Image: Fish Dissection - Swim bladder exposed
The other organs have been removed to expose the swim bladder at the top of the body cavity.
- Stuart Humphreys
- © Australian Museum
The swim bladder (also called the gas bladder or air bladder) is a flexible-walled, gas-filled sac located in the dorsal portion of body cavity. This organ controls the fish's buoyancy and in some species is important for hearing. Most of the swim bladder is not permeable to gases, because it is poorly vascularised (has few blood vessels) and is lined with sheets of guanine crystals.
A fish swimming in the water expends less energy if it is neutrally buoyant (that is, it neither sinks nor floats). If this fish starts to descend, the increased pressure from the water surrounding the fish results in a compression of the gas inside the swim bladder. The fish becomes negatively buoyant and will tend to sink. Conversely, if a fish swims into shallower water, there is a decrease in water pressure and so the gas in the swim bladder expands, and the fish tends to float upwards. The swim bladder helps to solve the problems associated with variations of pressure, and thus buoyancy.
If the fish becomes positively buoyant, and starts to float upwards, gas diffuses out of the swim bladder into the blood. This occurs at a site known as the oval. The gas in the blood is then removed from the body into the surrounding water at the gills.
Conversely if the fish becomes negatively buoyant, and starts to sink, air enters the swim bladder at a region called the gas gland. The way the fish does this involves three processes; the acidification of the blood, an increase in the concentration of lactate and hydrogen ions and the movement of blood through a complex structure called the rete mirabile (literally, the wonderful network). These complex processes are not discussed here. Refer to the reference below for more information.
Not all fishes have a swim bladder. Sharks for example do not have a swim bladder, and many species such as the Greynurse Shark, use a different strategy which includes having a large oily liver and specialised body shape to maintain buoyancy.
- Helfman G.S., Collette, B.B. & D.E. Facey. 1997. The Diversity of Fishes. Blackwell Science. Pp 528.