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Why do marine animals float?

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First of all we have to take into account the different phyla, and then give an example with a short explanation to go with each.

Mammals use blubber at times for minor buoyancy, but this primarily comes from the air stored in their body.

Fish on the other hand have what's called an air bladder which receives air from the blood; the amount of air is constantly regulated according to pressure and depth. How air, mainly oxygen because of nitrogen's volatility, is removed by the hemoglobin by a gas gland that produces lactic acid allowing the absorption of air into the swim bladder. It's placed near the spine; if the fish rushes to the surface, it risks bursting its bubble.

Most cephalopods have what's called a septa that's incorporated into their internal shell containing nitrogen and slime in a coiled fashion.

Birds such as penguins are buoyant due to the air in their body.

Reptiles float similar to a human's backstroke, but on their belly, keeping their head and back above the water.

Insects for another matter either use surface tension such as pond skippers or setae (dense, thick, bunch of hair) and possibly a plastron.

Finally, some amphibians' weight allows the lungs to keep the body afloat at the water's surface.

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