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Everything about marine vertebrates is fundamentally shaped by the demands of living underwater. Water is 800 times more dense than air, and 50 times more thick and viscous. Water also contains about 95 percent less available oxygen than air. The form, function and behavior of ocean animals reflect the ways they are adapted to life in this dense, oxygen-poor environment.
Few land animals can match the highly efficient feeding methods made possible by the density and viscosity of water. Most fishes have expandable heads with over 30 moveable bony parts. To collect food by suction, they rapidly expand the space in their mouths, creating a low-pressure area inside. When they open their mouths, water rushes in to fill the void—and everything floating in that water ends up inside their mouths. The food is swallowed, and the water flows out over the gills. Fishes can expand their mouth volume from six to 40 times in a fraction of a second, with some generating pressures so high they can suck a limpet off a rock.
Other marine vertebrates, such as whale sharks and manta rays, do not feed by suction. Instead, they filter tiny food particles suspended in the water around them. They swim with their huge mouths wide open. As water flows out over their gills, rows of long gill-rakers sieve the food particles from the water and pass it into the esophagus to be swallowed.
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| | TUBE-EYE
Stylephrus chordatus |
The tube-eye (Stylephorus chordatus) can increase the volume inside its mouth by 40 times. Its narrow mouth forces water to rush inside extremely rapidly, like water through a straw, at speeds up to 3.2 meters a second.
The parrotfish bites off chunks of coral with its powerful beak to eat the algae living inside the coral tissues. A single parrotfish may crush a ton of coral into white sand each year.
The archerfish spits droplets of water at terrestrial insects, causing them to fall into the water where it can reach them.
The cookie-cutter shark bites a circular plug of flesh from its victims.
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