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Many sharks can contract and dilate their pupils, like humans, something no teleost fish can do. Sharks have eyelids, but they do not blink because the surrounding water cleans their eyes. To protect their eyes some species have nictitating membranes. This membrane covers the eyes while hunting and when the shark is being attacked.
While the shark is moving, water passes through the mouth and over the gills in a process known as "ram ventilation". While at rest, most sharks pump water over their gills to ensure a constant supply of oxygenated water. A small number of species have lost the ability to pump water through their gills and must swim without rest.
While the majority of sharks are solely marine, a small number of shark species have adapted to live in freshwater. The river sharks (of the genus Glyphis) live in freshwater and coastal marine environments. The bull shark (Carcharhinus leucas), can swim between salt and fresh water, and are found in tropical rivers around the world.
Unlike beached whales and dolphins, which can potentially survive on land for an hour or more, sharks need a constant flow of water past their gills to breathe. Without it, most will die within ...
The entire body of a shark is a very efficient eating machine. Each organ has been fine-tuned for hunting and acquiring food. Sharks are built to feed: Here's why they are the ultimate eating machines
Shark Anatomy (50693674756) The gill slits of a whale shark flaring as it expels water from its pharyngeal cavity. In the shark anatomy image, it depicts the beginning half of the shark, including the gills. The shark gills are especially important and were evolved from the chordate pharyngeal gill slits synapomorphy.
Most sharks rely on ram ventilation, forcing water into the mouth and over the gills by rapidly swimming forward. In slow-moving or bottom dwelling species, especially among skates and rays, the spiracle may be enlarged, and the fish breathes by sucking water through this opening, instead of through the mouth. [6]
Young bull sharks leave the brackish water in which they are born and move out into the sea to breed. While is theoretically possible for bull sharks to live purely in fresh water, experiments conducted on bull sharks found that they died within four years. The stomach was opened and all that was found were two small, unidentifiable fishes.