The basic design of Chondrichthyans includes a skull, jaws with teeth, a vertebral column (backbone), and fins; but unlike the bony fishes, they lack ribs. Through millions of years of evolution many different species have evolved in varying shapes and sizes, each designed for the particular habitats where they are found and the types of prey they survive on. The upper jaw is loosely attached to the underside of the skull by ligaments and connective tissue.
Teeth vary in size and shape throughout the different species and these are embedded into the gums rather than being attached to the jaw – worn or damaged teeth are regularly replaced. The tooth design can vary from very sharp pointed teeth for catching and eating fish to flattened teeth for grinding. Whale Sharks and Manta Rays have hardened plates only, as they feed by capturing plankton in gill rakers, as they pass large volumes of water through the gills.
Chondrichthyans have three types of unpaired fins – the dorsal, anal and caudal; and two types of paired fins, the pectoral and pelvic. However, through evolution, some family groups like Dogfish, Sawsharks, Angel Sharks, and rays have lost the anal fin. Chimaeras and some rays have modified caudal fins which form long whip-like or ribbon-like tails.
In sharks and chimaeras, the pectoral fins extend from the body below and behind the head whereas in rays, the pectoral fins are attached at the back of the skull and part of the body stem and are greatly enlarged to form a body disc.
Body shape and method of movement through the water varies between the species. The body of most species of shark is shaped for hydrodynamic efficiency where swimming is achieved by side-to-side undulations of the tail. In rays, the body is highly flattened from the top and bottom, the tail is reduced in size, and movement through the water is achieved by undulation of the tips of the pectoral fins. Chimaeras move by rapid movement of the pectoral fins or by slow lateral movements of their tails. To aid hydrodynamics sharks, rays and skates skin is covered with small teeth-like structures called dermal denticles. These teeth-like structures are designed to reduce the amount of drag while swimming and therefore increase the efficiency while travelling through the water.
As with other fish, Chondrichthyans extract oxygen from the water to breathe using gills. Most species of sharks have 5 pairs of gill openings although there are several species have 6 or 7. Rays have 5 or 6 pairs of gill openings and chimaeras have 4 pairs of gill arches but only 1 pair of gill openings. The gill openings in sharks and chimeras are located on the side behind the head and in rays and skates they are located underneath.
Rays and most bottom dwelling sharks also have spiracles, these are openings which allow water to be pumped over the gills while stationary allowing them to rest on the seafloor. The other species of sharks need to continue to move through the water to breath. Chimaeras have large nostrils and take in water through these instead of the mouth.
Chondrichthyans have some of the most complex and finely tuned sensory systems of all animals. The eyes provide for an excellent field of vision and in some species it is believed they can see in colour, further adaptations give them the ability to function efficiently in low-light conditions. Most species have an acute sense of smell. Taste-receptor cells to determine palatability of prey occur on taste buds that cover a small bump in the mouth. Touch-receptor cells occurring near the skin surface respond to contact and small capsules deeper in the skin respond to the bending of the body and fins through the stretching and contracting of muscles.
Senses of balance, orientation, and body movement depend on a complex series of organs inside the ear. As the animal moves, fluids inside three semi-circular canals at right angles to each other press against hair cells, connected to a conglomerate of hard calcium granules, and stimulate nerve impulses. Hearing is acute, even though Chondrichthyans lack external ear openings. Clusters of sensory hair cells detect water movement or vibration. Those in the skin are called pit organs and can detect water movement relative to the body surface. To detect water acceleration, neuromasts also form an extensive network of small water-filled canals immediately below the surface of the skin in the lateral lines, which run across each side of the head and body.
Electro sense is present in all Chondrichthyans, but uncommon among other groups in the animal kingdom. Chondrichthyans electro sensory organs (Ampullae of Lorenzini) are able to detect weak electric fields generated by the movement of prey, predators, and ambient water in the Earth’s magnetic field. It is likely that these animals can navigate long distances by detecting slight variations in the Earth’s magnetic field caused by variation of metal deposits in their environment. The electro sense is so acute that Chondrichthyans animals can detect voltage gradients of less than 5 billionths of a volt across a distance of 1 cm. The Ampullae of Lorenzini are lined with hair cells attached to an insulated tube filled with a conductive jelly. A network of interconnected Ampullae located in pores on the head, lower jaw, and trunk enables the animal to measure voltage gradients at different positions on its body.
Torpedo rays have evolved paired kidney-shaped electric organs located on each side near the centre of the disc, capable of producing more than 200 volts. These are used for defence and stunning prey, and possibly also for electro-location of prey, predators, or mates, and for social communication.
There is still so much more to learn about these amazing animals.