Chondrichthyes

Chondrichthyes

The Chondrichthyes (Chondrichthyans), also called cartilaginous fish, are a group of very ancient aquatic vertebrates. Although they are not as numerous or diverse as bony fish, their morphological adaptability, swimming muscle tissue, sensory organs, and powerful predation habits and jaws indicate that they have been given a firm ecological status in the environment in which they live.

In this article we are going to tell you everything you need to know about the Chondrichthyes, their characteristics and biology.

Main characteristics of the Chondrichthyes

reproduction of cartilaginous fish

There are two types of cartilaginous fish. Next, we will describe its main characteristics:

Elasmobranchs

Sharks and rays belong to this group of animals. Some of them are carnivores, they locate their prey through their olfactory organs due to their poor visual development. Currently, there are more than 400 species of sharks in 8 orders and about 500 species of rays in 4 orders. As for sharks, most have the following characteristics:

  • Body: A spindle-shaped body with a pointed face with an abdomen in front. The tail of the body has an abnormal closed tail, that is, there are two different shapes and structures of leaves, one of which contains the end of the spine, and the front one has a pair of pectoral fins, a pair of pelvic fins, and two dorsal. odd fins. In males, the pelvic fins were previously modified as sexual organs for mating and are called glycoptera, pteropods, or genus.
  • Vision, skin and receptor organs: in relation to the mouth, they have uniform, ventral and anterior nostrils. The eyes do not have lids, although some species have nictitating membranes, with a stoma behind each eyelid. The skin is tough and in some species resembles sandpaper, it has plate-shaped scales, also called dermal scales, which are arranged in such a way as to reduce turbulence and face backward. They have neuromas throughout their bodies and heads, which are extremely sensitive receptors to vibrations and water currents. They also have special receptors that can detect prey through the electric field they emit, they are Lorenzini blisters on the head.
  • Teeth: The teeth do not merge with the lower jaw, there are two rows, the last row replaces the missing teeth in the first row, so new teeth can always grow. Depending on the species, these can have a serrated shape to cut food, sharp and have a gripping function, in the case of striped species, they have flat teeth that can be scratched on the surface.
  • Bones and Swimming: They have mineralized cartilage bones, not as bony as other fish. Also, they do not have a swim bladder, which causes them to constantly swim or stay on the bottom, otherwise they will sink. On the other hand, they have a huge liver, which contains lipids (squalene), which also prevents it from sinking.

Holocephalos

Within the Chondrichthyes we find this group that includes the chimeras. This small group is made up of approximately 47 species today. Anatomically it has a mixture of elasmobranch and bony fish characters:

  • The Body: They have a very strange shape, their body is elongated and their heads protruding, they have a classic structure that can support the females during mating. Its nose is like a rabbit and its tail is like a whip.
  • Jaws and teeth: They do not have teeth, but rather wide, flat plates. The upper jaw is completely fused with the skull, unlike the others, this is where its name comes from (holo = all, all and cephalo = head).
  • Size: They can be up to 2 meters long.
  • Defense: Its dorsal fin has a poisonous spine.
  • Foods: Their diet is based on crustaceans, mollusks, echinoderms, small fish, and algae, which are food mixtures that they grind up during feeding.

Swimming of the Chondrichthyes

chondrichthyans

Elasmobranchs have dermal scales, which allow them to reduce turbulence when swimming. On the other hand, along with their lipid-rich liver, their ability to swallow air, and their fins, they are excellent swimmers and these adaptations allow them to stay in the water. Weird fins can make you swing, and even fins can control you. On the other hand, the rear wing can control thrust and generate suspension force due to its unusual shape.

Manta rays are adapted to life underwater, the body is flat, with uniform fins that widen and merge with the head, acting like wings when swimming. Their teeth are flat, capable of scraping surfaces and grinding food, which are usually crustaceans, mollusks and small fish.

Their tails are whip-shaped, with one or more spines at the end, which are connected to the poisonous glands of certain species. They also have electrical organs on both sides of their heads, which can produce electrical shocks and stun their prey or predators.

Reproduction

Chondrichthyes evolution

Cartilaginous fish have internal fertilization and different reproductive modalities that we will see below:

  • Oviparous: They lay yolk-filled eggs immediately after fertilization. Many sharks and rays lay their eggs in the keratinous sac. Tendril-like filaments form at the end of the sac, which are used to adhere to the first solid object they touch. Embryos can form in 6 months to 2 years. Usually this pattern occurs in small, benthic species, which can lay up to 100 eggs.
  • Viviparous: They will develop a real placenta from which the embryo can eat. This mode of reproduction promoted their evolutionary success in this group. It occurs in almost 60% of cartilaginous fish and large active species.
  • Oviviparous: they retain the embryo in the fallopian tube during embryonic development and feed on its yolk sac until birth. In turn, it provides the embryos with different types of food, such as lecithin, where the embryo feeds on the egg yolk; tissue nutrition, where one or more embryos are nourished by the fluid (tissue nutrition) produced by the villi on the inner surface of the uterus. On the other hand, there are ovules, that is, embryos that feed on fertilized eggs when they are in the uterus. Finally, there are oleanders or cannibalism in the womb.

I hope that with this information you can learn more about the Chondrichthyes and their characteristics.


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