| Sharks (superorder Selachimorpha) are fish with a full cartilaginous skeleton[1] and a streamlined body. They respire with the use of five to seven gill slits.
Sharks have a covering of dermal denticles to protect their skin from damage and parasites and to improve fluid dynamics; they also have replaceable teeth.[1] Sharks include species ranging from the hand-sized pygmy shark, Euprotomicrus bispinatus, a deep sea species of only 22 centimetres (9 in) in length, to the whale shark, Rhincodon typus, the largest fish, which grows to a length of approximately 12 metres (39 ft) and which, like baleen whales, feeds only on plankton through filter feeding. The bull shark, Carcharhinus leucas, is the best known of several species to swim in both salt and fresh water and in deltas.
Etymology Until the 16th century,[10] sharks were known to mariners as "sea dogs".[11] According to the OED the name "shark" first came into use after Sir John Hawkins' sailors exhibited one in London in 1569 to refer to the large sharks of the Caribbean Sea, and later as a general term for all sharks. The name may have been derived from the Mayan word for fish, xoc, pronounced "shock" or "shawk".
Behaviour
Studies on the behaviour of sharks have only recently been carried out leading to little information on the subject, although this is changing. The classic view of the shark is that of a solitary hunter, ranging the oceans in search of food; however, this is only true for a few species, with most living far more sedentary, benthic lives. Even solitary sharks meet for breeding or on rich hunting grounds, which may lead them to cover thousands of miles in a year.[23] Migration patterns in sharks may be even more complex than in birds, with many sharks covering entire ocean basins.
Some sharks can be highly social, remaining in large schools, sometimes up to over 100 individuals for scalloped hammerheads congregating around seamounts and islands e.g. in the Gulf of California.[6] Cross-species social hierarchies exist with oceanic whitetip sharks dominating silky sharks of comparable size when feeding.
When approached too closely some sharks will perform a threat display to warn off the prospective predators. This usually consists of exaggerated swimming movements, and can vary in intensity according to the level of threat.[24]
Shark intelligence
Despite the common myth that sharks are instinct-driven "eating machines", recent studies have indicated that many species possess powerful problem-solving skills, social complexity and curiosity.The brain-mass-to-body-mass ratios of sharks are similar to those of mammals and other higher vertebrate species.[25]
In 1987, near Smitswinkle Bay, South Africa, a group of up to seven great white sharks worked together to relocate the partially beached body of a dead whale to deeper waters to feed.[26]
Sharks have even been known to engage in playful activities (a trait also observed in cetaceans and primates). Porbeagle sharks have been seen repeatedly rolling in kelp and have even been observed chasing an individual trailing a piece behind them.[27]
Shark sleep
Some say a shark never sleeps. It is unclear how sharks 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, they do not use their nares, but rather their spiracles. If a shark tried to use their nares while resting on the ocean floor, they 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 a spiny dogfish to continue to swim while sleeping. It is also possible that a shark can sleep with only parts of its brain in a manner similar to dolphins.[28] |
