2.1: Squirrel Fish - Biology

2.1: Squirrel Fish - Biology

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The fish species Holocentrus adscensionis, also known commonly as the squirrelfish or soldierfish, is found in the family Holocentridae. There are around 70 different species of the tropical reef squirrelfish. Squirrelfish are found distributed throughout the warm tropical waters of the Atlantic Ocean around the Caribbean, Bahamas, Florida, Bermuda, Turks and Caicos, and the Gulf of Mexico, where they typically remain at depths four feet to 40 feet where the waters are still warm but can go up to around 250 meters deep.

Squirrelfish have all five fins, a see-through pectoral fin, ventral, anal, and elongated dorsal and caudal tail fins. They also have fin spines along their spine with horizontal striped white lines along their back below them. In some species of squirrelfish, they have spines on their gill covers that are venomous that they use for self-protection. When the squirrelfish are juveniles they have more iridophores, cells that reflect light, which give them a silvery shimmer. When they transition into adults is when the red pigments of the chromatophores are more prominent distinguishing the young from the adults. The red orangey color helps them to blend in with the corals they sleep in during the day. The IUNC red list has classified squirrelfish globally as a species of least concern when they were assessed in January of 2013 but, global warming may have changed this trend as the corals are being bleached from being stressed as a result from warming ocean waters.

Squirrelfish are nocturnal carnivorous fish that hide in crevices of the coral reefs during the day to avoid predation. At night they swim through the reefs through seagrass beds hunting meroplankton, larvae and small crustaceans with the occasional small fish. When these fish are young, they tend to group together with one another which helps with protection and hunting, while adults prefer to establish their own territory and be alone. They are also able to communicate intra-specifically by producing sounds with their swim bladders. They make these sounds through vibrations to warn off predators or define their territory.

Stickleback fish provide genetic road map for rapid evolution

Three threespine sticklebacks, each about 2 1/2 inches long, that were captured from an Alaskan lake in 2010 as part of a three-decade study of their evolution from sea-faring fish to freshwater fish. Credit: Michael Bell

What happens when you dump an ocean fish into a freshwater lake?

That experiment has been performed naturally tens of thousands of times over millions of years as sea-faring threespine sticklebacks—which, like salmon, travel up rivers to spawn—have gotten stranded in lakes and had to evolve as permanent denizens of fresh water.

Michael Bell, currently a research associate in the University of California Museum of Paleontology at UC Berkeley, stumbled across one such natural experiment in 1990 in Alaska, and ever since has been studying the physical changes these fish undergo as they evolve and the genetic basis for these changes. He has even created his own experiments, seeding three Alaskan lakes with oceanic sticklebacks in 2009, 2011 and 2019 in order to track their evolution from oceanic fish to freshwater lake fish. This process appears to occur within decades—very unlike the slow evolution that Charles Darwin imagined—providing scientists a unique opportunity to actually observe vertebrate adaptation in nature.

The upshot of these 31 years of research is a study, published Friday, June 18, in the journal Science Advances, that details the genomic changes that drive sticklebacks' rapid evolution. The study, led by Bell, Krishna Veeramah at Stony Brook University and David Kingsley of Stanford University, sheds light on which genetic changes may underlie the evolutionary response to natural selection in other species.

"Our paper identifies specific genetic variants—DNA sequences—that occur at low frequency in marine populations of the threespine stickleback fish and are favored by natural selection when they colonize fresh water, which they have done countless times over the last 10 million years at least," Bell said.

Bell and his colleagues collected threespine sticklebacks (Gasterosteus aculeatus) from three of the Alaskan lakes each year and performed whole genome sequencing to track how their DNA evolved during adaptation to freshwater. They found hundreds of underlying genomic changes that form the basis of their rapid adaptation.

"We illustrated that the genomic features that were identified as important for rapid stickleback evolution can actually be used to predict the genomic location of where natural selection occurred in other species across the tree of life, such as Darwin's finches," said Veeramah. "This shows that the genomic mechanisms that govern evolution of freshwater stickleback underlie adaptation in species more generally."

"Older genes are larger than younger ones, and the larger, older genes tend to evolve faster than the younger, smaller ones," said Bell, who taught and conducted research at Stony Brook University for 40 years before retiring to California. "We identified similar trends in other species."

While the predictability of evolution is not an exact science, the authors believe their understanding of how the stickleback colonizes fresh water provide important insights into the genomics behind vertebrate evolution and provides insight into how evolution might proceed species-wide in the future.

Bell has been enamored of sticklebacks since the summer after his freshman year at UC San Diego, when he collected fossil sticklebacks in Nevada. He wrote his UCLA Ph.D. thesis on the fish's evolution and has been studying fossil and living sticklebacks ever since. It was a trip to Cook's Inlet in Alaska in 1990 that led to the current paper.

"I stumbled over the 35-year-old population in Loberg Lake during sampling for another study in 1990. It was an accident, but I recognized that rapid evolution must be occurring, so I followed up the next year and every year thereafter," he said. "In a sense, this (research project) is my baby and a unique evolutionary genomics paper."

Coelacanths may live nearly a century, five times longer than researchers expected

Adult coelacanth scales. Credit: Laurent Ballesta

Once thought to be extinct, lobe-finned coelacanths are enormous fish that live deep in the ocean. Now, researchers reporting in the journal Current Biology on June 17 have evidence that, in addition to their impressive size, coelacanths also can live for an impressively long time—perhaps nearly a century.

The researchers found that their oldest specimen was 84 years old. They also report that coelacanths live life extremely slowly in other ways, reaching maturity around the age of 55 and gestating their offspring for five years.

"Our most important finding is that the coelacanth's age was underestimated by a factor of five," says Kélig Mahé of IFREMER Channel and North Sea Fisheries Research Unit in Boulogne-sur-mer, France. "Our new age estimation allowed us to re-appraise the coelacanth's body growth, which happens to be one of the slowest among marine fish of similar size, as well as other life-history traits, showing that the coelacanth's life history is actually one of the slowest of all fish."

Earlier studies attempted to age coelacanths by directly observing growth rings on the scales of a small sample of 12 specimens. Those studies led to the notion that the fish didn't live more than 20 years. If that were the case, it would make coelacanths among the fastest-growing fish given their large size. That seemed surprising considering that the coelacanth's other known biological and ecological features, including slow metabolism and low fecundity, were more typical of fish with slow life histories and slow growth like most other deep-water species.

