Sharks have very distinctive tails

Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago, before the time of the dinosaurs.Since that time, sharks have diversified into 440 species, ranging in size from the small dwarf lanternshark, Etmopterus perryi, a deep sea species of only 17 centimetres in length, to the whale shark, Rhincodon typus, the largest fish, which reaches approximately 12 metres and which feeds only on plankton, squid, and small fish through filter feeding.Shark are generally do not live in freshwater, with a few exceptions such as the bull shark and the river shark which can live both in seawater and freshwater.They respire with the use of five to seven gill slits. Sharks have a covering of dermal denticles that protect their skin from damage and parasites and improve fluid dynamics so the shark can move faster. They have several sets of replaceable teeth.

Tails

Sharks have very distinctive tails. The tails of sharks vary considerably between species and are adapted to the lifestyle of the shark. The tail provides thrust and so speed and acceleration are dependent on tail shape. Different tail shapes have evolved in sharks adapted for different environments. Sharks possess a heterocercal caudal fin in which the dorsal portion is usually noticeably larger than the ventral portion. This is due to the fact that the shark's vertebral column extends into that dorsal portion, allowing for a greater surface area for muscle attachment which would then be used for more efficient locomotion among the negatively buoyant cartilaginous fishes.

Teeth

The teeth of sharks are embedded in the gums rather than directly fixed to the jaw, and are constantly replaced throughout the shark's life. Multiple rows of replacement teeth are grown in a groove on the inside of the jaw and moved forward in a "conveyor belt"; some sharks lose 30,000 or more teeth in their lifetime. The rate of tooth replacement varies from once every 8–10 days to several months.The shape of a shark's tooth depends on its diet: those that feed on mollusks and crustaceans have dense flattened teeth for crushing, those that feed on fish have needle-like teeth for gripping, and those that feed on larger prey such as mammals have pointed lower teeth for gripping and triangular upper teeth with serrated edges for cutting. The teeth of plankton-feeders such as the basking shark are greatly reduced and non-functional.

Behavior

The classic view describes a solitary hunter, ranging the oceans in search of food. However, this applies to only a few species, with most living far more sedentary, benthic lives. Even solitary sharks meet for breeding or at rich hunting grounds, which may lead them to cover thousands of miles in a year.Migration patterns in sharks may be even more complex than in birds, with many sharks covering entire ocean basins. However, shark behavior has only begun to be formally studied, so there is much more to learn.When approached too closely some sharks perform a threat display to warn off prospective predators. This usually consists of exaggerated swimming movements, and can vary in intensity according to the threat level.

Sleep

Some sharks can lie on the bottom while actively pumping water over their gills, but their eyes remain open and actively follow divers. When a shark is resting, it does not use its nares, but rather its spiracles. If a shark tried to use its nares while resting on the ocean floor, it would be sucking up sand rather than water. Many scientists believe this is one of the reasons sharks have spiracles. The spiny dogfish's spinal cord rather than its brain, coordinates swimming, so it is possible for spiny dogfish to continue to swim while sleeping.It is also possible that sharks sleep in a manner similar to dolphins,one cerebral hemisphere at a time, thus maintaining some consciousness and cerebral activity at all times.

Hearing

Although it is hard to test sharks' hearing, they may have a sharp sense of hearing and can possibly hear prey many miles away.A small opening on each side of their heads leads directly into the inner ear through a thin channel. The lateral line shows a similar arrangement, which is open to the environment via a series of openings called lateral line pores. This is a reminder of the common origin of these two vibration- and sound-detecting organs that are grouped together as the acoustico-lateralis system.