Sharks are cartilaginous hunters from lantern size to whale scale
Sharks are elasmobranch fishes with cartilaginous, ribless skeletons, dermal denticles, five to seven gill slits per side, and pectoral fins free of the head. They span the seventeen-centimetre dwarf lanternshark to the whale shark, the largest living fish at up to about eighteen metres.
They live in every sea, commonly down to 2,000 metres, though few enter fresh water. Several species, including bull, tiger, great white, mako, thresher, Greenland and hammerhead sharks, sit at the top of their food chains, while the whale shark and basking shark, among the biggest fish ever, filter plankton instead. Mariners called them sea dogs until the 16th century, a name that lingers in dogfish, and the word shark may come from Dutch schurk, villain, or from the Yucatec Maya xoc.
Their ancestry is ancient. Acanthodians, the so-called spiny sharks and the oldest members of the wider cartilaginous-fish lineage, appeared in the Early Silurian about 439 million years ago, and the first confirmed elasmobranchs followed in the Devonian. Modern sharks form the division Selachii, the sister group of rays and skates; molecular work has overturned an old idea that some sharks were closer to rays than to other sharks.
More than 500 species fall into thirteen orders, split between the galeomorph and squalomorph superorders. Ground sharks, the Carcharhiniformes, include the requiem sharks and hammerheads and shield their eyes with a nictitating membrane. Mackerel sharks, the Lamniformes, count the great white, makos, goblin and megamouth sharks and the extinct megalodon; carpet sharks include nurse sharks, wobbegongs and the whale shark; and the Hexanchiformes include the snakelike frilled shark.
People hunt them for meat and fins, and since 1970 shark populations have dropped by 71 percent, driven mostly by overfishing and finning.
Source: Shark
