By Dwight F. Davis, Mote Marine Laboratory Volunteer Sharks are among the world's most successful groups of animals, inhabiting the seas for more than 400 million years. And for more that 100 million years, they have been nearly unchanged in form. Together with their closest relatives, the skates, rays, and sawfishes, they are collectively known as elasmobranchs. Altogether, there are about 400 species of sharks. All have certain anatomical features in common, most notably the fact that their skeletons are made entirely of cartilage as contrasted to the bony fishes whose skeletons contain true bone.
Sharks are different in other ways, too.
They have the same five senses we have plus electrosensitivity which helps them find live prey that may be hidden. Also, unlike most bony fishes, sharks do not have wel-developed ribs or a swim bladder. Thus, sharks are slightly negatively buoyant. And despite the lift from a large oil-rich liver, they sink slowly unless they continue to swim. Like higher animals, sharks reproduce by internal fertilization.