amphibians, birds, and mammals.

Early gnathostomes were jawed fishes that possessed two sets of paired fins, which increased their ability to maneuver accurately. These paired fins were pectoral fins, located on the anterior body, and pelvic fins, on the posterior. The evolution of the jaw combined with paired fins permitted gnathostomes to expand from the sedentary suspension feeding of jawless fishes and become mobile predators. The gnathostomes’ ability to exploit new nutrient sources led to their evolutionary success during the Devonian period. Two early groups of gnathostomes were the acanthodians and placoderms, which arose in the late Silurian period and are now extinct. Most modern gnathostomes belong to the clades Chondrichthyes and Osteichthyes.

Placoderms: Dunkleosteous was an enormous placoderm from the Devonian period, 380–360 million years ago. It measured up to 10 meters in length and weighed up to 3.6 tons. As gnathostomes, they were more mobile and could exploit more food resources than the agnathostomes.

Chondrichthyes: Cartilaginous Fishes

The clade Chondrichthyes consists of sharks, rays, and skates, together with sawfishes and a few dozen species of fishes called chimaeras, or “ghost,” sharks. Chondrichthyes are jawed fishes that possess paired fins and a skeleton made of cartilage. This clade arose approximately 370 million years ago in the early or middle Devonian.

Hammerhead shark: Hammerhead sharks tend to school during the day and hunt prey at night. As members of Chondrichthyes, their skeletons are composed of cartilage.

Most cartilaginous fishes live in marine habitats, although a few species live in fresh water for part or all of their lives. Most sharks are carnivores that feed on live prey, either swallowing it whole or using their jaws and teeth to tear it into smaller pieces. Shark teeth probably evolved from the jagged scales that cover their skin called placoid scales. Some species of sharks and rays are suspension feeders that feed on plankton.

Sharks have well-developed sense organs that aid them in locating prey, including a keen sense of smell and electroreception. Organs called ampullae of Lorenzini enable sharks to detect the electromagnetic fields that are produced by all living things, including their prey. Only aquatic or amphibious animals possess electroreception. Sharks, together with most fishes and aquatic and larval amphibians, also have a sense organ called the lateral line, which is used to detect movement and vibration in the surrounding water. It is often considered homologous to “hearing” in terrestrial vertebrates. The lateral line is visible as a darker stripe that runs along the length of a fish’s body.

Rays and skates comprise more than 500 species and are closely related to sharks. They can be distinguished from sharks by their flattened bodies, pectoral fins that are enlarged and fused to the head, and gill slits on their ventral surface. Like sharks, rays and skates have a cartilaginous skeleton. Most species are marine and live on the sea floor, with nearly a worldwide distribution.

Osteichthyes: Bony Fishes

Members of the clade Osteichthyes, also called bony fish, are characterized by a bony skeleton. The vast majority of present-day fish belong to this group, which consists of approximately 30,000 species, making it the largest class of vertebrates in existence today.

Nearly all bony fish have an ossified skeleton with specialized bone cells (osteocytes) that produce and maintain a calcium phosphate matrix. A few groups of Osteichthyes, such as sturgeons and paddlefish, have primarily cartilaginous skeletons, but retain some bony elements. The skin of bony fish is often covered by overlapping scales. Skin glands secrete mucus that reduces drag when swimming and aids the fish in osmoregulation. Like sharks, bony fish have a lateral line system that detects vibrations in water. All bony fish use gills for gas exchange. Water is drawn over gills that are located in chambers covered and ventilated by a protective, muscular flap called the operculum. Many bony fish also have a swim bladder, a gas-filled organ that helps to control the buoyancy of the fish.

Bony fish are further divided into two extant clades: Actinopterygii (ray-finned fish) and Sarcopterygii (lobe-finned fish). Actinopterygii, the ray-finned fish include many familiar fish, such as tuna, bass, trout, and salmon, among others. Ray-finned fish are named for their fins that are webs of skin supported by bony spines called rays. In contrast, the fins of Sarcopterygii are fleshy and lobed, supported by bone. Although most members of this clade are extinct, living members include the less-familiar lungfishes and coelacanths. Early Sarcopterygii evolved into modern tetrapods, including reptiles, amphibians, birds, and mammals.

Actinopterygii and Sarcopterygii: The (a) sockeye salmon (Actinopterygii) and (b) coelacanth (Sarcopterygii) are both bony fishes of the Osteichthyes clade. The coelacanth, sometimes called a lobe-finned fish, was thought to have gone extinct in the Late Cretaceous period, 100 million years ago, until one was discovered in 1938 near the Comoros Islands between Africa and Madagascar.


15.27: Jawed Fishes - Biology

A Quick Course in Ichthyology

by Jason Buchheim
Director, Odyssey Expeditions

  • FISH Definition
  • FISHES- class agnatha
  • FISHES- Class Chondrichthyes
    • Shark Attack
    • REDUCING THE RISK
    1. ________________
    2. ________________
    3. ________________
    4. ________________
    5. ________________
    6. ________________
    7. Fish are fun!

    FISH : Any of a large group of cold-blooded, finned aquatic vertebrates. Fish are generally scaled and respire by passing water over gills.Modern fish are divided into three classes.

    I. AGNATHA, primitive jawless fish.Lampreys and Hagfish

    II. CHONDRICHTHYES, the jawed fish with cartilaginous skeletons. Sharks, Rays, Rat-Fishes

    III. OSTEICHTHYES, fish with bony skeletons.Lungfish, Trout, Bass, Salmon, Perch, Parrot Fish


    Fish come in all shapes and sizes, some are free swimming, while others rest on the bottom of the sea, some are herbivores and others are carnivores, and some lay eggs while others give live birth and parental care to their young.
    FISH: the members of a single species
    FISHES: more than one species of fish
    FISHES- class Agnatha

    • Primitive
    • No jaws
    • Cartilaginous skeleton
    • Scaleless skin
    • Oral sucker in place of jaws
    • Predators and filter feeders
    • anticoagulating saliva
    • fresh and salt water
    • some anadromous
    • Cartilaginous skeleton
    • Skin covered with denticles, not scales
    • Five to seven gill slits per side
    • No swim bladder
    • Internal fertilization
    • Spiral valve intestines
    • Five to seven gill arches
    • Cartilaginous jaws, loosely attached lower jaws

    In fact, most sharks are entirely incapable of this feat. The largest fish of all, the Whale Shark, which can reach sizes of up to 59 feet and weigh 88,000 lb., is a very calm and approachable plankton feeder. There are many species of sharks which can inflict severe bodily injury and require the utmost of respect. The most feared of all, the Great White Shark, has been responsible for most of the fatal shark attacks off the California and Australian coastlines. While the Great White gets all the notoriety, pound for pound, the Bull Shark is probably the most ferocious. The Great White generally attacks a person because it has confused it with its favorite food, the seals and sea lions, but the Bull Shark will attack a person just because they are there. Even with these dangerous animals roaming the ocean, your chances of getting attacked by a shark are very remote.

    Worldwide, there are only about three hundred documented shark attacks a year. The chances are much higher that you will be hit by a drunk driver while driving to the beach then they are that you will even encounter a dangerous shark when you get there. There are some activities that will greatly increase your chance of a shark attack, such as carrying speared fish with you while diving or collecting abalone in turbid waters. Statistics of 1,652 shark attacks show that males are much more likely to be attacked than females (10 to 1 ratio), this is probably because males are much more active in the water, surfing and going to deeper depths where sharks are more common.

    The presence of large numbers of fish, or fish behaving in an unusual manner, has been reported preceding many attacks. In 40 percent of the reported shark attacks, people were pole-fishing or spear-fishing in the area of an attack. A comparison of the number of people swimming to those fishing and spear-fishing seems to show that these two pastimes have by far the highest risk of inducing an attack. While swimming, the chance of drowning is more than 1,000 times greater than that of dying from a shark attack.

    Most shark attacks occur in shallow water, where most bathers are, and in 94 percent of the cases the attack was by an individual shark acting alone. About 10 percent of reported shark attacks are on divers since the number of divers in the water at one time must be much smaller than 10 percent of beach bathers, the odds of being attacked must be significantly greater for divers.

    Close passes were seldom made before the attack, and in the majority of the cases there was only one strike. Few attacks involved more than one bite. This indicates that in many cases the attacking shark mistook the victim for a more usual kind of food and did not attack any further when the error was discovered. It is fortunate that sharks, in most cases, do not consider humans to be suitable food. This information also refutes the long-standing notion that fresh human blood is a powerful attractant that excites sharks into a feeding frenzy. If this were so, the presence of blood would certainly have induced that attacking shark to strike the victim repeatedly. Most wounds occur on the appendages- the hands, arms, legs, and feet. Lacerations of varying severity are the most common types of injury. About 25 percent of attacks kill the victim. The most usual cause of death is shock, combined with a severe loss of blood.

