If they exhale, however, they sink. This means that the change in body volume during exhalation is enough to change the density of the animal from something slightly less than that of water to something greater. We can deduce from this that present day land animals did not evolve a mechanism for substantially altering their density when they emerged from the sea. Aquatic animals, however, need only to alter their density a little to stay afloat. Some fish, known as the teleost, or bony fishes, have evolved a mechanism, the swim bladder, to maintain neutral buoyancy.
Some jellyfish reduce their density by actively excluding sulfate from their tissues.
Some fish and crustaceans adjust their density by accumulating low-density lipids or waxes. Students will compare and contrast these two means of achieving buoyancy, and understand why one doesn t work for the other. Notice what happens to the fish as air fills its swim bladder. Watch the level of water in the submarine s ballast tank as you squeeze and hold the bulb leading to the submarine. Observe the reaction of the submarine as water is pumped out of its ballast tank.