What are Chondrichthyes?

Introduction:

 The Chondrichthyes are a class of fishes having well-defined jaws and a skeleton made up of cartilage. This class of fishes includes a  vast and diverse group of fishes including sharks, rays, skates, and chimeras. These are mostly marine fishes. 

 Other than Chondrichthyes there is a group of fishes which are bony fishes and all that is included in the class Osteichthyes.

Chondrichthyes is a class of fishes comprised in the classification named Gnathostomes because they possess jaws. This division( Gnathostomata) encloses all the vertebrates having jaws. It is divided into two upper classes named:

• Pisces: All that fishes have fins

• .Tetrapoda: All that fishes have limbs

Classification:

Kingdom: Animalia

Phylum: Chordata

Subphylum: Vertebrata

Division: Gnathostoma

Superclass: Pisces

Class: Chondrichthyes

Characteristics:

These are mostly marine fishes. These all fishes contain a pair of jaws and their jaws are very strong. Whale and sharks are the second most messy and largest vertebrate and also the largest among fish i.e. Some measure up to 15 m in length of the body. The mouth is present at the dorsally ventral side, mostly having an upper jaw large than the lower one. They comprise an endoskeleton made up of cartilage, and the deposition of calcium salts in their skeletons provides extra strength to them. The notochord in their bodies retains throughout their lives. Most of them have a heterocercal tail i.e.having an upper side larger than the lower one, so their tail has two lobes, the upper lobe is elongated which is stiffed due to the extension of vertebrae in it, and a shorter lower lobe and hence a distinct heterocercal tail is formed.

 They have placoid scales and the skin is covered by these minute tooth-like structures, providing them extra protection from predators. Their teeth are also altered placoid scales that are not connected to jawbones, whether they are entrenched in the tissue. They have many rows of teeth in their mouth and old ones fall and are continuously replaced by the new ones which are formed behind them. They possess 4-7 pairs of gills. Gaseous exchange in them occurs through gills when the water current passes over them.

They swim constantly in water to prevent from sinking because they don't have air bladders. They lack the ability to maintain their body temperatures and are called poikilotherms or cold-blooded animals. These are all almost predatory fishes i.e. they feed on other fishes,molluscus and crustaceans, etc. They also get their feed by filtering food particles like phytoplankton and zooplankton from the water current that passes from their mouth, gills, and pharynx.

They have a heart that is two-chambered and contains one article and one ventricle. They also contain a well-developed brain and a spinal cord, which is protected by vertebrae. They have Sense organs that are well developed. They retain the ability to detect their prey electrically by shack waves. Sharks possess electroreceptors present on their head, that can sense the electric current formed by the movement of prey. It also aids them in navigating their path., They can detect all kinds of vibration, motion, and water pressure surrounding them with the help of sensory cells in their lateral line.

Some of the Chondrichthyes possess electric organs and poison stings, which help them in their defense and predation. The digestive system of Chondrichthyes comprises a mouth, pharynx, stomach, intestine, and cloaca present on the lower belly side. It has a double function in females and also acts as a reproductive organ along with the elimination of nitrogenous wastes in their bodies.

 They have males and females separately and have internal fertilization. Skates and some sharks are oviparous but most sharks are ovoviviparous and some are viviparous.

Evolutionary Behaviour:

The primitive fossil remains of fish-like vertebrates are too small and spread to allow tracing the modern fishes specifically to their origins. It is thought that the ancestral fishes evolved first about 455 million years ago, toward the end of the Ordovician Period in the higher upper spaces of streams. At the end of the Silurian era and the start of the Devonian periods that observed, there appeared a highly diverse group of armor-plated fishes having jaw-like structures, bony skeletal elements, and paired fins. Paleontologists directed these extinct forms as a separate class named Placodermi.

During the start and end of the Devonian almost at the end of about 350 million years ago, these placoderms reached high in diversity as well as in numbers and almost completely died out. During their development, the placoderms evidently gave rise to the cartilaginous fishes, Osteichthyes, and the Chondrichthyes. Even though the borders of evolution still remain to be discovered but it is quite clear that the two groups, the Chondrichthyes and the Osteichthyes emerged independently The Chondrichthyes appeared after the emergence of the Osteichthyes.