All sharks are carnivorous and many people believe that sharks will eat just about anything; for a few species, such as the tiger shark, this is true. The vast majority of sharks, however, are far more specialised for particular prey items, and rarely stray from these. Some of the most specialised sharks have developed a filter feeding technique, which is employed by the whale, basking and megamouth sharks. These three shark species have evolved plankton feeding independently and use different strategies. Whale sharks feed using suction to take in large concentrations of plankton and small fishes. Basking sharks are ram-feeders, swimming steadily, with their mouth wide open, through plankton blooms. Megamouth sharks may make their suction feeding extra efficient with the use of luminescent tissue inside the mouth the attract prey in the deep ocean. This type of feeding was only possible through the evolution of gill rakers, long slender filaments that form a very efficient sieve, analogous to the baleen plates of the great whales. Plankton is trapped in these filaments and swallowed from time to time in huge mouthfuls. Teeth in these species are very small compared to the size of the animal, because they are not needed for feeding.
Other highly specialist feeders include the cookiecutter sharks, which feed on the flesh sliced out of other larger fish and marine mammals. The teeth in these sharks are enormous, compared to their size, with the teeth of the lower jaw being particularly sharp. Although they have never been observed feeding they are believed to latch onto their prey and use their thick lips to make a seal, twisting their bodies to rasp off flesh.
Some seabed dwelling species are highly effective as ambush predators. Angel sharks and wobbegongs are perfectly camouflaged for lying in wait in order to suck prey into their mouths. Many benthic sharks feed solely on crustaceans which they crush up with their flat molariform teeth.
Other sharks feed on squid or fishes, which are swallowed whole. The viper shark has teeth which can be pointed outwards to strike at and capture prey that is then swallowed intact. The great white and other large predators can either swallow small prey whole or take huge bites out of large animals. Thresher sharks use their long tails to stun shoaling fishes, and sawsharks may either stir prey up from the seabed or slash at swimming prey with their tooth-studded rostra.
Many sharks, including the whitetip reef shark are cooperative feeders and hunt in packs in order to herd and capture elusive prey. These social sharks are often highly migratory, travelling huge distances around ocean basins in large schools. These migrations may be partly necessary to find new food sources.
Digestion of the food can take a long time in sharks. The food moves from the mouth to the 'J' shaped stomach, where it is stored and initial digestion occurs. Unwanted items may never get any further than the stomach, and are coughed up again. Many sharks have the ability to turn their stomachs inside out and evert it out of their mouths in order to get rid of any unwanted contents.
One of the biggest differences in digestion in sharks when compared to mammals is the extremely short intestine. This short length is achieved by the spiral valve with multiple turns within a single short section instead of a very long tube-like intestine. The valve provides a very long surface area for the digestion of food, requiring it to pass around inside the apparently short gut until fully digested., when remaining waste products pass into the cloaca and vent.
The most obvious internal organ in sharks is the huge liver, which often fills most of the body cavity. In the basking shark, the liver makes up about a quarter of the body weight and may weigh up to a ton. In basking shark fisheries this was the major product as it contained up to 80% in weight of very high quality squalene oil.