    REDUCING THE RISK

    Swimmers and divers can reduce the chance of being attacked by following a few simple rules: Never swim in areas where sharks are known to be common. Never enter the water where people are fishing, either from the beach or from inshore boats. If there are a number of people in the water, do not separate yourself from them. There is safety in numbers. Avoid swimming near deep channels, or where shallow water suddenly becomes deeper. Do not swim alone, or at dusk or after dark, when sharks are feeding actively and are likely to be closer to the shore. Do not enter the water, or if in the water leave immediately, if large numbers of fish are seen, or if fish seem to be acting strangely. Be alert for unusual movements in the water. Do not wear a watch or other jewelry that shines and reflects light. Do not enter the water with an open wound, and women should not swim during their menstrual periods.

    FISHES- Chondrichthyes, Sharks

    Sharks are animals that are superbly adapted to their environment. Almost all are carnivores or scavengers, although the species that live close to the sea floor feed mostly on invertebrates. Most possess a keen sense of smell, a large brain, good eyesight, and highly specialized mouth and teeth. Their bodies are usually heavier than water, and they do not have an air filled swim bladder for buoyancy like most bony fishes. All sharks have an asymmetric tail fin, with the upper lobe being larger than the lower one. This feature, together with flattened pectoral fins, and an oil-filled liver compensates for the lack of a swim bladder. There are 344 known species of sharks living in all parts of the oceans, from shallow to deep water and from the tropics to the polar regions. A few even venture into fresh water and have been found in rivers and lakes. Contrary to popular belief, most sharks are harmless to humans. Sharks are classified into eight orders:

    1. Sawsharks (Pristophoriformes), one family, five sp.Live on the bottom in warm temperate or tropical seas. Easily recognized because of tube, blade like snouts. Bear live young.

    2. Dogfish Sharks (Squaliformes), three families, 73 sp. Bottom dwelling deep water sharks, distributed worldwide. Bear live young and eat bony fishes, crustaceans, squid and other sharks. Harmless to humans.

    3. Angel Sharks (Squatiniformes), one family, 13 sp. Flattened, bottom dwelling sharks. Found on continental shelves and upper slopes of cold temperate and tropical seas. Have very sharp, awl-like teeth that are used to impale small fish and crustaceans.

    4. Bullhead Sharks (Heterodontiformes), one family, 8 sp. Live on rocky reefs where there are plenty of cracks and crevices. Found in Pacific and Indian Ocean. Eat invertebrates.
    5. Gilled Sharks (Hexanchiformes), two families, five sp. Deep-water, bottom-dwelling sharks. Worldwide distribution. Only shark with six or seven gill slits. Bear live young and eat bony fish, crustaceans, and other sharks.

    6. Mackerel Sharks (Lamniformes), seven families, 16 sp. Small, highly diverse order. Found in tropical to cold temperate or even Arctic waters. Oceanic and coastal. Most very large, eat bony fish, other sharks, squid, and marine mammals. Includes the Mako and Great White and the plankton eating Megamouth and Basking Sharks.

    7. Carpet Sharks (Otectolobiformes) seven families, 31 sp. Warm tropical to temperate waters. All members except whale shark live on bottom. Flattened. Most eat small fishes and invertebrates. Whale shark is plankton feeder. Some bear live young and others lay eggs.

    8. Ground Sharks (Carcharhiniformes) 8 families, 193 sp. Largest order of sharks. Worldwide distribution, temperate and tropical waters. Most live near coast, although some found in deeper waters. Eat bony fishes, other sharks, squid, and small invertebrates. Includes the dangerous Tiger shark.

    Sharks have numerous structural and physiological features that make them unique among the fishes. They have a simple cartilaginous skeleton with no ribs, and a cartilaginous jaw, backbone, and cranium.

    Thick skin supports the flimsy skeleton. The skin is elastic and aids in movement when the tail is arched, it pulls on the skin, which pulls back like a rubber band. The jaws are not connected to the skull and become unhinged, protruding forward from the skull allowing for a wider gape when feeding. The teeth are ossified with minerals known as 'apatite'. They form a conveyer belt with as many as eight teeth in a row. When a shark looses a tooth, another one just pops up. Sharks go through up to 2,400 teeth a year.

    Sharks have placoid scales which are fixed, slightly ossified and layered. They are smooth to the touch in one direction and extremely course in another. Just rubbing a shark the wrong way can inflict serious wounds.

    All sharks, rays, and skates are carnivores. They have normal sensory modalities, a small brain (most of which is dedicated to the olfactory lobes giving them an acute sense of smell) and well developed eyes with color vision and adaptation to low light levels.

    Some sharks lay eggs (all skates and ratfish do), but most are ovoviviparous (all rays are). The young develop with their yolk sacks within the mother, but without a placenta or umbilical cord. Some sharks (the Great White) are oviphagous the young eat the other developing young and embryos inside their mother and only the fiercest is born! A few sharks (hammerheads and reef sharks) are viviparous like mammals, the young are nourished with a placenta within the mother. The gestation period is around 22 months and 2-80 pups are born per litter. Because most sharks are ovoviviparous or viviparous, they do not produce mass numbers of young like other fish do. They are slow to develop and for this reason shark population numbers have been decreasing rapidly due to the recent popularity of shark fin soup. Fishermen are taking many more sharks than the maximum sustainable yield will allow. Some sharks will soon be endangered species. Rays

    Rays in general are physiologically exactly like sharks except the rays pectoral fins are fussed to their heads. Their gills are ventrally located. They swim with their ventral fins, like wings. Their eyes are dorsally [top] located and have spericules behind them. The spericules are used to breathe in with.

    Rays are modified as bottom feeders, feeding on invertebrates found in the sand. Sometimes you can watch a ray making quite a ruckus on the sand bottom in search of the invertebrates.

    Manta rays are planktivores and cruise the open water filter feeding out small animals. Mantas are the largest of the rays.

    Electric rays swim with their caudal fin and use their modified pectoral fins to electrically shock and stun their prey.

    Sawfish look like sharks but have true fused pectoral fins and gills on the ventral surface.

    Stingrays have a toxin filled spine at the base of their tail. Stingrays are not the mean creatures roaming the waters to hurt swimmers, as many people believe them to be. Stingrays are actually very approachable and can be hand fed and petted, just don't step on them!

    FISHES- the BONY FISH, OSTIEICTHYES

    The bony fish comprise the largest section of the vertebrates, with over 20,000 species worldwide. They are called bony fish because their skeletons are calcified, making them much harder than the cartilage bones of the chondrichthyes. The bony fishes have great maneuverability and speed, highly specialized mouths equipped with protrusible jaws, and a swim bladder to control buoyancy.

    The bony fish have evolved to be of almost every imaginable shape and size, and exploit most marine and freshwater habitats on earth. Many of them have complex, recently evolved physiologies, organs, and behaviors for dealing with their environment in a sophisticated manner.

    Eels -Anguilliformes 597 spp

    Salmon -salmoniformes 350 spp

    Flyingfishes -Cyprinodontiformes 845 spp

    Silversides -Atheriniformes 235 spp

    Squirrelfishes -Beryciformes 164 spp

    Scorpionfishes -Scopaeniformes 1160 spp

    Flatfish -Pleuronectiformes 538 spp

    Triggerfish -Tetraodontiformes 329 spp

    Perch Like -Perciformes 7791 spp, largest order

    Deep Sea Fish -Stomiiformes 250 spp Gobies -Gobiesociformes 114 spp Trumpetfish -Syngnathiformes 257 spp

    FISH SEX- how fish reproduce

    Fish have come up with three modes of reproduction depending on the method they care for their eggs.

    • Ovopartity -- Lay undeveloped eggs, External fertilization (90% of bony fish), Internal fertilization (some sharks and rays)
    • Ovoviviparity - Internal development- without direct maternal nourishment-Advanced at birth (most sharks + rays)-Larval birth (some scorpeaniforms-rockfish)
    • Viviparity - Internal development- direct nourishment from mother-Fully advanced at birth (some sharks, surf perches)

    Parental care: In fishes, parental care is very rare as most fish are broadcast spawners, but there are a few instances of parental care. Male gobies guard the eggs in a nest until they are born. The male yellowhead jawfish actually guards the eggs by holding them in his mouth! Weird Fish Sex!

    Some fish are very kinky creatures by human standards, displaying behavior that would probably get a human incarcerated for a long time.