In the new study, Mahé, along with co-authors Bruno Ernande and Marc Herbin, took advantage of the fact that the French National Museum of Natural History (Muséum National d'Histoire Naturelle de Paris, MNHN) has one of the largest collections of coelacanths in the world, ranging from embryos in utero to individuals of almost two meters. They were able to examine 27 specimens in all. They also used new methods, including polarized light microscopy and scale interpretation technology mastered at IFREMER's Sclerochronology Centre, Boulogne-sur-mer, France, to estimate individuals' age and body growth more precisely than before.

While earlier studies relied on more readily visible calcified structures called macro-circuli to age the coelacanths much as counting growth rings can age a tree, the new approaches allowed the researchers to pick up on much tinier and nearly imperceptible circuli on the scales. Their findings suggest that the coelacanths actually are about five times older than was previously thought.

A coelacanth embryo with yolk sac from the MNHN collection. Credit: MNHN

"We demonstrated that these circuli were actually annual growth marks, whereas the previously observed macro-circuli were not," Mahé says. "It meant that the maximum longevity of coelacanth was five times longer than previously thought, hence around a century."

Their study of two embryos showed they were both about five years old. Using a growth model to back-calculate gestation length based on the size of offspring at birth, the researchers got the same answer. They now think that coelacanth offspring grow and develop for five years inside their mothers prior to birth.

"Coelacanth appears to have one of, if not the slowest life histories among marine fish, and close to those of deep-sea sharks and roughies," Mahé says.

The researchers say that their findings have implications for the coelacanth's conservation and future. They note that the African coelacanth is assessed as critically endangered in the Red List of Threatened Species of IUCN.

"Long-lived species characterized by slow life history and relatively low fecundity are known to be extremely vulnerable to perturbations of a natural or anthropic nature due to their very low replacement rate," Mahé says. "Our results thus suggest that it may be even more threatened than expected due to its peculiar life history. Consequently, these new pieces of information on coelacanths' biology and life history are essential to the conservation and management of this species."

In future studies, they plan to perform microchemistry analyses on coelacanth scales to find out whether a coelacanth's growth is related to temperature. The answer will provide some insight into the effects of global warming on this vulnerable species.

When your veins fill with ice

Most animals hate the cold. When winter comes around many species burrow underground to hibernate or migrate to lower latitudes where conditions are warmer.

But a few strange creatures do the opposite. They actually embrace the freezing conditions.

We are still unravelling the mysteries of these amazing animals that freeze. For one species in particular, doing so could prove significant. Several scientists are trying to work out how the Arctic ground squirrel (Spermophilus parryii) became the only known warm-blooded mammal to be able to tolerate subzero body temperatures. Solving the mystery could hold the key to freezing human organs for transplant without damaging them.

It might even provide a boost for the controversial field of cryonics, in which human corpses are put into deep freeze in the hope that they can be returned to life with future medical advances.

Subzero temperatures are a problem for all living things because water expands as it freezes to become ice. &ldquoWhen water in an animal's cells freezes, the ice crystals that form expand and physically rupture the cell, causing death,&rdquo explains David Denlinger, an entomologist at Ohio State University in Columbus. To survive freezing temperatures, animals must find a way to prevent ice from forming inside their cells.

Cryoprotectants, much like the antifreeze you put into your car radiator, prevent ice forming by reducing the freezing point of water

The most famous freeze-tolerant species is probably the wood frog (Rana sylvatica), which can survive subarctic temperatures for weeks at a time.

&ldquoFreeze tolerant organisms use various "tricks" to limit the damage that occurs when ice forms within tissues,&rdquo explains Jon Costanzo from the Laboratory for Ecophysiological Cryobiology at Miami University in Ohio. &ldquoOur team was the first to examine the extreme freeze tolerance in wood frogs living in subarctic Alaska,&rdquo he says. &ldquoThose frogs all survived freezing to -14C!&rdquo

The frogs are able to cope with freezing temperatures by producing &ldquocryoprotectants&rdquo &ndash substances that prevent ice crystals from forming inside their cells.

Cryoprotectants, much like the antifreeze you put into your car radiator, prevent ice forming by reducing the freezing point of water.

&ldquoCryoprotectants act by lowering the body's freezing point, preventing water freezing at temperatures well below 0C,&rdquo explains Denlinger. When a cryoprotectant chemical dissolves in water, it forms strong bonds with water molecules. Water molecules that are bonded to the cryoprotectant can no longer bond with other water molecules to form ice, meaning that the water can be cooled to subzero temperatures without freezing.

Wood frogs produce urea, glucose and glycogen to act as cryoprotectants in response to subzero temperatures, and researchers have now identified freeze-induced genes that are responsible for transporting glucose into cells.

The red flat bark beetle from Alaska can supercool its body fluids to -50C

Freezing vertebrates and insects also produce antifreeze proteins, which bind to ice crystals and prevent them growing. One compound, antifreeze glycolipid (AFGL) is used as a defence against freezing in organisms ranging from plants to beetles, and in 2014 scientists identified the same glycolipid as an antifreeze compound in the wood frog.

Most impressive among freezing animals may be the invertebrates. A host of arthropods from cockroaches to caterpillars can tolerate freezing temperatures for days at a time.

Many of these species use cryoprotectants and antifreeze proteins to protect their cells, and Denlinger says that body fluid freezing points (also called supercooling points) as low -25C are not uncommon.

One truly remarkable beetle, the red flat bark beetle (Cucujus clavipes puniceus) from Alaska, can supercool its body fluids to -50C. But there is huge variation between individuals &ndash some can tolerate body temperatures as low as -100C.

By dehydrating themselves, Antarctic midges are able to survive temperatures as low as -20C

These deep-supercooling beetles have higher levels of antifreeze proteins and cryoprotectants like glycerol, which help them to minimise ice formation even at such extreme temperatures.

But these remarkable adaptations do not explain the freezing feats of the ground squirrel. Arctic ground squirrels don&rsquot use cryoprotectants to protect their cells, and no antifreeze compounds have been identified in their blood.

