Shark
Sharks are a member of Class Chondrichthyes, which includes the rays and skates. In general, sharks are characterized as large, marine, cartilaginous predators. There are over 500 different species and there are many exceptions to the above characterization. Sharks include everything from a thumb sized deep sea species, up to the whale shark, which can get to 60 feet in length but feeds only on plankton like the great whales. Some, like a sub-species of the bull shark, can even live in freshwater lakes. A few of the larger species actually are homeothermic, able to maintain their body temperature at a level above the ocean's temperature.
Most sharks are predatory however and include the well known species such as the great white, mako, blue, hammerhead, and bull sharks among others.
Sharks are fished commercially and recreationally. Some are fished simply for the sport of landing a good fighting fish (mako sharks for instance), others for food (blacktip, mako and others), and some species for other products. Sharks generally reach sexual maturity slowly and produce very few offspring in comparison to other fishes that are harvested. This has caused concern among biologists regarding the increase in effort applied to catching sharks over time, and many species are considered to be threatened.