    • Hermaphroditism : Some fish individuals are both males and females, either simultaneously or sequentially. There is no genetic or physical reason why hermaphroditism should not be present. About 21 families of fish are hermaphrodites.
    • Simultaneous hermaphrodite : There are some instances where being a member of both sexes could have its advantages. Imagine all the dates that you could have! In the deep sea, the low light levels and limited food supply make for a very low population density meaning that potential mates are few and far between. Members of the fish family Salmoniformes (eg salmon) and Serranidae (hamlets) are simultaneous hermaphrodites they can spawn with any individual encountered.
    • Sequential hermaphrodite: Very strange life histories develop in species whose individuals may change sex at some time in their life. They may change from being males to females (protandry) or females to males (protogyny).

    A classic example of protogyny is found in the wrasses and parrotfishes. The males in these species form harems, with one large male sequestering and defending a group of smaller females. The male enjoys spectacular reproductive success, as it has many females to mate with. The females also enjoy a limited reproductive success, producing as many eggs as they can, all fertilized by the one male. The male has the advantage over the females it has many females producing eggs for him to fertilize, whereas the females only have themselves. It is great to be the king!

    The weird sex stuff comes in when we analyze what the reproductive success of a smaller male may be. As only the largest male, the 'SuperMale' gets to mate with the females, a smaller male would enjoy zero reproductive success. There is no advantage to being a small male, and this is where the hermaphrodism comes in. If all the smaller fish were females, they could all enjoy a limited reproductive success while they are growing. If the male dies, the one that has grown to be the largest female will change sexes and become the male, in turn enjoying a much greater reproductive success than if she did not switch. So there are no small males and everything is all said and done, but wait! Evolution has a keen ability in finding weaknesses in any system, and it has done so with the parrotfish. In nature, we do find smaller male parrotfish, why should this be so? It has to do with the kind of thing that if a parrotfish was a human, could get the parrotfish into a great deal of trouble. The 'supermale' has to run around all of the time keeping track of and protecting all of his females as well capturing and eating food himself, so he does not necessarily have time to pay close attention to the details. When parrotfish mate, they form a spawning aggregation where the supermale will release his sperm into the water and the many females release their eggs. The sperm and egg find each other in the water column and fertilization takes place, and this is where the weakness of the system lays. Along comes the smaller male, who has evolved to look just like a female. Most of the time the smaller male will make itself completely inconspicuous by behaving just like the females, but during the spawning aggregations, he will be releasing sperm instead of eggs. The supermale will probably not even know that he has been conned. Everything gets really mixed up as males are changing into females changing into males. FISH- Schooling Behavior

    Everyone has heard of a school of fish, an aggregation of fish hanging out together but why, they are obviously not learning reading, writing, and arithmetic. Schools of fish may be either polarized (with all the fish facing the same direction) or non polarized (all going every which way)

    There are some factors that can make it advantageous to hang out with other fish.

      A. Confusion effect. A large school of fish may be able to confuse a potential predator into thinking that the school is actually a much larger organism.

    B. Dilution affect. If a fish hangs out with a lot of other fish and a predator does come around, the predator must usually select one prey item. With so many choices, the chances are that it will not be you. This is known as the 'selfish herd'.

    Enhanced Foraging: A school of fish may have better abilities to acquire food. With many more eyes to detect food, many more meals may be found but there would also be many more mouths to feed. By working as a team, the school may be able to take larger food items than any one individual could manage to capture.

    Migration: The migration abilities of fish in schools may possibly be enhanced due to better navigation, etc. Hydrodynamic efficiency: Due to the complex hydrodynamic properties of water (properties the fish probably discovered only by accident), a fish may gain a swimming advantage by being in a school. The slipstream from the fish ahead of it may make it easier to pass through the water. Good for all the fish except for the ones in front.

    The density of water makes it very difficult to move in, but fish can move very smoothly and quickly.

    A swimming fish is relying on its skeleton for framework, its muscles for power, and its fins for thrust and direction.

    The skeleton of a fish is the most complex in all vertebrates. The skull acts as a fulcrum, the relatively stable part of the fish. The vertebral column acts as levers that operate for the movement of the fish.

    The muscles provide the power for swimming and constitute up to 80% of the fish itself. The muscles are arranged in multiple directions (myomeres) that allow the fish to move in any direction. A sinusoidal wave passes down from the head to the tail. The fins provide a platform to exert the thrust from the muscles onto the water.

    Diagram of forces when a fish swims.

    Thrust- force in animal's direction

    Lift- force opposite in right angles to the thrust

    Drag- force opposite the direction of movement

    • Cruisers: These are the fish that swim almost continuously in search for food, such as the tuna. Red Muscle- richly vascularized (blood-carrying capacity), rich in myoglobin (oxygen holder and transferor into the muscles active sites) * able to sustain continuous aerobic movement.
    • Burst Swimmers: These fish usually stay relatively in the same place such as most reef fish.
    • Caudal fin-- provides thrust, and control the fishes direction
    • Pectorals-- act mostly as rudders and hydroplanes to control yaw and pitch. Also act as very important brakes by causing drag.
    • Pelvic fins-- mostly controls pitch
    • Dorsal/anal-- control roll
    • A tuna fish which has a fusiform similar to a torpedo can cruise through the water at very high speeds.
    • The attenuated shape of the eel allows it to wiggle into small crevices where it hunts prey.
    • The depressed shape of the angler fish is advantageous for its "sit and wait" strategy of hunting.
    • The compressed shape found on many reef fishes such as the butter fish gives the fish great agility for movement around the reef and can support sudden bursts of acceleration.
    • Ectothermic: fish derive their heat from the environment
    • Poikilothermic : fish conform to the heat in the environment

    They maintain a higher body temperature through the use of a specialized counter-current heat exchanger called a reta mirabile. These are dense capillary beds within the swimming muscle that run next to the veins leaving the muscles. Blood passes through the veins and arteries in a counter current (opposite) direction. The heat produced from the muscle contraction flows from the exiting veins into the incoming arteries and is recycled.

    Why should they bother having an elevated body temperature? To increase the speed of the fish. The higher the body temperature, the greater the muscular power. Thirty degrees Celsius is the optimum temperature for muscular speed. With increased speed, the tuna can capture the slower, cold blooded fish it prey upon. Tuna have been clocked at record speed of 50-70 mph!

    Bony fish have swim bladders to help them maintain buoyancy in the water. The swim bladder is a sac inside the abdomen that contains gas. This sac may be open or closed to the gut. If you have ever caught a fish and wondered why its eyes are bulging out of its head, it is because the air in the swim bladder has expanded and is pushing against the back of the eye. Oxygen is the largest percentage of gas in the bladder nitrogen and carbon dioxide also fill in passively.

    Physoclistous- swim bladder is closed to the gut. The gas gets in through a special gas gland in the front of the swim bladder. Gas leaves the bladder through an oval body in the back of the swim bladder. The system works in a pretty miraculous way. Oval body, filled by venous blood -gasses leave here

    Gas gland, fed by arterial blood -gasses enter here

    inside the spots= giant secretory cells- secrete lactate -in capillary clusters rete mirabile

    Increased lactate levels from the giant secretory cells lower the surrounding pH, causing the blood hemoglobin to dump off its oxygen. The oxygen diffuses back into the incoming capillary, increasing the partial pressure of oxygen in the incoming capillary. This continues until the partial pressure of the oxygen in the capillary is higher than that of the swim bladder (which has a high concentration of oxygen). This complex system is necessary because the concentration of oxygen is higher in the swim bladder than it is in the blood, so simple diffusion would tend to pull the oxygen out of the bladder instead of pushing it in. If the fish wants more buoyancy, it must tell its secretory cells to release more lactate. Since oxygen diffuses easily with oxygen-poor venous blood, the gas can be forced out.

    *Fish that migrate vertically tend to have high oxygen levels in their bladders because it fills in faster and leaves faster.

    *Fish that maintain a stable depth tend to have more nitrogen because it is inert, enters slowly, and exits slowly.

    How in the heck can a fish, which is underwater, breath if there is no air? When we go under water, we have to bring air with us to survive. Whales and dolphins have lungs that store air from the surface. Fish don't have lungs, and they rarely ever venture into the air, so how do they survive. We all know it has something to do with gills, but what exactly.

    The water surrounding a fish contains a small percentage of dissolved oxygen. In the surface waters there can be about 5 ml. of oxygen per liter of water. This is much less than the 210 ml. of oxygen per liter of air that we breath, so the fish must use a special system for concentrating the oxygen in the water to meet their physiological needs. Here it comes again, a counter current exchange system, similar to the one we found in the fish's swim bladder and in the tuna's muscles.

    The circulation of blood in fish is simple. The heart only has two chambers, in contrast to our heart which has four. This is because the fish heart only pumps blood in one direction. The blood enters the heart through a vein and exits through a vein on its way to the gills. In the gills, the blood picks up oxygen from the surrounding water and leaves the gills in arteries, which go to the body. The oxygen is used in the body and goes back to the heart. A very simple closed-circle circulatory system.