&ldquoMammals do not flood their bodies with cryoprotectants,&rdquo says Brian Barnes from the Institute of Arctic Biology at the University of Alaska Fairbanks, who has been studying the hibernation of the Arctic ground squirrel for over two decades. Something else is going on inside the squirrels&rsquo bodies.

The Antarctic midge (Belgica antarctica) has come up with an alternative solution to cope with freezing temperatures. &ldquoThe Antarctic midge does not make cryoprotectants nor antifreeze proteins,&rdquo says Denlinger. &ldquoInstead, it simply gets rid of its body water.&rdquo

If there&rsquos no water, there can be no ice

Most insects can survive losing 20-30% of their water, but the Antarctic midge can lose up to 70% and still survive, he says. By dehydrating themselves, Antarctic midges are able to survive temperatures as low as -20C.

While frozen, their metabolism stops and they appear lifeless. &ldquoWhen it has lost so much water, it does not appear to be alive,&rdquo says Denlinger, &ldquobut when you add water it quickly becomes fully hydrated again and goes on its merry way.&rdquo

&ldquoThe midge has the ability of cryoprotective dehydration," says Shin Goto, who studies animal physiology at the Osaka City University in Japan. He explains that when their habitat freezes, the surrounding ice draws water from the midges&rsquo highly permeable bodies, preventing any ice from forming inside their delicate tissues.

Dehydration is an effective way to prevent ice formation &ndash if there&rsquos no water, there can be no ice. But no mammal can survive such extreme dehydration. Cryoprotective dehydration definitely doesn&rsquot explain the mystery of the Antarctic ground squirrel&rsquos super-cool stunt.

Prevention isn&rsquot the only way to deal with ice. Another trick used by wood frogs and other freeze-tolerant animals is to produce substances known as ice-nucleating agents, which actively encourage ice formation.

Although counter-intuitive, ice-nucleating compounds produced in the right place ensure that ice forms between cells, not inside them. This &lsquoextracellular ice&rsquo is less harmful because its sharp crystals are kept away from delicate machinery inside the cell.

These frozen animals have evolved adaptations to survive a lack of oxygen

An ice nucleator could be almost anything: a speck of dust or even a bacterial cell. Inside freeze-tolerant organisms they are thought to be proteins or fats, although scientists are yet to isolate these natural ice-nucleators in the laboratory.

&ldquoIce nucleators are molecular mimics of ice,&rdquo says Barnes. They have a similar structure to ice crystals, enabling them to act as a seed to start the ice-crystal formation process. Wood frog blood contains ice-nucleating proteins, and ice-nucleators have been identified in insects, molluscs and even plants.

When frozen, wood frogs and other freeze-tolerant vertebrates are forced to shut their systems down &ndash they have no heartbeat or breathing. So as well as adapting to having ice inside their bodies, these frozen animals have evolved adaptations to survive a lack of oxygen, known as anoxia.

Painted turtle (Chrysemys picta marginata) hatchlings produce antioxidants and proteins that bind iron - part of the body&rsquos usual response to a lack of oxygen &ndash in reaction to freezing temperatures. A study published in 2015 confirmed a similar defence mechanism to anoxia in dehydrated Antarctic midges, whose larvae increase the expression of antioxidants when frozen.

The Arctic ground squirrel can survive body temperatures to -3C, but they do not freeze

But like cryoprotective dehydration, ice-nucleation is not a strategy that would work for ground squirrels. While invertebrates like the Antarctic midge, and cold-blooded animals like the wood frog, have evolved to tolerate some freezing inside their bodies, this just isn&rsquot a possibility for mammals.

In fact, it is the very absence of ice-nucleators that turns out to be key to the squirrel&rsquos unique abilities.

&ldquoArctic ground squirrels cleanse their bodies of would be ice-nucleators before entering hibernation,&rdquo says Barnes. Although the exact mechanism remains a mystery, Barnes&rsquo research suggests the ground squirrels may achieve this by producing masking agents, which neutralise ice-nucleators before ice has a chance to form around them.

&ldquoThe Arctic ground squirrel can survive body temperatures to -3C, but they do not freeze&rdquo, he explains. &ldquoInstead body fluids within the arctic ground squirrel enter a &lsquosupercooled&rsquo state&rdquo.

Without any effective nucleators to get the ice crystals started, water in the squirrel&rsquos blood simply can&rsquot freeze.

Whatever the strategy for tolerating subzero body temperatures, the strain of this physiological feat is evident. Most freeze-tolerant species can only pull off the trick once a year. When tested in spring, wood frogs were only able to tolerate temperatures down to -5C still impressive, but nothing compared to their autumnal feats.

&ldquoWe don't fully understand why freeze tolerance is so abruptly reduced in spring, but a limited capacity for cryoprotectant production is probably at issue,&rdquo explains Costanzo. &ldquoBoth major cryoprotectants, glucose and urea, are at lower levels in spring frogs&rdquo &ndash and these spring frogs also suffer more ice damage when artificially frozen.

Understanding these processes could help us develop new techniques to freeze human organs for storage prior to transplantation

This may be a common problem one study found that four species of freeze-tolerant frog all showed the same loss of freeze-tolerance in spring, linked to a reduced ability to produce cryoprotectants.

In fact, most freeze-tolerant species benefit from at least a short period of acclimatisation to an oncoming freeze. Sudden drops in temperature can be bad news even for the hardiest of species.

The Isabella tiger moth (Pyrrharctia isabella) lives in the Arctic and produces caterpillars that can survive air temperatures down to -20C &ndash supercooling their body fluids as low as -10C. For these caterpillars, 12 weeks of acclimatisation reduced the freezing point of their body fluids by nearly 2C. It gave them time to physiologically adapt by producing glycerol, proline and amino acids to act as cryoprotectants.

The Arctic ground squirrel is also far better at coping with the cold during the winter months. &ldquoBlood sampled from hibernating animals can supercool to much lower levels than blood sample from animals in summer,&rdquo says Barnes, although the exact mechanism remains a mystery.

He suspects it may be linked to larger scale seasonal changes in the squirrels. &ldquoWe are currently studying how they time their annual cycles of hibernation.&rdquo He says these are determined by an internal &ldquocalendar&rdquo in the brain, which sets the timing of the physiological changes that adapt them for the winter months.

