Chondrichthyes

The chondrichthyes (Chondrichthyes, from the Greek χονδρος khóndros, "cartilage" and ιχθύς ikhthýs, "fish") are a class of aquatic vertebrates known as cartilaginous fish, a name that refers to the fact that their skeleton is made of cartilage, unlike bony fish (osteichthyans), which have bone.

This class includes subclasses elasmobranchs (sharks, rays, and mantas) and holocephalans (chimaeras).

General characteristics

Chondrichthyans present a mosaic of evolved and primitive characters. Among the primitive features, its basic anatomy stands out. Among the evolved features, two stand out: the suspension and structure of the fins and the structure and composition of the jaws and dentition. The latter in different species appears with more evolved or more primitive forms. Another highly advanced trait is your immune system. Since the Mesozoic, shark specializations have become clear, placing them at the top of marine trophic webs. There is a characteristic tendency towards gigantism in this group, as a measure to avoid predation. The typical shark has a length of two meters, and a typical ray of one meter. These proportions are very large by the standard of vertebrates.

The teeth are not fused to the jaw and are replaced by new ones continuously, quickly and in series thanks to a cavity along the edge of the jaw. This allows them to always have new teeth in front of those that are breaking, wearing down and detaching. There are serrated teeth, with a cutting function; sharp teeth, with a grasping function and flat teeth (in many stripes) to grind the food.

They swim with the aid of fins and breathe through gills throughout their lives; these are exposed to the outside directly through 5 or 7 gill slits in the case of rays or sharks, and 1 exclusively in chimaeras.

They differ from other fish in having a skeleton made up mainly of cartilage and not bone. They lack an operculum and swim bladder. The lack of the latter forces them to constantly swim or perch on the bottom (as some rays and sharks do), unable to maintain a static position in the water column.

Senses

They have a keen sense of smell. They are able to detect blood and follow its trail until they find it. Their eyesight is less acute: they can detect light and shadow in the water. The organs that they have in the lateral lines and snout (Lorenzini ampullae) allow them to capture the electrical stimuli of the muscular contractions of bony fish.

Classification

Subclass Elasmobranchii

Subclass Holocephali

Evolution

Biochronology and phylogeny of the condrictios.

CladoelacheA Devonian condrictio.

Chondrichthyans made their appearance during the Late Devonian period. The most primitive fossils that are known correspond to the genus Cladoselache. Throughout history, chondrichthyans have had two great periods of evolutionary radiation, the first took place until the beginning of the Permian, to later go through a phase of decline that lasted 100 million years; the second great radiation occurred towards the Cretaceous.

Anatomy

External Anatomy

External anatomy of an elasmobranchio condríctio.

Elasmobranchs are laterally compressed and have a spindle-shaped appearance with unpaired and paired fins. The unpaired fins are the dorsal (or dorsal), the caudal (which is heterocercal) and the anal. The paired fins are the pectorals and the pelvics, these always being inserted behind the pectorals.

They have various openings throughout the body, such as the nostrils, the mouth, the gill slits, the cloacal pit, the blowhole (which is the rest of the first gill slit) and the orbital eye sockets.

The integument has placoid scales (also called dermal denticles) that form in the dermis and emerge through the epidermis. The layers of the placoid scales from outside to inside, like the layers of the teeth to which they are homologous, are: enamel, dentin, and pulp. Throughout evolution, these scales have been lightening and simplifying. They are oriented backwards, and have various functions such as making the skin flexible and at the same time very resistant and minimizing the friction and viscosity of the aquatic environment due to the reduction of turbulence generated by locomotion.

Swimming

Good locomotor skills are important to function effectively. The movement of the fish is due to the fact that the water is incompressible, and when a force is exerted on it, it responds with an equal and opposite vector (R1). R1 has a lateral component (L1) and a frontal component (F1). By exerting successive lateral forces, and inserting those from the opposite direction, the frontal components add up and the lateral components cancel each other out. This is the wave motion. The beating movement is also used, which propels water backwards to move the animal.

The direction of motion must be controlled, which requires control of three yaw motions: pitch, roll, and yaw.

The unpaired fins oppose their surface to roll and yaw. The paired fins oppose (control) pitching.

The movement of the caudal fin is also involved. In sharks the caudal fin is heterocercal. A heterocercal fin, when beating, not only produces thrust, but also a suspension force (lift), which controls the position in the water column. There are, however, doubts about the true function of the caudal heterocerca. Bony fish that have a heterocercal fin do not appear to derive any buoyancy from it, according to our data. Sharks lack a swim bladder, but they still manage to have a density roughly equal to that of seawater thanks to their liver. The lift that the caudal fin would produce when beating would allow them to control the depth to which they swim. To control their buoyancy, sharks swallow air, but the most important thing in this regard is the liver, which is very large and has a high amount of lipids (squalene). The effect of the liver means that the correction for density when submerging a shark is sufficient so that it does not sink in seawater.

Sharks are able, by controlling the lipid storage-reabsorption balance in the liver, to control their buoyancy. Benthic species do not have as rich lipid droplets in the liver.

Playback

Egg; note the clay that allows you to adhere to the vegetation.

Fertilization is internal. The pterygopods of the fins of the males are copulatory organs. The male introduces one of the pterygopods into the female's cloaca, where there is a suction effect of the sperm towards the oviduct. A strong tendency is observed in the whole group towards ovoviviparism. In certain cases oviparism or even viviparism occurs.

The phenomenon of "oophagia" (cannibalism inside the uterus, where an embryo eats the remains of eggs).