    • The blood flows thorough the gill filaments and secondary lamellae in the opposite direction from the water passing the gills. This is very important for getting all of the available oxygen out of the water and into the blood.
    • If the blood flowed in the same direction as the water passing it, then the blood would only be able to get half of the available oxygen from the water. The blood and water would reach an equilibrium in oxygen content and diffusion would no longer take place.
    • By having the blood flow in the opposite direction, the gradient is always such that the water has more available oxygen than the blood, and oxygen diffusion continues to take place after the blood has acquired more than 50% of the water's oxygen content. The countercurrent exchange system gives fish an 80-90% efficiency in acquiring oxygen.
    • When fish are taken out of the water, they suffocate. This is not because they cannot breathe the oxygen available in the air, but because their gill arches collapse and there is not enough surface area for diffusion to take place. There are actually some fish that can survive out of the water, such as the walking catfish (which have modified lamellae allowing them to breathe air.
    • It is possible for a fish to suffocate in the water. This could happen when the oxygen in the water has been used up by another biotic source such as bacteria decomposing a red tide.

    --Ram Ventilation: Swim through the water and open your mouth. Very simple, but the fish must swim continuously in order to breathe, not so simple.

    Successful survival in any environment depends upon an organism's ability to acquire information from its environment through its senses. Fish have many of the same senses that we have, they can see, smell, touch, feel, and taste, and they have developed some senses that we don't have, such as electroreception. Fish can sense light, chemicals, vibrations and electricity.

    Light: photoreception [Vision]. Fish have a very keen sense of vision, which helps them to find food, shelter, mates, and avoid predators. Fish vision is on par with our own vision many can see in color, and some can see in extremely dim light.

    Fish eyes are different from our own. Their lenses are perfectly spherical, which enables them to see underwater because it has a higher refractive index to help them focus. They focus by moving the lens in and out instead of stretching it like we do. They cannot dilate or contract their pupils because the lens bulges through the iris. As the depth at which fish are found increases, the resident fish's eye sizes increase in order to gather the dimmer light. This process continues until the end of the photic zone, where eye size drops off as their is no light to see with. Nocturnal fish tend to have larger eyes then diurnal fish. Just look at a squirrelfish, and you will see this to be so. Some fish have a special eye structure known as the Tapetum lucidum, which amplifies the incoming light. It is a layer of guanine crystals which glow at night. Photons which pass the retina get bounced back to be detected again. If the photons are still not absorbed, they are reflected back out of the eye. On a night dive, you may see these reflections as you shine your light around!

    Chemicals: chemoreception [Smell and Taste]. Chemoreception is very well developed in the fishes, especially the sharks and eels which rely upon this to detect their prey. Fish have two nostrils on each side of their head, and there is no connection between the nostrils and the throat. The olfactory rosette is the organ that detects the chemicals. The size of the rosette is proportional to the fish's ability to smell. Some fish (such as sharks, rays, eels, and salmon) can detect chemical levels as low as 1 part per billion.

    Fish also have the ability to taste. They have taste buds on their lips, tongue, and all over their mouths. Some fish, such as the goatfish or catfish, have barbels, which are whiskers that have taste structures. Goatfish can be seen digging through the sand with their barbels looking for invertebrate worms to eat and can taste them before they even reach their mouths.

    Vibrations: mechanoreception [Hearing and touch]. Have you ever seen a fish's ear. Probably not, but they do have them, located within their bodies as well as a lateral line system that actually lets them feel their surroundings.

    Fish do not have external ears, but sound vibrations readily transmit from the water through the fish's body to its internal ears. The ears are divided into two sections, an upper section (pars superior) and a lower section (utriculus) The pars superior is divided into three semicircular canals and give the fish its sense of balance. It is fluid-filled with sensory hairs. The sensory hairs detect the rotational acceleration of the fluid. The canals are arranged so that one gives yaw, another pitch, and the last- roll. The utriculus gives the fish its ability to hear. It has two large otoliths which vibrate with the sound and stimulate surrounding hair cells.

    Fish posses another sense of mechanoreception that is kind of like a cross between hearing and touch. The organ responsible for this is the neuromast, a cluster of hair cells which have their hairs linked in a glob of jelly known as 'cupala'. All fish posses free neuromasts, which come in contact directly with the water. Most fish have a series of neuromasts not in direct contact with the water. These are arranged linearly and form the fishes lateral lines. A free neuromast gives the fish directional input.

    A lateral line receives signals stimulated in a sequence, and gives the fish much more information (feeling the other fish around it for polarized schooling, and short-range prey detection 'the sense of distant touch').

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    Key Terms

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      • Authors: Samantha Fowler, Rebecca Roush, James Wise
      • Publisher/website: OpenStax
      • Book title: Concepts of Biology
      • Publication date: Apr 25, 2013
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      • Book URL: https://openstax.org/books/concepts-biology/pages/1-introduction
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      Gnathostomes: Jawed Fishes

      Gnathostomes or “jaw-mouths” are vertebrates that possess jaws. One of the most significant developments in early vertebrate evolution was the development of the jaw, which is a hinged structure attached to the cranium that allows an animal to grasp and tear its food. The evolution of jaws allowed early gnathostomes to exploit food resources that were unavailable to jawless fishes.

      Early gnathostomes also possessed two sets of paired fins, allowing the fishes to maneuver accurately. Pectoral fins are typically located on the anterior body, and pelvic fins on the posterior. Evolution of the jaw and paired fins permitted gnathostomes to expand from the sedentary suspension feeding of jawless fishes to become mobile predators. The ability of gnathostomes to exploit new nutrient sources likely is one reason that they replaced most jawless fishes during the Devonian period. Two early groups of gnathostomes were the acanthodians and placoderms (Figure), which arose in the late Silurian period and are now extinct. Most modern fishes are gnathostomes that belong to the clades Chondrichthyes and Osteichthyes.

      Dunkleosteous was an enormous placoderm from the Devonian period, 380–360 million years ago. It measured up to 10 meters in length and weighed up to 3.6 tons. (credit: Nobu Tamura)


      Contents

      Skeleton Edit

      The skeleton is cartilaginous. The notochord is gradually replaced by a vertebral column during development, except in Holocephali, where the notochord stays intact. In some deepwater sharks, the column is reduced. [3]

      As they do not have bone marrow, red blood cells are produced in the spleen and the epigonal organ (special tissue around the gonads, which is also thought to play a role in the immune system). They are also produced in the Leydig's organ, which is only found in certain cartilaginous fishes. The subclass Holocephali, which is a very specialized group, lacks both the Leydig's and epigonal organs.

      Appendages Edit

      Apart from electric rays, which have a thick and flabby body, with soft, loose skin, chondrichthyans have tough skin covered with dermal teeth (again, Holocephali is an exception, as the teeth are lost in adults, only kept on the clasping organ seen on the caudal ventral surface of the male), also called placoid scales (or dermal denticles), making it feel like sandpaper. In most species, all dermal denticles are oriented in one direction, making the skin feel very smooth if rubbed in one direction and very rough if rubbed in the other.

      Originally, the pectoral and pelvic girdles, which do not contain any dermal elements, did not connect. In later forms, each pair of fins became ventrally connected in the middle when scapulocoracoid and puboischiadic bars evolved. In rays, the pectoral fins are connected to the head and are very flexible.

      One of the primary characteristics present in most sharks is the heterocercal tail, which aids in locomotion. [4]

      Body covering Edit

      Chondrichthyans have tooth-like scales called dermal denticles or placoid scales. Denticles usually provide protection, and in most cases, streamlining. Mucous glands exist in some species, as well.

      It is assumed that their oral teeth evolved from dermal denticles that migrated into the mouth, but it could be the other way around, as the teleost bony fish Denticeps clupeoides has most of its head covered by dermal teeth (as does, probably, Atherion elymus, another bony fish). This is most likely a secondary evolved characteristic, which means there is not necessarily a connection between the teeth and the original dermal scales.

      The old placoderms did not have teeth at all, but had sharp bony plates in their mouth. Thus, it is unknown whether the dermal or oral teeth evolved first. It has even been suggested [ by whom? ] that the original bony plates of all vertebrates are now gone and that the present scales are just modified teeth, even if both the teeth and body armor had a common origin a long time ago. However, there is currently no evidence of this.

      Respiratory system Edit

      All chondrichthyans breathe through five to seven pairs of gills, depending on the species. In general, pelagic species must keep swimming to keep oxygenated water moving through their gills, whilst demersal species can actively pump water in through their spiracles and out through their gills. However, this is only a general rule and many species differ.