Across the animal kingdom, creatures living in the coldest parts of the world have developed adaptations to cope with freezing conditions. Understanding these processes could help us develop new techniques to freeze human organs for storage prior to transplantation. What&rsquos more, learning how animals freeze may be the key to making human cryopreservation a reality.

If we are to cryopreserve human cells successfully, we need to understand how warm-blooded organisms like the squirrel cope

Currently, organs for transplant are chilled, but not frozen. This means they are viable for just a few hours, and so an organ that could save someone&rsquos life can&rsquot always reach the patient in time. Increasing the viability of transplanted organs by freezing them could revolutionise organ transplantation &ndash and cold-tolerant animals like the arctic ground squirrel could hold the secrets to making this possible.

In fact, research into freeze-tolerance is already yielding results.

Antifreeze proteins identified in fish and insects are an obvious target for improving cryopreservation techniques, and in 2005 a team of researchers at the University of California, Berkeley and Sheba Medical Center in Israel, successfully used antifreeze proteins isolated from Antarctic fish to freeze and preserve rat hearts for 21 hours. These hearts were then successfully transplanted into recipient rats, where they continued to beat for at least 24 hours.

Tissue preservation companies are now investigating whether insect antifreeze proteins, which are more effective, could act as potential cryoprotectants for human organ preservation.

What we can learn about freezing humans from insects and frogs may be dwarfed by what the arctic ground squirrel can teach us, though. Freezing an insect or amphibian is one thing, but if we are to cryopreserve human cells successfully, we need to understand how warm-blooded organisms like the squirrel cope with subzero temperatures.

Even the most impressive animals can only cope with temperatures down to about -50C

If we could identify the ground squirrel&rsquos technique for removing ice-nucleators, and apply that to humans, we might be able to supercool cells and organs without even a single crystal of ice forming inside.

Some also hope freezing technology may one day help us to preserve whole humans. In fact, cryonics methods are already beginning to be informed by our understanding of the protective mechanisms used by freeze tolerant animals.

For instance, some cryonics companies dehydrate bodies, replacing the blood with a solution of cryoprotectants such as glycerol and dimethyl sulfoxide, and finally deep-freezing them, in the hope that doing so won&rsquot damage the human tissues or prevent reanimation in the future.

But even the most impressive animals can only cope with temperatures down to about -50C, nowhere near the deep-freezing temperatures used in whole body cryopreservation.

Shin Goto sees potential for research into insect cryoprotectants and anti-freeze proteins to improve cryopreservation of human cells, tissues and organs. But he says he doubts that we will ever be able to freeze and thaw whole humans. &ldquoA human is too large in size to freeze and thaw,&rdquo he says.

David Denlinger agrees, saying &ldquoI think that is pretty unlikely, but I do hold out hope for freezing and thawing human tissues.&rdquo He adds that cryoprotective dehydration as seen in the Antarctic midge has &ldquotremendous implications for the field of organ storage&rdquo.

Learning how animals freeze may be the key to making human cryopreservation a reality

&ldquoIf we truly understood how [the midge] does this, we could possibly adopt such mechanisms to store human tissues and organs,&rdquo he says.

Three decades of research into freeze-tolerant fish, frogs and insects has taught us a lot about how to survive subzero temperatures. But many mysteries remain.

The arctic ground squirrel, the only mammal to survive subzero body temperatures for extended periods of time, could offer our best chance of safely entering the world of subzero tissue preservation. It&rsquos just about possible that, one day, a squirrel could save your life.

Biology of Finfish and Shellfish

They simply scatter their eggs in the environment and they do not have specialized reproductive structures. This group is further divided into two categories.

1. Pelagic Spawners

These are known as pelagophils. They spawn in open waters and this strategy is exhibited by many schooling fish like sardines, mackerels, and tunas. In addition to pelagic fishes some of the demersal benthic fishes, also release pelagic eggs. These pelagic eggs are buoyant and are planktonic in nature. Eggs contain oil globule and lot of water content to ensure floatation.

But these pelagic spawners have some disadvantages.

2. Benthic spawners

These fishes deposit the eggs on the substratum and eggs are adhesive. They release their eggs on known area and they are mass spawners and there is no courtship behaviour and also they do not care for eggs and young ones. They lay eggs in long strings or thick thread. The benthic spawners are broadly classified into 3 categories.

Biology and population dynamics of the freshwater puffer fish, Tetraodon lineatus (Linnaeus, 1758), from the River Nile, Aswan, Egypt

A fresh water pufferfish Tetraodon lineatus in Egypt has limited information available. A total of 350 specimens were collected monthly from the river Nile, Aswan (2019). Specimens were 13.5–38.5 cm in total length with an average of 22.16 ± 5.02 cm and a total weight of 61.7–1456 g with an average of 339.86 ± 248.85 g. The length-weight relationship showed negative allometric growth, and both absolute and relative conditions decreased after March towards September, represents the spawning period. The sex ratio of 1 male to 1.28 females showed the predomination of females (56.23%) over males (43.77%) over the year. Males dominated the first length classes of 14–19 cm. The gonadosomatic index (GSI) was elevated from April to August, with its highest peak in June, and the unimodal curve indicated spawning once per year. The hepatosomatic index showed an inverse trend against the increase of GSI. Length at maturity was 18.7 cm for males and 18.2 for females. The lifespan of the fish was determined by the counting the growth rings of vertebrae No 3 was five years for both sexes at mean lengths of 14.5, 21.2, 26.3, 31.2, and 34.2 cm for 1st, 2nd, 3rd, 4th, and 5th, years respectively. Age group III was the most abundant followed by age group II. Asymptotic length was 46.29 cm, condition was 0.242 year −1 , and to was −0.609 year −1 , growth performance was 2.71, and longevity was 11.78 years. Our results are among the first biology and population dynamics data on the freshwater pufferfish in Egypt, and they indicate a satisfactory growth rate and ability to live longer than their age at maturity. These data could be utilized as the basis for scientific studies of physiology and toxicology, and useful in further management.

Food Chain in Ecosystem (Explained with Diagrams)

For an ecosystem to work there has to be a flow of energy within it. The organisms of the ecosystem need energy in the form of food.

The ultimate source of this energy is the sun. Producers like green plants trap solar energy and convert it into the chemical energy of food. When a primary consumer eats the producer, a part of this energy is passed on to it.

