Sharks, skates, and rays make up Chondrichthyes, or "cartilaginous fish." First appearing on Earth almost 450 million years ago, cartilaginous fish today include both fearsome predators and harmless mollusc-eaters. Members of Chondrichthyes all lack true bone and have a skeleton made of cartilage (the flexible material you can feel in your nose and ears). Only their teeth, and sometimes their vertebrae, are calcified; this calcified cartilage has a different structure from that of true bone.
Key Features of Chondrichthyes
- Resemble bony fish and terrestrial vertebrates in having jaws and paired appendages.
- An electroreceptive system is well developed.
- Endoskeleton entirely cartilaginous.
- No swim bladder or lung.
- Respiration by means of five to seven pairs of gills with separate and exposed gill slits, no operculum.
Sharks do not have swim bladders, and if they were to stop moving they would probably sink to the bottom of the ocean; therefore, they are constantly in motion. Sharks usually are scavengers, eating injured fishes, carrion, garbage, and other waste from ships as well as animals such as seals, turtles, birds, whales, crabs, and a wide range of fishes. The whale shark is the largest shark and also the largest fish in the sea, measuring up to 15 m (49 ft) in length; the cookie-cutter shark measures less than 50 cm (19 in) in length.
Skates and Rays
Skates are a family of flat-bodied rays found in warm and temperate seas. Rays have broad, flattened bodies, with eyes located on the upper surface of the body, while the mouth and gills are located on the lower surface. The upper surface of the body is dark colored, while the lower surface is light colored. This is called counter shading and is a technique many marine animals use for protection as well as for predation. Counter shading makes the animal almost invisible because when another animal looks down at them, they are camouflaged with the darkness of the sea bottom. When looked up at from underneath, the animal is camouflaged with the light from the sun. Great White Sharks use this technique for predation.
Orr, Robert T. 1971. Vertebrate Biology (3rd edition) pp. 5-56. W.B. Saunders Company Philadelphia, PA
Introduction to the Chondrichthyes