      A spiracle is a small hole found behind each eye. These can be tiny and circular, such as found on the nurse shark (Ginglymostoma cirratum), to extended and slit-like, such as found on the wobbegongs (Orectolobidae). Many larger, pelagic species, such as the mackerel sharks (Lamnidae) and the thresher sharks (Alopiidae), no longer possess them.

      Nervous system Edit

      In chondrichthyans, the nervous system is composed of a small brain, 8-10 pairs of cranial nerves, and a spinal chord with spinal nerves. [5] They have several sensory organs which provide information to be processed. Ampullae of Lorenzini are a network of small jelly filled pores called electroreceptors which help the fish sense electric fields in water. This aids in finding prey, navigation, and sensing temperature. The Lateral line system has modified epithelial cells located externally which sense motion, vibration, and pressure in the water around them. Most species have large well-developed eyes. Also, they have very powerful nostrils and olfactory organs. Their inner ears consist of 3 large semicircular canals which aid in balance and orientation. Their sound detecting apparatus has limited range and is typically more powerful at lower frequencies. Some species have electric organs which can be used for defense and predation. They have relatively simple brains with the forebrain not greatly enlarged. The structure and formation of myelin in their nervous systems are nearly identical to that of tetrapods, which has led evolutionary biologists to believe that Chondrichthyes were a cornerstone group in the evolutionary timeline of myelin development. [6]

      Immune system Edit

      Like all other jawed vertebrates, members of Chondrichthyes have an adaptive immune system. [7]

      Fertilization is internal. Development is usually live birth (ovoviviparous species) but can be through eggs (oviparous). Some rare species are viviparous. There is no parental care after birth however, some chondrichthyans do guard their eggs.

      Capture-induced premature birth and abortion (collectively called capture-induced parturition) occurs frequently in sharks/rays when fished. [8] Capture-induced parturition is often mistaken for natural birth by recreational fishers and is rarely considered in commercial fisheries management despite being shown to occur in at least 12% of live bearing sharks and rays (88 species to date). [8]

      The class Chondrichthyes has two subclasses: the subclass Elasmobranchii (sharks, rays, skates, and sawfish) and the subclass Holocephali (chimaeras). To see the full list of the species, click here.

      The fossil record of the Holocephali starts in the Devonian period. The record is extensive, but most fossils are teeth, and the body forms of numerous species are not known, or at best poorly understood.

      • †Order MongolepidiformesKaratajüte-Talimaa & Novitskaya, 1990
      • †Order OmalodontiformesTurner, 1997
      • †Order CoronodontiformesZangerl, 1981
      • †Order SymmoriiformesZangerl, 1981
      • Subclass Holocephali
        • †Superorder Paraselachimorpha
          • †Order DesmiodontiformesZangerl, 1981
          • †Order PolysentoriformesCappetta, 1993
          • †Order OrodontiformesZangerl, 1981
          • †Order PetalodontiformesZangerl, 1981
          • †Order HelodontiformesPatterson, 1965
          • †Order IniopterygiformesZanger, 1973
          • †Order DebeeriiformesGrogan & Lund, 2000
          • †Order EugeneodontiformesZangerl, 1981
          • †Order Psammodontiformes* Obruchev, 1953
          • †Order CopodontiformesObručhev, 1953
          • †Order Squalorajiformes
          • †Order ChondrenchelyiformesMoy-Thomas, 1939
          • †Order Menaspiformes
          • †Order CochliodontiformesObručhev, 1953
          • Order ChimaeriformesBerg, 1940 sensu Obručhev, 1953 (chimaeras)
          • Plesioselachus
          • †Order AntarctilamniformesGinter, Liao & Valenzuela-Rios, 2008
          • †Order ElegestolepidiformesAndreev et al., 2016
          • †Order LugalepididaKaratajute-Talimaa, 1997
          • †Order SquatinactiformesZangerl, 1981
          • †Order ProtacrodontiformesZangerl, 1981
          • †Infraclass Cladoselachimorpha
            • †Order CladoselachiformesDean, 1909
            • †Order BransonelliformesHampe & Ivanov, 2007
            • †Order XenacanthiformesBerg, 1940
            • †Order AltholepidiformesAndreev et al., 2015
            • †Order Polymerolepidiformes
            • †Order Ptychodontiformes
            • †Order CtenacanthiformesZangerl, 1981
            • †Division Hybodonta
              • †Order HybodontiformesOwen, 1846
              • Subdivision Selachii (modern sharks)
                • Superorder Galeomorphi Compagno, 1977
                  • Order Heterodontiformes (bullhead sharks)
                  • Order Orectolobiformes (carpet sharks)
                  • Order Lamniformes (mackerel sharks)
                  • Order Carcharhiniformes (ground sharks)
                  • Order Chlamydoselachiformes
                  • Order Hexanchiformes (frilled and cow sharks)
                  • Order Squaliformes (dogfish sharks)
                  • †Order Protospinaciformes
                  • †Order Synechodontiformes
                  • Order Squatiniformes (angel sharks)
                  • Order Pristiophoriformes (sawsharks)
                  • Order Torpediniformes (electric rays)
                  • Order Pristiformes (sawfishes)
                  • Order Rajiformes (skates and guitarfishes)
                  • Order Myliobatiformes (stingrays and relatives)

                  Cartilaginous fish are considered to have evolved from acanthodians. [ by whom? ] Originally assumed [ by whom? ] to be closely related to bony fish or a polyphyletic assemblage leading to both groups, the discovery of Entelognathus and several examinations of acanthodian characteristics indicate that bony fish evolved directly from placoderm like ancestors, while acanthodians represent a paraphyletic assemblage leading to Chondrichthyes. Some characteristics previously thought to be exclusive to acanthodians are also present in basal cartilaginous fish. [13] In particular, new phylogenetic studies find cartilaginous fish to be well nested among acanthodians, with Doliodus and Tamiobatis being the closest relatives to Chondrichthyes. [14] Recent studies vindicate this, as Doliodus had a mosaic of chondrichthyian and acanthodiian traits. [15]

                  Dating back to the Middle and Late Ordovician Period, many isolated scales, made of dentine and bone, have a structure and growth form that is chondrichthyan-like. They may be the remains of stem-chondrichthyans, but their classification remains uncertain. [16] [17] [18]

                  The earliest unequivocal fossils of cartilaginous fishes first appeared in the fossil record by about 430 million years ago, during the middle Wenlock Epoch of the Silurian period. [19] The radiation of elasmobranches in the chart on the right is divided into the taxa: Cladoselache, Eugeneodontiformes, Symmoriida, Xenacanthiformes, Ctenacanthiformes, Hybodontiformes, Galeomorphi, Squaliformes and Batoidea.

                  By the start of the Early Devonian, 419 million years ago, jawed fishes had divided into three distinct groups: the now extinct placoderms (a paraphyletic assemblage of ancient armoured fishes), the bony fishes, and the clade that includes spiny sharks and early cartilaginous fish. The modern bony fishes, class Osteichthyes, appeared in the late Silurian or early Devonian, about 416 million years ago. The first abundant genus of shark, Cladoselache, appeared in the oceans during the Devonian Period. The first Cartilaginous fishes evolved from Doliodus-like spiny shark ancestors.

                  A Bayesian analysis of molecular data suggests that the Holocephali and Elasmoblanchii diverged in the Silurian ( 421 million years ago ) and that the sharks and rays/skates split in the Carboniferous ( 306 million years ago ).

                  Devonian Devonian (419–359 mya)
                  Cladoselache Cladoselache was the first abundant genus of primitive shark, appearing about 370 Ma. [20] It grew to 6 feet (1.8 m) long, with anatomical features similar to modern mackerel sharks. It had a streamlined body almost entirely devoid of scales, with five to seven gill slits and a short, rounded snout that had a terminal mouth opening at the front of the skull. [20] It had a very weak jaw joint compared with modern-day sharks, but it compensated for that with very strong jaw-closing muscles. Its teeth were multi-cusped and smooth-edged, making them suitable for grasping, but not tearing or chewing. Cladoselache therefore probably seized prey by the tail and swallowed it whole. [20] It had powerful keels that extended onto the side of the tail stalk and a semi-lunate tail fin, with the superior lobe about the same size as the inferior. This combination helped with its speed and agility which was useful when trying to outswim its probable predator, the heavily armoured 10 metres (33 ft) long placoderm fish Dunkleosteus. [20]
                  Carbon-
                  iferous
                  Carboniferous (359–299 Ma): Sharks underwent a major evolutionary radiation during the Carboniferous. [21] It is believed that this evolutionary radiation occurred because the decline of the placoderms at the end of the Devonian period caused many environmental niches to become unoccupied and allowed new organisms to evolve and fill these niches. [21]
                  Orthacanthus senckenbergianus The first 15 million years of the Carboniferous has very few terrestrial fossils. This gap in the fossil record, is called Romer's gap after the American palaentologist Alfred Romer. While it has long been debated whether the gap is a result of fossilisation or relates to an actual event, recent work indicates that the gap period saw a drop in atmospheric oxygen levels, indicating some sort of ecological collapse. [22] The gap saw the demise of the Devonian fish-like ichthyostegalian labyrinthodonts, and the rise of the more advanced temnospondyl and reptiliomorphan amphibians that so typify the Carboniferous terrestrial vertebrate fauna.