The primary consumer is then eaten by a secondary consumer. And the secondary consumer may be eaten by a tertiary consumer, and so on. In this way energy gets transferred from one consumer to the next higher level of consumer. A series of organisms through which food energy flows in an ecosystem is called a food chain. It may also be defined as follows.

A food chain in an ecosystem is a series of organisms in which each organism feeds on the one below it in the series.

In a forest ecosystem, grass is eaten by a deer, which in turn is eaten by a tiger. The grass, deer and tiger form a food chain (Figure 8.2). In this food chain, energy flows from the grass (producer) to the deer (primary consumer) to the tiger (secondary consumer).

A food chain in a grassland ecosystem may consist of grasses and other plants, grasshoppers, frogs, snakes and hawks (Figure 8.3).

In a freshwater aquatic ecosystem like a pond, the organisms in the food chain include algae, small animals, insects and their larvae, small fish, big fish and a fish-eating bird or animal (Figure 8.4).

A food chain always begins with producers. Herbivores (plant-eaters) come next in the chain. They are consumed by carnivores (flesh-eaters). A few food chains can be long and may extend to the fourth, fifth or even sixth order of consumers.

Centropomus undecimalis

Common Snook. Photo © George Burgess

These golden yellow fish have a very distinct black lateral line, sloping forehead, and protruding lower jaw. They are a very popular game fish that prefer near-shore vegetative habitats like river mouths and salt marshes, growing to over 40 inches long at times. These hermaphrodites change from male to female as they mature and their pelvic and caudal fins are noticeably more yellow during spawn.

Order – Perciformes Family – Centropomidae Genus – Centropomus Species – undecimalis

Common Names

The English language common names for this species are common snook, linesiders, pike, sargeant fish, snook and thin snook. Other common names include chyk (Russian), alimindelig robalo (Danish), almindelig snook (Danish), bicudo (Portuguese), cambriacu (Portuguese), camburiacu (Portuguese), camorim (Portuguese), camorim-acu (Portuguese), camuri (Portuguese), camurim (Portuguese), camurim branco (Portuguese), camurim-acu (Portuguese), camuri-cabo-de-machado (Portuguese), camurim-preto (Portuguese), camurimpema (Portuguese), camuripeba (Portuguese), camuripema (Portuguese), cangoropeba (Portuguese), canjurupeba (Portuguese), esalho (Portuguese), falso-robalo-branco (Portuguese), rabalão, robalo (Portuguese), rabalo-bicudo (Portuguese), robalo-branco (Portuguese), robalo-camurim (Portuguese), robalo-de-galha (Portuguese), robalo-estoque (Portuguese), robalo-flecha (Portuguese), robalo-flexa (Portuguese), rolão (Portuguese), bima (Papiamento), sapat’i solda (Papiamento), snoekoe (Papiamento), binnensnoek (Dutch), snoek (Dutch), brochet (French), brochet de mer (French), crossie blanc (French), loubine (French), kamuli (Galibi), loubin gran lanmè (Creole, French), lubi (Palicur), pakiyau (Wayana), quéquere (Spanish), robalito (Spanish), róbalo (Spanish), robalo (Spanish), robalo blanco (Spanish), róbalo blanco (Spanish), róbalo común (Spanish), robalo (Swedish), signokou (Djuka), snoek (Sranan), snoekoe (Sranan), snook (German) and zuchwiak (Polish).

Importance to Humans

Common snook is a hugely popular recreational fish in the Gulf Coast due to its fighting ability and culinary value. The commercial harvest of common snook is prohibited throughout Texas and Florida (USA).

Conservation Status

Common snook are commercially exploited throughout most of their range except in Texas and Florida were they support a large recreational fishery. The Florida state legislation declared common snook a gamefish in 1957 and prohibited its sale. There has been no legal commercial harvest of common snook in Florida since this piece of legislation was enacted. Common snook is also commercially protected in Texas. In January 1999, the Florida Fish and Wildlife Conservation Commission implemented a slot limit, which means fish can only be recreationally harvested between the sizes of 26 to 34 inches (66 to 86 cm) in order to protect the larger breeding females. Common snook fishing is closed between December 15 and January 31 statewide in Florida and a bag limit of 1 or 2 fish per person per day depending on the area is strictly enforced (FWC fishing regulations). The common snook is not listed on the IUCN Red list.

Geographical Distribution

World distribution map for the snook

Common snook are the most widely distributed species within the Centropomus genus and have been reported as far north as New York (USA) and throughout the Gulf of Mexico. Common snook are abundant along the Atlantic coast of Florida from Cape Canaveral south through the Keys and Dry Tortugas, and north to Cedar Key on the gulf coast. Common snook occur infrequently along the coast of Texas to Galveston and then more or less continuously south to Rio de Janeiro, Brazil.


Juvenile common snook are generally restricted to the protection of riverine and estuary environments. These environments offer shallow water and an overhanging vegetative shoreline. Juvenile common snook can survive in waters with lower oxygen levels than adults. Adult common snook inhabit many environments including mangrove forests, beaches, river mouths, nearshore reefs, salt marshes and sea grass meadows. Adult common snook appear to be less sensitive to cold water temperatures than larvae or small juveniles. The lower lethal limit of water temperature is 48.2°-57.2° F (9°-14° C) for juveniles and 42.8°-53.6° F (6°-12° C) for adults.


Common snook. Photo courtesy National Park Service

Distinctive Features
Common snook have a slender body and a distinct lateral line. The dorsal fins are high and divided and the anal spines are relatively short. The common snook has a sloping forehead with a large mouth and a protruding lower jaw. Adult common snook can grow to over 47.24 inches (120 cm) in total length , which is larger than any other species in this family.

Coloration of the common snook is golden yellow with a distinct black lateral line and pale yellow pelvic fins.

The common snook lacks teeth on the maxillae. There is a band of fine teeth located on the premaillae and the dentaries. The palatine bone has a narrow band of teeth on it and the ectopterygoid bone located behind the palate may or may not have a tooth patch associated with it.

