Chondrichthyes

Chondrichthyan belongs to the class Chondrichthyes is also known as chondrichthyan. The chondrichthyan meaning has a diverse group of cartilaginous fishes. The best Chondrichthyes examples are skates, chimaeras, sharks, and rays. The Chondrichthyan classes are widely dividing the living fish groups into two great groups. The other fishes are Osteichthians or bony fishes. The name Selachii is also used for denoting the group containing sharks.  

The fishes under Chondrichthyes have many unique, physiological, structural, behavioral, and biochemical characteristics. This attracts many scientists to make research on the class Chondrichthyes. Some sharks and stingrays are frightening and dangerous to human beings. At the same time, they are fascinating. So, people in a tropical region, who depend on the sea are figured prominently in folklore. The hazards caused by the shark are very real and sensational, so people are attempting to distinguish them as dangerous and harmless species. 

Taxonomy of Chondrichthyes

The Selachii denotes the vertebrate category of fishes and their skeletons are primarily composed of cartilages. Many ichthyologists are working on Selachii and some authorities are even considering Selachii as a class or subclass that contains all the modern rays and sharks. But some other authorities restricted the use of this name for the order of modern sharks and certain extinct ancestral forms. In the latter system, the rays include guitarfishes, mantas, stingrays, sawfishes, electric rays, and skates are ranked separately. 

The species under Holocephali (chimaeras) classes holds many similarities to Chondrichthyes sharks and rays in their skeletal structure, physiology, and internal organs. Ichthyologists went through all the similarities among the modern and ancient rays, sharks, and chimaeras, which are under the class Chondrichthyes, and further noticed the cartilaginous fishes. After analyzing all these systems the skates, sharks, and rays are classified and grouped into one subclass. The chimaeras and elasmobranchs are classified and kept under the subclass Holocephali. But the rank of the two group classes implies a degree of distinctness equal to the birds (Aves),  reptiles (Reptilia), amphibians (Amphibia), and mammals (Mammalia).

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Distributions of Chondrichthyes 

Most of the rays and sharks are marine fishes. But some species under this class enter estuaries and travel miles together in the rivers and some species remain in freshwaters as permanent residents. Most of the species live in relatively shallow waters of continental margins or around offshore islands. And other few species roam far out in the vast spaces of the oceans. Further, some species live at great depth in the midwater or at the bottom. But most of the Chondrichthyes are surface swimmers and are inhabited in the bottom of shallow waters. 

Most of the countries’ fisheries are mainly dependent on sharks and rays. As these fishes have higher value than bony fishes and fisheries are specifically targeting elasmobranchs as a primary commercial resource. Fishers are roughly getting 750,000 metric tons as their annual yields and most of them are sold fresh and some are salted and processed for future use. Many fishers are harvesting the rays and sharks on a wide scale, which may reduce the population on a large scale and they remain as a commercial extinction species. The huge scale acquisition of sharks and rays is disturbing the entire food chain in marine ecosystems. 

Chondrichthyes Characteristics

Physical Appearance of Chondrichthyes 

Elasmobranchs are the sub-class under Chondrichthyes. Vertebrates of elasmobranch fishes are different from bony fishes in many aspects. The skeletons of the cartilage fishes are calcified because they lack true bones except in the root of teeth. These species have five to seven fully developed gill clefts, which are opened separately to the exterior part. Most of the rays and sharks have an opening in each eye called a spiracle. This spiracle is a modified first-gill cleft. These species have rigid dorsal fins or fins and fin spines and they are not erectile. Only a few species have scales and they look like teeth structures called dermal denticles. Each of the denticles consists of a hollow cone pulp cavity, which is covered externally by a layer of hard enamel-like substances called vitrodentine. The scales in cartilaginous fishes do not grow throughout life as in bony fishes. But they will grow only for a limited size. Here, the new scales will start growing between the existing scales as the body grows. 

The teeth of the Chondrichthyes vary from species to species. The teeth of sawfishes look like beaks. The stingrays have stinging spines and the teeth in the mouth of fishes are structurally modified scales. The teeth arranged in rows in the fish’s mouth are not firmly attached to the jaws but are embedded in a fibrous membrane lying over the jaws. When a fish loses their tooth by breaking or worn or lost, it will get replaced by one moving forward from the next behind the row. The base or innermost row has rudimentary teeth and tooth buds, which helps to develop and move forward when the new teeth are needed. The spiral membranous fold or spiral valves are extended from the intestine of all rays, sharks, and chimaeras. 

The rays are externally different from sharks, they have the gill opening on their lower surface. The eyes of the rays are located on the dorsal surface and the edges of the pectoral fins are attached to the sides of the head near the front of gill openings.  Some rays don’t have scales and others may contain thorns, tubercles, or prickles, which are all the modified scales. Tails of some ray species are long and their saw-toothed spines are equipped with poison glands. The electric rays contain electric organs, through which they can administer electric shocks to their prey or enemies. 

