Shark research is a relatively new field, so that considerable gaps still remain to be filled in many areas of knowledge.
Sharks are extremely intelligent animals and show highly developed social behaviour. Most species live in coastal waters, with a few favouring open waters. Our knowledge of deep-sea shark species is still extremely limited. Sharks may be solitary hunters or form schools; the latter behaviour is exhibited by the striking hammerhead shark (Sphyrna). Many species of shark must remain in constant motion in order to take in sufficient oxygen through their gills. Others, such as the whitetip reef shark (Triaenodon obesus) or leopard shark (Triakis semifasciata), are capable of "resting" on the ocean floor.
All sharks are carnivores or meat-eaters. The majority hunt fish,crabs, molluscs and seals. The largest sharks, such as the megamouth shark (Megachasma pelagios) and whale shark (Rhincodon typus), subsist entirely on a diet of animal plankton. Human beings are not among the prey of sharks.
Sharks do not devour prey at random. When a shark spots an animal which resembles its preferred prey, it first engages in a "taste bite". The shark's excellent sense of taste enables it to decide whether the potential prey is edible. The taste bite is primarily designed to weaken the prey - a technique used particularly by great white sharks.
As yet little is known about shark reproduction. They reach sexual maturity at a relatively late age and produce few young. This low reproduction rate, combined with extensive shark hunting activities by humans, has led to a dramatic decline in many shark populations.
In all species, the female is fertilized internally by the male. The majority of shark species give birth to living young from eggs which hatch within the female's body (ovoviviparous). The gestation period varies from species to species, from six months to almost two years. In some species of shark, the young develop in the female's uterus and are born live (viviparous). Around thirty per cent of sharks lay eggs (oviparous). The females attach the small number of tough egg sacs to plants or other objects to prevent them from floating away.