Buoyancy enables fish to float in the water without expending a great deal of energy. Most fish use an organ called the swim bladder to maintain buoyancy, but some species have special adaptations to enable them to remain buoyant in different environments. Natural buoyancy enables fish to elude predators, find food and seek mates easily.
Neutral Buoyancy Explained
Fish maintain a state called neutral buoyancy in order to float in the water. Neutral buoyancy means that upward force of water is equal to the downward pull of gravity. Without neutral buoyancy, fish would sink to the bottom or float to the surface. They wouldn't be able to propel themselves through the water at will. Fish maintain neutral buoyancy automatically by using one or more natural adaptations.
The Swim Bladder
Most bony fish use a special organ called a swim bladder to maintain neutral buoyancy. The swim bladder inflates with air and displaces the fish's weight. Fish inflate the swim bladder in one of two ways -- by gulping air directly into the bladder or through automatic dilation and contraction of blood vessels, which balances the level of gases entering and leaving the swim bladder.
Some fish gulp water into their stomachs to create a temporary swim bladder. Sand tiger sharks, for example, rise to the surface, gulp as much water as they need, then use the weight of the water to float through their environment as needed. Some fish use their fins to displace water similarly to the way a bird flapping his wings displaces air in order to fly. The downward pressure of water from the fins offsets the pull of gravity, enabling the fish to float.
Liver and Skeletal Adaptations for Buoyancy
Sharks, skates and rays have other adaptations that create buoyancy. These cartilaginous fish have cartilage instead of bones, which makes their entire bodies lighter and more flexible in H2O. Their livers also store much more low-density oils than other mammals. According to the Shark Trust, a shark's liver is 25 percent of its body weight, compared to 5 percent of a mammal's weight. A shark's large, oil-filled liver helps it achieve neutral buoyancy similar to the way the swim bladder helps tropical fish remain buoyant.
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