                  The Carboniferous seas were inhabited by many fish, mainly Elasmobranchs (sharks and their relatives). These included some, like Psammodus, with crushing pavement-like teeth adapted for grinding the shells of brachiopods, crustaceans, and other marine organisms. Other sharks had piercing teeth, such as the Symmoriida some, the petalodonts, had peculiar cycloid cutting teeth. Most of the sharks were marine, but the Xenacanthida invaded fresh waters of the coal swamps. Among the bony fish, the Palaeonisciformes found in coastal waters also appear to have migrated to rivers. Sarcopterygian fish were also prominent, and one group, the Rhizodonts, reached very large size.

                  Most species of Carboniferous marine fish have been described largely from teeth, fin spines and dermal ossicles, with smaller freshwater fish preserved whole. Freshwater fish were abundant, and include the genera Ctenodus, Uronemus, Acanthodes, Cheirodus, and Gyracanthus.

                  As a result of the evolutionary radiation, carboniferous sharks assumed a wide variety of bizarre shapes e.g., sharks belonging to the family Stethacanthidae possessed a flat brush-like dorsal fin with a patch of denticles on its top. [21] Stethacanthus' unusual fin may have been used in mating rituals. [21] Apart from the fins, Stethacanthidae resembled Falcatus (below).

                  Megalodon is an extinct species of shark that lived about 28 to 1.5 Ma. It looked much like a stocky version of the great white shark, but was much larger with fossil lengths reaching 20.3 metres (67 ft). [32] Found in all oceans [33] it was one of the largest and most powerful predators in vertebrate history, [32] and probably had a profound impact on marine life. [34]


                  Jawless Fishes

                  Jawless fishes are craniates that represent an ancient vertebrate lineage that arose over one half-billion years ago. In the past, the hagfishes and lampreys were classified together as agnathans. Today, hagfishes and lampreys are recognized as separate clades, primarily because lampreys are true vertebrates, whereas hagfishes are not. A defining feature is the lack of paired lateral appendages (fins). Some of the earliest jawless fishes were the ostracoderms (which translates to “shell-skin”). Ostracoderms were vertebrate fishes encased in bony armor, unlike present-day jawless fishes, which lack bone in their scales.

                  Myxini: Hagfishes

                  Figure 1. Pacific hagfish are scavengers that live on the ocean floor. (credit: Linda Snook, NOAA/CBNMS)

                  The clade Myxini includes at least 20 species of hagfishes. Hagfishes are eel-like scavengers that live on the ocean floor and feed on dead invertebrates, other fishes, and marine mammals (Figure 1). Hagfishes are entirely marine and are found in oceans around the world, except for the polar regions. A unique feature of these animals is the slime glands beneath the skin that release mucus through surface pores. This mucus allows the hagfish to escape from the grip of predators. Hagfish can also twist their bodies in a knot to feed and sometimes eat carcasses from the inside out.

                  The skeleton of a hagfish is composed of cartilage, which includes a cartilaginous notochord that runs the length of the body. This notochord provides support to the hagfish’s body. Hagfishes do not replace the notochord with a vertebral column during development, as do true vertebrates.

                  Petromyzontidae: Lampreys

                  Figure 2. These parasitic sea lampreys attach to their lake trout host by suction and use their rough tongues to rasp away flesh in order to feed on the trout’s blood. (credit: USGS)

                  The clade Petromyzontidae includes approximately 35–40 or more species of lampreys. Lampreys are similar to hagfishes in size and shape however, lampreys possess some vertebral elements. Lampreys lack paired appendages and bone, as do the hagfishes. As adults, lampreys are characterized by a toothed, funnel-like sucking mouth. Many species have a parasitic stage of their life cycle during which they are ectoparasites of fishes (Figure 2).

                  Lampreys live primarily in coastal and fresh waters, and have a worldwide distribution, except for in the tropics and polar regions. Some species are marine, but all species spawn in fresh water. Eggs are fertilized externally, and the larvae distinctly differ from the adult form, spending 3 to 15 years as suspension feeders. Once they attain sexual maturity, the adults reproduce and die within days.

                  Lampreys possess a notochord as adults however, this notochord is surrounded by a cartilaginous structure called an arcualia, which may resemble an evolutionarily early form of the vertebral column.


                  Contents

                  Bony fish are characterized by a relatively stable pattern of cranial bones, rooted, medial insertion of mandibular muscle in the lower jaw. The head and pectoral girdles are covered with large dermal bones. The eyeball is supported by a sclerotic ring of four small bones, but this characteristic has been lost or modified in many modern species. The labyrinth in the inner ear contains large otoliths. The braincase, or neurocranium, is frequently divided into anterior and posterior sections divided by a fissure.

                  Early bony fish had simple lungs (a pouch on either side of the esophagus) which helped them breathe in low-oxygen water. In many bony fish these have evolved into swim bladders, which help the body create a neutral balance between sinking and floating. (The lungs of amphibians, reptiles, birds, and mammals were inherited from their bony fish ancestors.) [12] [13] [14] They do not have fin spines, but instead support the fin with lepidotrichia (bone fin rays). They also have an operculum, which helps them breathe without having to swim.

                  Bony fish have no placoid scales. Mucus glands coat the body. Most have smooth and overlapping ganoid, cycloid or ctenoid scales.

                  Traditionally, Osteichthyes was considered a class, recognised on the presence of a swim bladder, only three pairs of gill arches hidden behind a bony operculum, and a predominately bony skeleton. [15] Under this classification system, Osteichthyes was considered paraphyletic with regard to land vertebrates, as the common ancestor of all osteichthyans includes tetrapods amongst its descendants. While the largest subclass, Actinopterygii (ray-finned fish), is monophyletic, with the inclusion of the smaller sub-class Sarcopterygii, Osteichthyes was regarded as paraphyletic.

                  This has led to the current cladistic classification which splits the Osteichthyes into two full classes. Under this scheme Osteichthyes is monophyletic, as it includes the tetrapods making it a synonym of the clade Euteleostomi. Most bony fish belong to the ray-finned fish (Actinopterygii).

                  Actinopterygii
                  ray-finned fish
                  Actinopterygii, members of which are known as ray-finned fishes, is a class or subclass of the bony fishes. The ray-finned fishes are so called because they possess lepidotrichia or "fin rays", their fins being webs of skin supported by bony or horny spines ("rays"), as opposed to the fleshy, lobed fins that characterize the class Sarcopterygii which also possess lepidotrichia. These actinopterygian fin rays attach directly to the proximal or basal skeletal elements, the radials, which represent the link or connection between these fins and the internal skeleton (e.g., pelvic and pectoral girdles). In terms of numbers, actinopterygians are the dominant class of vertebrates, comprising nearly 99% of the over 30,000 species of fish (Davis, Brian 2010). They are ubiquitous throughout freshwater and marine environments from the deep sea to the highest mountain streams. Extant species can range in size from Paedocypris, at 8 mm (0.3 in), to the massive ocean sunfish, at 2,300 kg (5,070 lb), and the long-bodied oarfish, to at least 11 m (36 ft).
                  Sarcopterygii
                  lobe-finned fish
                  Sarcopterygii (fleshy fin), members of which are known as lobe-finned fish, is a group of the bony fishes. Traditionally, it is a class or subclass that excludes Tetrapoda, a group of typically terrestrial vertebrates that descends from lobe-finned fish. However, under modern cladistic classification schemes, Sarcopterygii is a clade that includes the tetrapods. The living sarcopterygians are the coelacanths, lungfish, and the tetrapods. Early lobe-finned fishes had fleshy, lobed, paired fins, joined to the body by a single bone. [16] Their fins differ from those of all other fish in that each is borne on a fleshy, lobelike, scaly stalk extending from the body. Pectoral and pelvic fins have articulations resembling those of tetrapod limbs. These fins evolved into legs of the first tetrapod land vertebrates, amphibians. They also possess two dorsal fins with separate bases, as opposed to the single dorsal fin of actinopterygians (ray-finned fish). The braincase of sarcoptergygians primitively has a hinge line, but this is lost in tetrapods and lungfish. Many early lobe-finned fishes have a symmetrical tail. All lobe-finned fishes possess teeth covered with true enamel.

