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All About Sharks!
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Sharks vary greatly in their diets, but they are all carnivores (meat-eaters). The method of obtaining the prey and the prey itself differ greatly among the different types of sharks.
Many sharks (like the great white, blue shark, mako, tiger shark, and hammerhead) are swift, sleek predators that eat fish, squid, other sharks, and marine mammals (like sea lions, seals, and smaller whales).
Some bottom-dwelling (benthic) sharks (like the zebra horn shark, angel shark, and wobbegong) are slow-swimming predators that crush and eat shellfish (crabs and clams) from the ocean floor.
Some large sharks (like the whale shark, the basking shark and the megamouth shark) are filter feeders that sieve tiny bits of plankton and small animals from the water through their gills as they swim with wide-open mouths.
Do Sharks Drink Water?
Unlike bony marine fish, sharks don't drink water. The blood and tissues of sharks (and other cartilaginous fish) are extremely salty, containing high concentrations of urea, trimethalamine oxide, and other salts. Sharks and their relatives are slightly hyperosmotic to sea water (that is, sharks are even saltier than the sea).
Sharks absorb (pure) water from the sea by osmosis. Osmosis is a process in which water flows across a semi-permeable membrane from an area of higher concentration of water to an area of lower concentration of water, effectively moving to equalize concentrations. In the case of sharks vs. sea water, water flows from the sea water into the even-saltier shark via gill tissues.
This system makes most sharks unable to live in fresh water. The exceptions are bull sharks and some stingrays, which can lower their salt concentrations to enter fresh water.
Sharks can secrete any extra salts through the rectal gland.
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