Size, Age & Growth
Common snook on the Atlantic coast of Florida commonly grow to larger sizes than common snook on the gulf coast of Florida. The largest observed sizes for females on the Atlantic and gulf coasts are 43.5 inches and 40.6 inches (110.5 and 103.2 cm) respectively. The world record for a common snook caught on hook and line is a 53-pound 10-ounce (24.28 kg) in Parismina Ranch, Costa Rica.

Theoretical longevity estimates from age and growth studies suggest that common snook can live to about twenty years old. On the Atlantic coast, the oldest sampled common snook was an eighteen-year-old female and the oldest male was fifteen. On the gulf coast, the oldest common snook sampled was a fifteen year old female and the oldest male was twelve.

Food Habits
Larval and small common snook eat mainly copepods and microcrustaceans. As common snook grow larger they eat fish, shrimp, crabs and zooplankton. This change in food habits occurs at around 1.8 inches (4.5 cm) standard length and continues throughout adulthood.

Common snook in an aquarium. Photo courtesy NOAA

Common snook are protandric hermaphrodites, changing from male to female after maturation. This transition is identified by the presence of both male and female sex cells in the gonads and takes place when they grow to between 9.4-2.4 inches (24.0-82.4 cm) fork length which corresponds to 1-7 years of age. A study conducted in 2000, indicated that the sex ratios for common snook ages 0 – 2 are significantly skewed between the east and west coasts of Florida (USA) due to protrandry and differences in growth and mortality rates. The majority of small common snook are male and most large snook are female. Males reach sexual maturity during their first year at 5.9-7.9 inches (15.0-20.0 cm) fork length. Research shows that female gonads mature directly from the mature male gonads shortly after spawning. The probability that a common snook of a particular size will be a female increases with length or age.

The common snook are preyed upon by dolphins, birds including osprey and heron, and larger fish.

Three species of myxosporeans [Myxobolus centropomi (trophozoites in gill, psuedobranch and under scales), Ceratomyxa choleospora (trophozoites and spores in gallbladder, spores in fecal casts) and Fabespora sp. (spores released in feces)] have been found to parasitize common snook in Florida.


Bloch first described Centropomus undecimalis in 1792 in Jamaica. Synonyms include Sciaena undecimalis Bloch 1792, Centropomus undecim-radiatus Lacèpéde 1802, Perca loubina Lacèpéde 1802, Sphyraena aureoviridis Lacèpéde 1803, Centropomus appedniculatus Poey 1860 and Centropomus argenteus Regan 1904. The genus name Centropomus is derived from the Greek word “kentron” meaning to sting and the Greek “poma, -atos” meaning cover, operculum.

Allopatric Speciation

A geographically continuous population has a gene pool that is relatively homogeneous. Gene flow, the movement of alleles across a species’ range, is relatively free because individuals can move and then mate with individuals in their new location. Thus, an allele’s frequency at one end of a distribution will be similar to the allele’s frequency at the other end. When populations become geographically discontinuous, it prevents alleles’ free-flow. When that separation lasts for a period of time, the two populations are able to evolve along different trajectories. Thus, their allele frequencies at numerous genetic loci gradually become increasingly different as new alleles independently arise by mutation in each population. Typically, environmental conditions, such as climate, resources, predators, and competitors for the two populations will differ causing natural selection to favor divergent adaptations in each group.

Isolation of populations leading to allopatric speciation can occur in a variety of ways: a river forming a new branch, erosion creating a new valley, a group of organisms traveling to a new location without the ability to return, or seeds floating over the ocean to an island. The nature of the geographic separation necessary to isolate populations depends entirely on the organism’s biology and its potential for dispersal. If two flying insect populations took up residence in separate nearby valleys, chances are, individuals from each population would fly back and forth continuing gene flow. However, if a new lake divided two rodent populations continued gene flow would be unlikely therefore, speciation would be more likely.

Biologists group allopatric processes into two categories: dispersal and vicariance. Dispersal is when a few members of a species move to a new geographical area, and vicariance is when a natural situation arises to physically divide organisms.

Figure 2. The northern spotted owl and the Mexican spotted owl inhabit geographically separate locations with different climates and ecosystems. The owl is an example of allopatric speciation. (credit “northern spotted owl”: modification of work by John and Karen Hollingsworth credit “Mexican spotted owl”: modification of work by Bill Radke)

Scientists have documented numerous cases of allopatric speciation taking place. For example, along the west coast of the United States, two separate spotted owl subspecies exist. The northern spotted owl has genetic and phenotypic differences from its close relative: the Mexican spotted owl, which lives in the south (Figure 2).

Additionally, scientists have found that the further the distance between two groups that once were the same species, the more likely it is that speciation will occur. This seems logical because as the distance increases, the various environmental factors would likely have less in common than locations in close proximity. Consider the two owls: in the north, the climate is cooler than in the south. The types of organisms in each ecosystem differ, as do their behaviors and habits. Also, the hunting habits and prey choices of the southern owls vary from the northern owls. These variances can lead to evolved differences in the owls, and speciation likely will occur.

Adaptive Radiation

In some cases, a population of one species disperses throughout an area, and each finds a distinct niche or isolated habitat. Over time, the varied demands of their new lifestyles lead to multiple speciation events originating from a single species. We call this adaptive radiation because many adaptations evolve from a single point of origin thus, causing the species to radiate into several new ones. Island archipelagos like the Hawaiian Islands provide an ideal context for adaptive radiation events because water surrounds each island which leads to geographical isolation for many organisms. The Hawaiian honeycreeper illustrates one example of adaptive radiation. From a single species, the founder species, numerous species have evolved, including the six in Figure 3.

Figure 3. The honeycreeper birds illustrate adaptive radiation. From one original species of bird, multiple others evolved, each with its own distinctive characteristics.

Notice the differences in the species’ beaks in Figure 3. Evolution in response to natural selection based on specific food sources in each new habitat led to evolution of a different beak suited to the specific food source. The seed-eating bird has a thicker, stronger beak which is suited to break hard nuts. The nectar-eating birds have long beaks to dip into flowers to reach the nectar. The insect-eating birds have beaks like swords, appropriate for stabbing and impaling insects. Darwin’s finches are another example of adaptive radiation in an archipelago.

Watch this video to see how scientists use evidence to understand how birds evolved.