                  A phylogeny of living Osteichthyes, including the tetrapods, is shown in the cladogram. [4] [17] [18] [19]

                  Whole-genome duplication took place in the ancestral Osteichthyes. [20]

                  All bony fish possess gills. For the majority this is their sole or main means of respiration. Lungfish and other osteichthyan species are capable of respiration through lungs or vascularized swim bladders. Other species can respire through their skin, intestines, and/or stomach. [21]

                  Osteichthyes are primitively ectothermic (cold blooded), meaning that their body temperature is dependent on that of the water. But some of the larger marine osteichthyids, such as the opah, [22] [23] swordfish [24] [25] and tuna [26] [27] have independently evolved various levels of endothermy. Bony fish can be any type of heterotroph: numerous species of omnivore, carnivore, herbivore, filter-feeder or detritivore are documented.

                  Some bony fish are hermaphrodites, and a number of species exhibit parthenogenesis. Fertilization is usually external, but can be internal. Development is usually oviparous (egg-laying) but can be ovoviviparous, or viviparous. Although there is usually no parental care after birth, before birth parents may scatter, hide, guard or brood eggs, with sea horses being notable in that the males undergo a form of "pregnancy", brooding eggs deposited in a ventral pouch by a female.

                  The ocean sunfish is the heaviest bony fish in the world, [28] while the longest is the king of herrings, a type of oarfish. Specimens of ocean sunfish have been observed up to 3.3 metres (11 ft) in length and weighing up to 2,303 kilograms (5,077 lb). Other very large bony fish include the Atlantic blue marlin, some specimens of which have been recorded as in excess of 820 kilograms (1,810 lb), the black marlin, some sturgeon species, and the giant and goliath grouper, which both can exceed 300 kilograms (660 lb) in weight. In contrast, Paedocypris progenetica and the stout infantfish can measure less than 8 millimetres (0.31 in). [29] [30] The Beluga sturgeon is the largest species of freshwater bony fish extant today, and Arapaima gigas is among the largest of the freshwater fish. The largest bony fish ever was Leedsichthys, which dwarfed the beluga sturgeon as well as the ocean sunfish, giant grouper and all the other giant bony fishes alive today.

                  Cartilaginous fishes can be further divided into sharks, rays and chimaeras. In the table below, the comparison is made between sharks and bony fishes. For the further differences with rays, see sharks versus rays.


                  Vertebrate jaw design locked early: Study on initial diversification of jaws sheds light on early vertebrate feeding ecology

                  More than 99 per cent of modern vertebrates (animals with a backbone, including humans) have jaws, yet 420 million years ago, jawless, toothless armour-plated fishes dominated the seas, lakes, and rivers. There were no vertebrates yet on land and the recently evolved jawed fishes were minor players in this alien world, some sporting unusual jaw shapes and structures that bear little physical resemblance to modern animals.

                  The researchers, led by Dr Philip Anderson of Bristol's School of Earth Sciences, applied concepts from physics and engineering to unravel the potential feeding functions of these unusual, early vertebrate jaw designs, and compared this data to patterns of diversity in both jawed and jawless fishes. While it has long been assumed that jawed fishes were better adapted, and therefore directly out-competed and replaced their jawless neighbours during this tumultuous time, this assertion has never been tested.

                  Dr Anderson said: "Surprisingly, our results indicate that long-held assumptions concerning the replacement of jawless fishes by newly evolved jawed forms are likely wrong. The variety of feeding mechanisms in early jawed animals appears to have had little to no affect on the diversity of jawless fishes, which shared ecological space with the jawed fishes for at least 30 million years before beginning to notably decline. When the jawless fishes do decline, we see no indication that their jawed cousins took up new functional roles, calling into question old ideas of ecological replacement.

                  "Furthermore, jawed vertebrates achieved a stable diversity in their feeding apparatus early in their evolution, and maintained this diversity in the face of major environmental changes during the Devonian period. Previous studies have suggested that the rise of major jawed vertebrate ecological diversity is tied to a documented oxygenation event 400 million years ago, but our results place the first burst of diversification of jawed vertebrates well before that.

                  "The groups which comprise the majority of modern fish diversity (ray-finned fishes), as well as our own fish ancestors (early tetrapods), are restricted to only a few types of jaws and feeding ecologies, while bizarre, extinct groups (such as placoderms and a surprising number of extinct lungfishes) show a wide range of feeding ecologies that at the time dominated the jawed vertebrate world. It is interesting to speculate what modern jawed vertebrates might have looked like if these diverse groups hadn't been severely diminished (extinct in the case of the placoderms) after the Devonian."

                  The research group hopes that these new methods for assessing the variation in functional systems (such as feeding apparatus), will be applied to the study of other extinct groups during times of dramatic transitions, such as mass extinctions and evolutionary radiations.


                  29.2 Fishes

                  By the end of this section, you will be able to do the following:

                  • Describe the difference between jawless and jawed fishes
                  • Discuss the distinguishing features of sharks and rays compared to other modern fishes

                  Modern fishes include an estimated 31,000 species, by far the most of all clades within the Vertebrata. Fishes were the earliest vertebrates, with jawless species being the earliest forms and jawed species evolving later. They are active feeders, rather than sessile, suspension feeders. The Agnatha (jawless fishes)—the hagfishes and lampreys—have a distinct cranium and complex sense organs including eyes, that distinguish them from the invertebrate chordates, the urochordates and cephalochordates.

                  Jawless Fishes: Superclass Agnatha

                  Jawless fishes (Agnatha) are craniates representing an ancient vertebrate lineage that arose over 550 million years ago. In the past, hagfishes and lampreys were sometimes recognized as separate clades within the Agnatha, primarily because lampreys were regarded as true vertebrates, whereas hagfishes were not. However, recent molecular data, both from rRNA and mtDNA, as well as embryological data, provide strong support for the hypothesis that living agnathans—previously called cyclostomes—are monophyletic, and thus share recent common ancestry. The discussion below, for convenience, separates the modern “cyclostomes” into the class Myxini and class Petromyzontida. The defining features of the living jawless fishes are the lack of jaws and lack of paired lateral appendages (fins). They also lack internal ossification and scales, although these are not defining features of the clade.

                  Some of the earliest jawless fishes were the armored ostracoderms (which translates to “shell-skin”): vertebrate fishes encased in bony armor—unlike present-day jawless fishes, which lack bone in their scales. Some ostracoderms, also unlike living jawless fishes, may have had paired fins. We should note, however, that the “ostracoderms” represent an assemblage of heavily armored extinct jawless fishes that may not form a natural evolutionary group. Fossils of the genus Haikouichthys from China, with an age of about 530 million years, show many typical vertebrate characteristics including paired eyes, auditory capsules, and rudimentary vertebrae.

                  Class Myxini: Hagfishes

                  The class Myxini includes at least 70 species of hagfishes—eel-like scavengers that live on the ocean floor and feed on living or dead invertebrates, fishes, and marine mammals (Figure 29.9). Although they are almost completely blind, sensory barbels around the mouth help them locate food by smell and touch. They feed using keratinized teeth on a movable cartilaginous plate in the mouth, which rasp pieces of flesh from their prey. These feeding structures allow the gills to be used exclusively for respiration, not for filter feeding as in the urochordates and cephalochordates. Hagfishes are entirely marine and are found in oceans around the world, except for the polar regions. Unique slime glands beneath the skin release a milky mucus (through surface pores) that upon contact with water becomes incredibly slippery, making the animal almost impossible to hold. This slippery mucus thus allows the hagfish to escape from the grip of predators. Hagfish can also twist their bodies into a knot, which provides additional leverage to feed. Sometimes hagfish enter the bodies of dead animals and eat carcasses from the inside out! Interestingly, they do not have a stomach!

                  Hagfishes have a cartilaginous skull, as well as a fibrous and cartilaginous skeleton, but the major supportive structure is the notochord that runs the length of the body. In hagfishes, the notochord is not replaced by the vertebral column, as it is in true vertebrates, and thus they may (morphologically) represent a sister group to the true vertebrates, making them the most basal clade among the skull-bearing chordates.

                  Class Petromyzontida: Lampreys

                  The class Petromyzontida includes approximately 40 species of lampreys, which are superficially similar to hagfishes in size and shape. However, lampreys possess extrinsic eye muscles, at least two semicircular canals, and a true cerebellum, as well as simple vertebral elements, called arcualia—cartilaginous structures arranged above the notochord. These features are also shared with the gnathostomes—vertebrates with jawed mouths and paired appendages (see below). Lampreys also have a dorsal tubular nerve cord with a well-differentiated brain, a small cerebellum, and 10 pairs of nerves. The classification of lampreys is still debated, but they clearly represent one of the oldest divergences of the vertebrate lineage. Lampreys lack paired appendages, as do the hagfishes, although they have one or two fleshy dorsal fins. As adults, lampreys are characterized by a rasping tongue within a toothed, funnel-like sucking mouth. Many species have a parasitic stage of their life cycle during which they are fish ectoparasites (some call them predators because they attack and eventually fall off) (Figure 29.10).

