Cartilaginous Fishes

Abstract

The class Chondrichthyes or cartilaginous fishes comprises about 800 species belonging to two major radiations that have diverged over 350 million years ago, i. e. the elasmobranchs and the holocephalians (Compagno 1977; Carroll 1988). Fossil evidence suggests that living holocephalians are a much older form than modern elasmobranchs, which arose approximately 200 million years ago (Carroll 1988). The living elasmobranchs (sharks, skates and rays) comprise four major superorders: Squalomorphii, Galeomorphii, Squatinomorphii and Batoidea (Table 12.1). The elasmobranchs are widely distributed, as they are marine in habitat, except for one species, Carcharinus leucas, which lives in Lake Nicaragua and in estuaries of large rivers, such as the Ganges, the Mississippi and the Zambesi. There are about 350 described living species of sharks, ranging in size from the giant whale shark (up to 15 m long) and basking shark (up to 10 m long) to tiny ones such as Squaliolus laticaudus, which in adulthood measures 15 cm in length. Paradoxically, the largest sharks mentioned above are plankton feeders and quite harmless. Of the 350 species of living sharks, no more than 35 have been implicated in attacks on humans (Gilbert 1984).