The chimaeras contain only one external gill opening. In the adult chimaeras, the skin of each side of the head looks smooth and they don’t have scales.  They consist of six pairs of grinding plate teeth. The dorsal fin and spine of chimaeras are erectile.  Like male rays and sharks, male chimeras also contain claspers through which they transfer sperm to the female. In addition, the male chimaeras have an erectile clasping device called tantaculum in front of each pelvic fin. But most of the other species have tantaculum on the top of the head.  

Senses in Chondrichthyes 

Though Chondrichthyes sharks have lower order of intelligence. Sharks, rays, and chimaeras are surviving successfully in the environment for a long period of geologic time. They are well trained to find their own prey and they can discriminate in specific locations, respond to various temperatures, and react to repelling substances in water according to the seasonal migrations. The cartilaginous fish characteristics are assisting them to maintain equilibrium. 

The sense of smell is highly developed in Chondrichthyes to locate their prey and guide the predator. The eyes of fishes are structurally and functionally adapted to observe the environment and can determine the colour of the object. Fast predators like sharks have more acute vision and some deep diver’s eyes have a maximum selection of ambient light. 

The cartilaginous fish characteristics use hearing apparatus, which are located in the auditory capsule of the cranium, which includes semicircular canals. These are responsible for maintaining equilibrium. The sharks are very sensitive to sounds of low frequency and possess extraordinary faculty for directional hearing. But they are not sensitive to smell. 

Taste buds of some species are located in the floors, sides, throats, and roof of the mouth as well as the tongue of fishes. While experimenting with large sharks, most are preferred tuna fishes. Sometimes, they will attack their own kind. 

The skins of sharks, chimaeras, and rays gather information such as vibration to low frequencies, salinity, temperature, pressure, and minute electrical stimuli. These sensory organs are located in the lateral line system. A group of pores is called ampullary organs, they are found on the head, around the jaws, and snout to detect the electrical impulse caused by other species.  

Respiration in Chondrichthyes 

Cartilaginous fish characteristics of breathing vary from species to species. Usually, sharks breathe through opening the mouth, while expanding the mouth throat cavity and by contracting the gill pouches to close the gill slits.  When their mouth closed, the bucco-pharyngeal cavity contracted while dilating the gill pouches drew the excess water over the gills. Further, the exchange of oxygen and carbon dioxide takes place in gills. 

Most of the rays take water through the spiracles when they close by contraction at their anterior margins; these bear a spiracular valve and rudimentary gill filaments. The fold-like membrane present on the roof and floor of the mouth prevents the excess water from passing through the throat and it directs them to the gill openings. The skates found on the lower surface of the head slightly above the bottom will also inhale some water through the mouth. The mantas have small spiracles, which live near the surface, respire chiefly through the mouth. 

Chimaeras will intake water chiefly through the nostrils. They keep their mouths closed for the most part. The water will reach the mouth primarily through grooves and leading them through the nostrils.

Reproduction of Chondrichthyes 

The best Chondrichthyes examples are rays, sharks, and chimaeras. All-female species all produce large yolk-rich eggs, which are fertilized internally. The male species are equipped with two copulatory organs namely, claspers and inner edges of the pelvic fins. Each clasper contains a groove for guiding sperm. Rays and sharks will mate and are probably characteristic of the entire group. During mating, a male catches a female’s pectoral fins with their teeth to bring her in position to insert his clasper through a cloaca cavity and into an oviduct trube. Most of the male species use only one clasper at a time. Then, the sperm enters the anterior and reaches the end of the oviduct to fertilize the eggs. Then the eggs move down the oviduct past the shell gland. Most of the sharks, guitarfishes, and other chimeras are oviparous, which means egg-laying species. The eggs are first coated with a shell gland, which is a  temporary membranous capsule that lasts only during early development. 

Some species of sharks are viviparous, the yolk sac develops folds and projections, which interdigitate with folds of the uterine wall. Thus the yolk sac placenta transfers the essential nutrition materials  to the fetus 

Growth of Chondrichthyes 

The growth of sharks will vary from species to species and depends on the climatic conditions..etc. The growth can also be measured by the statistical analysis of the length collected from the samples kept in aquariums and samples collected from various species from various regions. All studies show that the growth of Chondrichthyes is at a slow rate. The greenland shark will attain 6.5 meters and grow only about 7.5mmm per year. The Australian school shark can grow only about  80 mm in its first year and it grows 30 mm (1 inch) in its 12th year and in the 22nd year their maximum length will be about 1.6 meters.