Can divergence occur if no physical barriers are in place to separate individuals who continue to live and reproduce in the same habitat? The answer is yes. We call the process of speciation within the same space sympatric. The prefix “sym” means same, so “sympatric” means “same homeland” in contrast to “allopatric” meaning “other homeland.” Scientists have proposed and studied many mechanisms.

One form of sympatric speciation can begin with a serious chromosomal error during cell division. In a normal cell division event chromosomes replicate, pair up, and then separate so that each new cell has the same number of chromosomes. However, sometimes the pairs separate and the end cell product has too many or too few individual chromosomes in a condition that we call aneuploidy (Figure 4).

Figure 4. Aneuploidy results when the gametes have too many or too few chromosomes due to nondisjunction during meiosis. In the example shown here, the resulting offspring will have 2n+1 or 2n−1 chromosomes

Practice Question

In Figure 4, which is most likely to survive, offspring with 2n+1 chromosomes or offspring with 2n-1 chromosomes?

Polyploidy is a condition in which a cell or organism has an extra set, or sets, of chromosomes. Scientists have identified two main types of polyploidy that can lead to reproductive isolation of an individual in the polyploidy state. Reproductive isolation is the inability to interbreed. In some cases, a polyploid individual will have two or more complete sets of chromosomes from its own species in a condition that we call autopolyploidy (Figure 5). The prefix “auto-” means “self,” so the term means multiple chromosomes from one’s own species. Polyploidy results from an error in meiosis in which all of the chromosomes move into one cell instead of separating.

Figure 5. Autopolyploidy results when mitosis is not followed by cytokinesis.

For example, if a plant species with 2n = 6 produces autopolyploid gametes that are also diploid (2n = 6, when they should be n = 3), the gametes now have twice as many chromosomes as they should have. These new gametes will be incompatible with the normal gametes that this plant species produces. However, they could either self-pollinate or reproduce with other autopolyploid plants with gametes having the same diploid number. In this way, sympatric speciation can occur quickly by forming offspring with 4n that we call a tetraploid. These individuals would immediately be able to reproduce only with those of this new kind and not those of the ancestral species.

The other form of polyploidy occurs when individuals of two different species reproduce to form a viable offspring called an allopolyploid. The prefix “allo-” means “other” (recall from allopatric): therefore, an allopolyploid occurs when gametes from two different species combine. Figure 6 illustrates one possible way an allopolyploid can form. Notice how it takes two generations, or two reproductive acts, before the viable fertile hybrid results.

Figure 6. Alloploidy results when two species mate to produce viable offspring. In the example shown, a normal gamete from one species fuses with a polyploidy gamete from another. Two matings are necessary to produce viable offspring.

The cultivated forms of wheat, cotton, and tobacco plants are all allopolyploids. Although polyploidy occurs occasionally in animals, it takes place most commonly in plants. (Animals with any of the types of chromosomal aberrations that we describe here are unlikely to survive and produce normal offspring.) Scientists have discovered more than half of all plant species studied relate back to a species evolved through polyploidy. With such a high rate of polyploidy in plants, some scientists hypothesize that this mechanism takes place more as an adaptation than as an error.

Outdoor Annual

The Texas Parks and Wildlife Outdoor Annual includes regulations for recreational freshwater and saltwater fishing and hunting in Texas. While Texas Parks and Wildlife Department (TPWD) strives to provide accurate information in the Outdoor Annual, hunting and fishing regulations may change due to legislative or Texas Parks and Wildlife Commission actions. The Outdoor Annual Mobile App may automatically update to reflect published changes upon establishment of a data connection, but users should independently check the associated statutes and regulations to verify their accuracy. For commercial fishing regulations, see the "Commercial Fishing Guide". For more detailed information on game and fish regulations, contact the Texas Parks and Wildlife Law Enforcement offices, or the Texas Parks and Wildlife Department at 800-792-1112 or 512-389-4800 (Mon–Fri 8AM – 5PM).

TPWD receives funds from the USFWS. TPWD prohibits discrimination on the basis of race, color, religion, national origin, disability, age, and gender, pursuant to state and federal law. To request an accommodation or obtain information in an alternative format, please contact TPWD on a Text Telephone (TTY) at 512-389-8915 or by Relay Texas at 7-1-1 or 800-735-2989 or by email at [email protected] If you believe you have been discriminated against by TPWD, please contact TPWD, 4200 Smith School Road, Austin, TX 78744, or the U.S. Fish and Wildlife Service, Office for Diversity and Workforce Management, 5275 Leesburg Pike, Falls Church, VA 22041.

Results and discussion

Prior to starting the experiments, the focal fish swam around the tank and showed no unusual reactions to the covered mirror. Immediately after initial exposure to the mirror, seven of 10 fish responded aggressively to their reflection, attacking it and exhibiting mouth fighting (Fig 1 and S1 Video [45,46]), suggesting that the focal fish viewed the reflection as a conspecific rival. The frequency of mouth fighting was highest on day 1 and decreased rapidly thereafter, with zero occurrences by day 7 and almost no aggression throughout the remainder of the experimental period (Fig 1A cf. a similar decrease in aggression seen in chimpanzees and shown in Figure 2 of [1]). This initially high and subsequently decreasing aggression is consistent with phase (i) of the mark test as reported in other taxa.

As mouth fighting towards the mirror reflection decreased, the incidence of atypical behaviours (e.g., swimming upside-down, a highly unusual behaviour typically never observed in cleaner wrasse Table 1 and S2 and S3 Videos) significantly increased and was highest on days 3 to 5 (Fig 1A). On days 3 and 4, the estimated average frequency of these atypical behaviours across the seven individuals was extremely high—36 times per hour. Each of these atypical behaviours was of short duration (≤1 s), often consisting of rapid actions with sudden onset within 5 cm of the mirror, and could be loosely grouped into five types (Table 1). While it is possible to interpret these behaviours as a different form of aggression or social communication, they have not been recorded in any previous studies of social behaviour in this species [46] and were not likely to be part of a courtship display because all of the subject fish were females. Moreover, we did not observe these behaviours in our own control experiments when presenting a conspecific across a clear divide (Fig 1C), further demonstrating they were unlikely to be forms of social communication.

