A swim bladder, also known as a gas bladder or air bladder, is a gas-filled internal organ found in the majority of bony fish. This sac-like organ is located in the dorsal part of the body cavity, between the digestive tube and the vertebral column. The walls of the bladder are flexible, allowing them to contract and expand. These walls have very few blood vessels and are lined with guanine crystals, which makes them impermeable to gases. It is derived from an out-pocketing of the digestive tube during the fish’s development.
The Purpose of a Swim Bladder
The primary purpose of the swim bladder is to control buoyancy, allowing a fish to maintain its desired depth without expending significant energy. By adjusting the gas in the bladder, a fish can achieve neutral buoyancy, a state where gravity is balanced by the buoyant force. This allows the fish to hover effortlessly in the water column, conserving energy for activities like hunting, evading predators, and resting.
The swim bladder also serves secondary functions in some species. Its position in the dorsal part of the body helps with lateral stability, as its expansion lowers the fish’s center of mass. For certain fish, like carp and catfish, the swim bladder acts as a resonating chamber to produce or receive sound. In these fish, the bladder is connected to the inner ear by small bones, transmitting vibrations to enhance hearing.
How Fish Adjust Buoyancy
Fish use two distinct methods for regulating gas volume in their swim bladders. The first type, physostomous, is found in fish like trout, carp, and eels. These fish retain a connection, called the pneumatic duct, between their swim bladder and gut. This duct allows them to control buoyancy by gulping air at the surface to fill the bladder or by “burping” air to release it, enabling rapid adjustments.
The second type, physoclistous, is found in more evolutionarily derived fish where the pneumatic duct is lost after the early life stages. A “gas gland” secretes gas, primarily oxygen, from the blood into the bladder. This process is facilitated by a network of capillaries called the rete mirabile, which acidifies the blood to release oxygen from hemoglobin. To decrease gas volume, a specialized area called the oval allows gas to be reabsorbed into the bloodstream.
When Swim Bladders Go Wrong
Disruptions to the swim bladder’s function can lead to buoyancy problems, often called “swim bladder disease” by aquarium enthusiasts. This condition is not a single disease but a symptom of an underlying issue affecting the organ. A fish with a buoyancy disorder may float uncontrollably to the surface, sink to the bottom, or struggle to swim upright, sometimes floating upside down or on its side.
Physical injuries, sudden changes in water pressure, or internal problems can impact the swim bladder. Common causes in aquarium fish include constipation or an enlarged stomach from overeating, which puts physical pressure on the bladder. Bacterial or parasitic infections can also lead to inflammation of the organ, and poor water quality can cause stress that disrupts buoyancy control.
Fish Without Swim Bladders
Not all fish possess a swim bladder. Cartilaginous fish, such as sharks and rays, never evolved this organ. Many sharks have large livers rich in low-density oils, which provide some lift, though it is not adjustable like a gas bladder. They must also rely on “dynamic lift,” generated by continuous swimming, where water moving over their fins acts much like an airplane wing to prevent sinking.
Some bony fish, particularly bottom-dwellers like flounders, gobies, and sculpins, have lost their swim bladders over evolutionary time. For these species, being negatively buoyant is an advantage, allowing them to remain firmly on the seafloor without expending energy. This adaptation is suited to their lifestyle of hiding from predators and ambushing prey from the substrate.