                  Lampreys live primarily in coastal and freshwater environments, and have a worldwide distribution, except for the tropics and polar regions. Some species are marine, but all species spawn in fresh water. Interestingly, northern lampreys in the family Petromyzontidae, have the highest number of chromosomes (164 to 174) among the vertebrates. Eggs are fertilized externally, and the larvae (called ammocoetes) differ greatly from the adult form, closely resembling the adult cephalocordate amphioxus. After spending three to 15 years as suspension feeders in rivers and streams, they attain sexual maturity. Shortly afterward, the adults swim upstream, reproduce, and die within days.

                  Gnathostomes: Jawed Fishes

                  Gnathostomes , or “jaw-mouths,” are vertebrates that possess true jaws—a milestone in the evolution of the vertebrates. In fact, one of the most significant developments in early vertebrate evolution was the development of the jaw: a hinged structure attached to the cranium that allows an animal to grasp and tear its food. Jaws were probably derived from the first pair of gill arches supporting the gills of jawless fishes.

                  Early gnathostomes also possessed two sets of paired fins, allowing the fishes to maneuver accurately. Pectoral fins are typically located on the anterior body, and pelvic fins on the posterior. Evolution of the jaw and paired fins permitted gnathostomes to expand their food options from the scavenging and suspension feeding of jawless fishes to active predation. The ability of gnathostomes to exploit new nutrient sources probably contributed to their replacing most jawless fishes during the Devonian period. Two early groups of gnathostomes were the acanthodians and placoderms (Figure 29.11), which arose in the late Silurian period and are now extinct. Most modern fishes are gnathostomes that belong to the clades Chondrichthyes and Osteichthyes (which include the class Actinoptertygii and class Sarcopterygii).

                  Class Chondrichthyes: Cartilaginous Fishes

                  The class Chondrichthyes (about 1,000 species) is a morphologically diverse clade, consisting of subclass Elasmobranchii (sharks [Figure 29.12], rays, and skates, together with the obscure and critically endangered sawfishes), and a few dozen species of fishes called chimaeras, or “ghost sharks” in the subclass Holocephali. Chondrichthyes are jawed fishes that possess paired fins and a skeleton made of cartilage. This clade arose approximately 370 million years ago in the early or middle Devonian. They are thought to be descended from the placoderms, which had endoskeletons made of bone thus, the lighter cartilaginous skeleton of Chondrichthyes is a secondarily derived evolutionary development. Parts of shark skeleton are strengthened by granules of calcium carbonate, but this is not the same as bone.

                  Most cartilaginous fishes live in marine habitats, with a few species living in fresh water for a part or all of their lives. Most sharks are carnivores that feed on live prey, either swallowing it whole or using their jaws and teeth to tear it into smaller pieces. Sharks have abrasive skin covered with tooth-like scales called placoid scales. Shark teeth probably evolved from rows of these scales lining the mouth. A few species of sharks and rays, like the enormous whale shark (Figure 29.13), are suspension feeders that feed on plankton. The sawfishes have an extended rostrum that looks like a double-edged saw. The rostrum is covered with electrosensitive pores that allow the sawfish to detect slight movements of prey hiding in the muddy sea floor. The teeth in the rostrum are actually modified tooth-like structures called denticles, similar to scales.

                  Sharks have well-developed sense organs that aid them in locating prey, including a keen sense of smell and the ability to detect electromagnetic fields. Electroreceptors called ampullae of Lorenzini allow sharks to detect the electromagnetic fields that are produced by all living things, including their prey. (Electroreception has only been observed in aquatic or amphibious animals and sharks have perhaps the most sensitive electroreceptors of any animal.) Sharks, together with most fishes and aquatic and larval amphibians, also have a row of sensory structures called the lateral line , which is used to detect movement and vibration in the surrounding water, and is often considered to be functionally similar to the sense of “hearing” in terrestrial vertebrates. The lateral line is visible as a darker stripe that runs along the length of a fish’s body. Sharks have no mechanism for maintaining neutral buoyancy and must swim continuously to stay suspended in the water. Some must also swim in order to ventilate their gills but others have muscular pumps in their mouths to keep water flowing over the gills.

                  Sharks reproduce sexually, and eggs are fertilized internally. Most species are ovoviviparous: The fertilized egg is retained in the oviduct of the mother’s body and the embryo is nourished by the egg yolk. The eggs hatch in the uterus, and young are born alive and fully functional. Some species of sharks are oviparous: They lay eggs that hatch outside of the mother’s body. Embryos are protected by a shark egg case or “mermaid’s purse” (Figure 29.14) that has the consistency of leather. The shark egg case has tentacles that snag in seaweed and give the newborn shark cover. A few species of sharks, e.g., tiger sharks and hammerheads, are viviparous: the yolk sac that initially contains the egg yolk and transfers its nutrients to the growing embryo becomes attached to the oviduct of the female, and nutrients are transferred directly from the mother to the growing embryo. In both viviparous and ovoviviparous sharks, gas exchange uses this yolk sac transport.

                  In general, the Chondrichthyes have a fusiform or dorsoventrally flattened body, a heterocercal caudal fin or tail (unequally sized fin lobes, with the tail vertebrae extending into the larger upper lobe) paired pectoral and pelvic fins (in males these may be modified as claspers), exposed gill slits (elasmobranch), and an intestine with a spiral valve that condenses the length of the intestine. They also have three pairs of semicircular canals, and excellent senses of smell, vibration, vision, and electroreception. A very large lobed liver produces squalene oil (a lightweight biochemical precursor to steroids) that serves to aid in buoyancy (because with a specific gravity of 0.855, it is lighter than that of water).

                  Rays and skates comprise more than 500 species. They are closely related to sharks but can be distinguished from sharks by their flattened bodies, pectoral fins that are enlarged and fused to the head, and gill slits on their ventral surface (Figure 29.15). Like sharks, rays and skates have a cartilaginous skeleton. Most species are marine and live on the sea floor, with nearly a worldwide distribution.

                  Unlike the stereotypical sharks and rays, the Holocephali (chimaeras or ratfish) have a diphycercal tail (equally sized fin lobes, with the tail vertebrae located between them), lack scales (lost secondarily in evolution), and have teeth modified as grinding plates that are used to feed on mollusks and other invertebrates (Figure 29.15b). Unlike sharks with elasmobranch or naked gills, chimaeras have four pairs of gills covered by an operculum. Many species have a pearly iridescence and are extremely pretty.

                  Osteichthyes: Bony Fishes

                  Members of the clade Osteichthyes , also called bony fishes, are characterized by a bony skeleton. The vast majority of present-day fishes belong to this group, which consists of approximately 30,000 species, making it the largest class of vertebrates in existence today.

                  Nearly all bony fishes have an ossified skeleton with specialized bone cells (osteocytes) that produce and maintain a calcium phosphate matrix. This characteristic has been reversed only in a few groups of Osteichthyes, such as sturgeons and paddlefish, which have primarily cartilaginous skeletons. The skin of bony fishes is often covered by overlapping scales, and glands in the skin secrete mucus that reduces drag when swimming and aids the fish in osmoregulation. Like sharks, bony fishes have a lateral line system that detects vibrations in water.

                  All bony fishes use gills to breathe. Water is drawn over gills that are located in chambers covered and ventilated by a protective, muscular flap called the operculum. Many bony fishes also have a swim bladder , a gas-filled organ derived as a pouch from the gut. The swim bladder helps to control the buoyancy of the fish. In most bony fish, the gases of the swim bladder are exchanged directly with the blood. The swim bladder is believed to be homologous to the lungs of lungfish and the lungs of land vertebrates.

                  Bony fishes are further divided into two extant clades: Class Actinopterygii (ray-finned fishes) and Class Sarcopterygii (lobe-finned fishes).

                  Actinopterygii (Figure 29.16a), the ray-finned fishes, include many familiar fishes—tuna, bass, trout, and salmon among others—and represent about half of all vertebrate species. Ray-finned fishes are named for the fan of slender bones that supports their fins.

                  In contrast, the fins of Sarcopterygii (Figure 29.16b) are fleshy and lobed, supported by bones that are similar in type and arrangement to the bones in the limbs of early tetrapods. The few extant members of this clade include several species of lungfishes and the less familiar coelacanths, which were thought to be extinct until living specimens were discovered between Africa and Madagascar. Currently, two species of coelocanths have been described.


                  Watch the video: FISHESZOOLOGYVertebratescomplete concepts about fishesFish concepts u0026 classificationFish facts (May 2022).