These atypical behaviours were individually specific, with each fish performing one or two types of behaviour (Table 1 Fisher’s exact probability test for count data with simulated P value based on 2,000 replicates of P = 0.0005). Crucially, these behaviours occurred only upon exposure to the mirror and were not observed in the absence of the mirror (i.e., before mirror presentation) or during conspecific controls. Almost all of the behaviours ceased by day 10 (Fig 1A) and were rarely observed thereafter. These behaviours were different from the previously documented contingency-testing behaviours of great apes, elephants, and magpies [1,4,7], but given the taxonomic distance between them, this could hardly be otherwise. While primates and elephants may perform more anthropomorphic behaviours such as changing facial expression or moving the hands, legs, or trunk in front of the mirror, wrasse and other fishes cannot perform behaviours that are so easily interpreted by a human observer. Nevertheless, behaviours such as upside-down swimming are indeed unusual for a healthy fish and could represent alternative indices of contingency that are within the behavioural repertoire of the study species. Moreover, the atypical movements observed in cleaner wrasse were consistent with behaviour previously interpreted as contingency testing in other species [1,4,5,7] in that these behaviours were atypical and idiosyncratic, repetitive, displayed only in front of the mirror, absent in the absence of a mirror, shown after a phase of initial social (here aggressive) behaviour, displayed over a short period of time, and distinct from aggressive behaviour. Although we reserve judgement as to whether these behaviours should unequivocally be interpreted as evidence that these fish are examining and perceiving the reflection as a representation of self, we nevertheless argue that on an objective basis, these behaviours fulfil the criteria as presented for contingency testing and are consistent with phase (ii) of MSR as presented for other taxa [1,4,5,7].

In phase (iii), species that pass the mark test increase the amount of time spent in front of the mirror in nonaggressive postures, apparently visually exploring their own bodies [1,4,5,7]. This interpretation is again rife with pitfalls because it requires an assessment of the intentionality of nonhuman animal behaviour. An agnostic approach is to simply measure the amount of time animals spend in postures that could reflect the body in the mirror [2], giving an upper measurement of the time in which animals could observe their reflection while making no inferences about the intentionality of the act. We observed an increase in the amount of time spent in nonaggressive postures while close to the mirror (distance of <5 cm), peaking on day 5 after mirror presentation and remaining consistently higher than days 1 to 4 (Wilcoxon sign-ranked test, T = 36, P = 0.008 Fig 1A). Although we did not observe directed viewing behaviour as seen in chimpanzees and elephants, this would in any case be difficult given challenges of assessing gaze direction in animals like fish (although see [45] for a recent technological solution). We therefore consider that in terms of time spent in postures that would facilitate viewing the mirror reflection, this behaviour was consistent with phase (iii) of MSR.

Species with MSR distinguish their own reflection from real animals viewed behind glass [e.g., 29]. When we exposed naïve cleaner wrasse to conspecifics behind glass, we observed fundamentally different responses towards their mirror image (S1 Text). Aggressive behaviour frequency towards real fish was generally low yet did not diminish appreciably during the 2-wk testing period (Fig 1C). Time spent within 5 cm of the glass in the presence of conspecifics was also higher than that in the presence of the mirror. Importantly, no atypical or idiosyncratic behaviour (that might be considered contingency testing) was exhibited towards conspecifics. These behaviours were only observed upon exposure to the mirror. Similar to many previous MSR studies [1,4,5,7], not all individuals we tested passed through every phase of the test. After the initial presentation of the mirror, three fish showed low levels of aggression and rarely performed atypical behaviours during period E1 (Fig 1B). Instead, these three individuals spent relatively longer periods in front of the mirror, as is typically observed during phase (iii), and we conclude these fish failed the test (but see S1 Text for an alternative explanation).

In the second part of the experiment, we used a modified standard mark test protocol to assess reactions to visible (pigmented) or sham (transparent) marks. We used subcutaneously injected elastomer (see Materials and methods) to apply a small amount of colour below the skin surface, a widely used procedure that has been repeatedly shown not to affect fish behaviour [51–54, Northwest Marine Technology]. Moreover, the combined use of coloured and transparent sham marks provided an internal control for the effects of application, including irritation or tactile sensations around the marking site. Nevertheless, the procedure certainly resulted in higher tactile stimulation than, e.g., paint marks on elephant skin, necessarily so because of the requirements of provisioning marks in the aquatic environment and on animals covered in a protective mucus coating. We must therefore consider recent studies showing that visual–somatosensory training induced self-directed behaviour in rhesus monkeys [10,11] that could not be achieved through visual stimuli alone. Our study differs in that we do not provide direct somatic stimulation during the mark test and that we observed no response during our sham-mark phases, which also used a subcutaneous injection. However, given the nature of the mark application, we cannot rule out that a combination of visual and tactile cues produces the behavioural responses we describe, and our test might therefore be considered more similar to the modified tactile–visual mark test than the original mark test.

Fish were marked at night while under anaesthesia, and they swam normally the next morning in the no-mirror condition. After the initial settlement period ‘E1’ (i.e., the initial 2 wk of phases i–iii), we evaluated behaviour during periods ‘E2’ (no mark), ‘E3’ (injection with transparent sham mark), ‘E4’ (injection with coloured mark with no mirror present), and ‘E5’ (coloured mark with mirror present) during a subsequent 2-wk period. The sham and coloured marks were applied on the right side of the head of two fish, on the left side of the head of two other fish, and under the throat in a further four fish these areas were only visible in the mirror. Each mark was in the form of a small brown mark with the intention of mimicking a natural ectoparasite in colour, size, and shape.

We first examined whether fish assumed postures in front of the mirror that would reflect the marked site by categorising all body postures performed within 5 cm of the mirror into three categories: postures exposing the right side of the head to the mirror, postures exposing the left side of the head, and frontal–vertical postures exposing the head, throat, and underside to the mirror. These postures would reflect the right face mark, the left face mark, and the throat mark, respectively. We predicted that if fish were attempting to observe the coloured marks on body parts reflected in the mirror, they would assume postures that facilitated this observation of the mark significantly more frequently during E5 (mirror, colour mark) than in E2 (mirror, no mark) or E3 (mirror, transparent sham mark). Two independent analyses of the videos were conducted (by MK and JA), as well as two further blind analyses by unrelated researchers of a subset (15%) of the videos the frequencies were highly correlated between the analyses (r = 0.988).

Watch the video: Squirrelfish lecture Chapter 6 (June 2022).


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