Sharks

Sharks are fish that have cartilage skeletons, a torpedo-shaped body, five to seven gills, and small, tooth-like scales. Sharks are relatively non-aggressive towards humans, despite the fact that many people fear shark attacks—a fear that has been encouraged by such movies as Jaws.

In May 2009, researchers solved one of most persistent scientific mysteries surrounding sharks. The basking shark, a hulking fish known for being particularly non-aggressive, has baffled scientists by disappearing every winter. By tracking the elusive giant, researchers finally determined that the slow-moving sharks migrate from northern waters, where they spend most of their time near the surface, to deep in the Caribbean, where they "hide out" at depths of up to 3,000 feet or more.WIRED: Enormous Sharks Secret Hideout Finally Discovered (May 7, 2009)

The skeleton of the shark is made out of cartilage, with makes it more flexible than other vertebrate fish. Sharks breath through a set of gills, which extracts oxygen from the water as it passes over them. Most sharks have the ability to pump water over their gills while not moving. However there are a few species that do not have this ability, and because of this they must be in continuous motion their entire life in order to breath. Sharks teeth have a unique characteristic as well, in that they are constantly being replaced. They are not lodged in the cartilage jawbone, but rather embedded in the flesh of the shark. Some species replace their teeth ever eight to ten days, and can replace upwards of 30,000 teeth in their lifetime.

The life cycle of a shark can last up to 100 years, with the average species living for about 20 to 30 years. Reproduction is accomplished using a modified type of pelvic fin in the male to fulfill the role of a male sexual organ. Females give birth to a dozen or so pups on average, which can be born live or protected by an egg 'sack'.