Ground Sharks

This order is the largest, most diverse and widespread group of sharks. It contains at least 291 species in 12 families. Most are small and harmless to people, but this order also includes some of the largest predatory sharks.

This order displays a very wide range of appearances, from strange bottom-dwelling, deepsea sharks, to typical large sharks. All have two spineless dorsal fins and an anal fin. A long mouth extends to or behind the eyes, which are protected by nictitating lower eyelids. Nasoral grooves are usually absent (or broad and shallow, when present in a few catsharks). If barbels are present, these are developed from the anterior nasal flaps of nostrils. Teeth variably differentiated along jaws, with anterior teeth not separated by small intermediate teeth or a gap from lateral teeth. Intestine usually has a spiral or scroll valve.

They occur worldwide, from cold to tropical seas, ranging from intertidal zones to the deep ocean, including pelagic habitats in the open sea. Some species are poor swimmers and remain restricted to small areas of the seabed, whereas others are strong, long-distance swimmers and highly migratory.

Reproductive strategies are highly varied and remain poorly known in some groups. Many species are oviparous (egg-laying), some deposit eggs on the seabed with embryos developing for up to a year before hatching, others retain the eggs until close to hatching. More evolution- arily advanced families retain foetuses inside the female, nourished by the yolk-sac or by a placenta, until live young are born (viviparity).