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The sharks, streaks and chimeras (deep-water fish, also called ratfish) of this class (from the Greek chondros = cartilage + ichthys = fish) are the earliest living vertebrates with complete and separate vertebrae, movable jaws and even fins.
This group is ancient and represented by numerous fossil remains. Belong to some of the largest and most efficient marine predators. They all have a cartilage skeleton, specialized teeth that renew themselves over a lifetime, and a skin thickly covered with tooth-shaped scales.
Virtually all are marine, although there are shark and ray species that regularly penetrate estuaries and rivers, and in tropical regions freshwater species.
All cartilaginous fish are predators, although the filters also ingest phytoplankton. In this case there are rigid projections of gill arches, which act as filters. Much of their diet consists of live prey, although they also consume corpses when available.
Most sharks are no more than 2.5 m in length but some reach 12 m and the whale shark 18 m, which are the largest living vertebrates, except for whales.
The rays are equally small, about 60-90 cm long, but the manta ray reaches 5 m in length and 6 m in wingspan.
Sharks, with their spindle-shaped and aerodynamic body, are of great biological interest because they have basic anatomical characteristics present in embryos of superior vertebrates.
No real bones but made up of tough and flexible cartilage, more or less reinforced by limestone deposits, the skeleton consists of a skull attached to a spine and pectoral and pelvic girdles. The mandible (not fused to the skull) and the maxilla are present. The notochord is persistent in intervertebral spaces. Some species have hard spines, all similar to those of bony fish. This type of skeleton only supports animals over 10 meters long in the aquatic environment, whose density is higher than air.
The skin is hard and is covered with tooth-like scales (composed of an enamel-coated dentin plate in the dermis) with a back-oriented spine, as well as numerous mucous glands. This coating gives the skin a sandpaper texture, which makes the animal more hydrodynamic. Some species of rays have large and thorny scales, while others do not have scales at all.
Distinct brain and highly developed sensory organs that allow them to locate prey even when far away or buried deep in the mud. These organs include:
Nostrils: located ventrally at the rounded end of the head, capable of detecting molecules dissolved in water in minimal concentrations;
Ears: with three semicircular canals arranged perpendicular to each other (functioning as a balancing organ, as in all upper vertebrates);
Eyes: lateral and without eyelids, whose retina usually only contains rods (providing black-and-white vision but well adapted to low light);
Sideline: a thin groove along the flanks containing many small openings contains pressure-sensitive nerve cells (something like a sense of touch at a distance);
Lorenzini Ampoules: located in the ventral area of the head, are other sensory channels connected to small ampoules that contain electroreceptors capable of detecting the electrical currents of the muscles of other organisms;Continues after advertising
The mouth is ventral with rows of enamel-coated teeth (developed from placoid scales). The teeth are implanted in the flesh rather than the jaw, and are continually replaced from the back of the mouth as they are lost.
The shape of the teeth reveals the eating habits of the animals, sharpened, jagged teeth in sharks that use them to grasp and cut, and small, tile-shaped rays that use them to break the shells and shells of shellfish and shellfish. that feed on the bottom.
The intestine has a spiral valve (to increase the absorption area) and a large, very oil-rich liver that gives it great buoyancy, sometimes making up 20% of body weight. However, in some species this is not enough, as if they stop swimming they will sink. The anus opens to the cloaca.
Heart with 2 chambers (atrium and ventricle) through which only venous blood circulates.
The gills are attached to the wall of 5 to 7 pairs of gill sacs, each with an individual slit-shaped opening, opening in front of the pectoral fin in sharks or on the ventral surface of the rays. In the chimeras there is only one gill slit.
The nostrils do not communicate with the oral cavity but with the pharynx.
Gill sacs may contract to expel water or, as with most sharks, the animal uses a kind of jet breathing, actively swimming with its mouth and gills open, maintaining a steady flow of water. For this reason, sharks often drown when caught in lost fishing nets.
There is usually a pair of spiracles behind the eyes, in connection with the pharynx, which, in benthic species, allow water to enter the gills without debris. There is no swimming bladder;
Sharks and rays have separate sexes, typically even gonads, where the ducts open in the cloaca and the fertilization is internal. The clasper, modified ventral fins, are introduced into the female's cloaca and the sperm drips through the canal formed by the two joined structures.
Can be oviparous (eggs are released wrapped in semi-rigid capsules), viviparous (youngsters develop within a placenta-like structure, allowing them to be fed directly from the mother's body) or ovoviviparous (retain eggs inside the female, fully hatched pups born, tail first), produce eggs are very rich in calf but without embryonic attachments.
Development is straightforward, there are never larval states. Puppies are born with functional teeth and are able to hunt immediately, although due to their size they are potential prey themselves.
The egg sac of a squalid shark, popularly known as the "Mermaid Bag"
Sharks are persecuted either out of sheer ignorance or to obtain their fins (for soup and use in "aphrodisiac" Asian potions) or accidentally killed in trawls. Today, large numbers of species are in serious danger of extinction.
With the increase of the human population and the reduction of the bone fish schools, cartilaginous fishes have been fished in large numbers. About 100 million sharks and the like are killed each year, of which about 6 million are blue sharks, killed only by their fins.
As these animals are fundamental to the correct "functioning" of the marine ecosystem, this killing must in the short term lead to very serious imbalances.