Shark

Hexanchiformes

Characteristics

Sharks have keen olfactory senses, with some species able to detect as little as one part per million blood in seawater. They are even more attracted to the chemicals found in the gut of many species, and often linger near or in sewage outfalls. Some species, such as Nurse Sharks, have external barbels that greatly increase their ability to sense prey. Shark eyes are similar to the eyes of other vertebrates, including similar lenses, corneas and retinas, though their eyesight is well adapted to their marine environment. Some sharks have stronger nocturnal adaptations, allowing them to see in dark conditions, and some have nictitating membranes to protect the eyes during predation. Sharks generally rely on their superior sense of smell to find prey, but once they are close they also use the lateral lines running along their sides to sense movement in the water and also employ special sensory pores on their heads to detect electrical pulses created by the muscles of prey. Their teeth are not attached to the jaw, but embedded in their flesh, and in many species are constantly replaced throughout the shark's life. The lower teeth are primarily used for holding prey, while the top are used for cutting into it. (Gilbertson, 7.3)

Related Topics:
Olfactory - Senses - Species - Part per million - Blood - Nurse Shark - Barbels - Prey - Eye - Vertebrates - Lens - Cornea - Retina - Marine - Nocturnal - Nictitating membrane - Lateral line - Electrical - Pulse - Jaw

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There are exceptions to the "large", "marine" (as in 'ocean-going') and "predatory" portions of the characterization. Sharks include everything from the hand-sized pygmy shark, a deep sea species, to the whale shark, the largest fish (although sharks are not closely related to bony fish) which is believed to grow to a maximum length of 18m (59 feet) and which, like the great whales, feeds only on plankton. The bull shark is a unique species in that it can swim in both salt water ocean and fresh water rivers (and in lake Nicaragua). A few of the larger species, the Mako and White Shark, are mildly homeothermic, able to maintain their body temperature at a level above the ocean's temperature. This is possible because of the presence of the rete mirabile, a counter current exchange mechanism that reduces the loss of body heat.

Related Topics:
Pygmy shark - Whale shark - Fish - Great whale - Plankton - Bull shark - Lake Nicaragua - Mako - White Shark - Homeothermic - Rete mirabile

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In many cases, a shark will suffocate if it stops active swimming. Sharks extract their oxygen from seawater as it passes over their gills. Because of their size and the nature of their metabolism, sharks have a higher demand for oxygen than most fish and they cannot rely on ambient water current to provide an adequate supply of oxygenated water. If a shark stops swimming, the necessary water circulation for respiration becomes too low and the animal suffocates. Some sharks, like the blacktip reef shark and nurse shark, can pump water over their gills as they rest. Because sharks have no swim bladders, they sink when they stop swimming; a resting shark sinks to the sea bed.

Related Topics:
Blacktip reef shark - Nurse shark

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A shark if inverted enters a natural state of paralysis. This state is called 'tonic' and the shark usually becomes dull and unresponsive for a while. Researchers use this condition for handling sharks safely. The condition is termed tonic immobility.

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A popular myth is that sharks are immune to disease and cancer, this is untrue and there are disease and parasites that affect sharks, and the evidence that sharks are at least resistant to cancer and disease is mostly anecdotal and there have been few if any scientific or statistical studies that have shown sharks to have heightened immunity to disease. http://news.nationalgeographic.com/news/2003/08/0820_030820_sharkcancer.html

Related Topics:
Cancer - Parasites - Anecdotal - Statistical

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