The Great Barrier Reef is home to over 1500 different species of marine fish, from the brightly colored wrasses to the ever popular anemone fish. The fish that live in this saltwater environment must overcome challenges to enable them to survive. Firstly they have a skeleton made of bone, a pair of gills, they reproduce by spawning and they have a swim bladder. The swim bladder (also called the gas bladder or air bladder) is an oval shaped sac that is used to regulate the fishes buoyancy in the water column.
The problem of buoyancy is solved by the fish in the marine environment by using their swim bladder. This oval shaped sac is filled with gas allowing the fish to control their buoyancy. When gas is added to the swim bladder the fish becomes less dense overall (the fish will rise), when the gas is removed the fish becomes more dense (the fish will sink).
The addition and removal of gasses allows the density of the fish to equal the density of the water thus maintain neutral buoyancy (weightless). Being weightless in a marine environment is much less energy demanding than being heavy or light.
SCUBA divers will be aware that when diving neutral buoyancy is achieved by adding or removing air from their buoyancy control device. This is the same principle the fish use to regulate their buoyancy. Adding or removing ions to the gas in the swim bladder from blood vessels around its outside controls the gasses density. More ions makes the gas more dense and less ions makes the gas less dense.
However not all fish on the Great Barrier Reef have swim bladders. This is most common in fish that don’t move around a lot and ambush prey from below. An example is the Lizardfish which lay on a high point on the reef waiting to ambush unsuspecting prey such as Damsel fish. Sharks also do not have a swim bladder, they use a different strategy which includes having a large oily liver and specialized body shape to maintain buoyancy.
So next time you are snorkelling on the Great Barrier Reef take some time and consider the processes occurring that allow the fish to freely move around in the water column.