Swim Bladder
Most fish have an organ called the swim bladder. It is a flexible, gas-filled sac located in the
dorsal or top portion of the body cavity and helps to control the fish's buoyancy. Since fish have
a density that is heavier than water, they need this swim bladder to not sink when they stop
swimming. There are two types of swim bladders, the physostomous and physoclistous.
Fish with a physostomous swim bladder are usually found in shallow water, though some are
in deeper water. They will expel bubbles as they swim closer to the surface of the water and then
they will swallow air at the surface before diving back down into the water. This air is passed
into the guts and then they will force it into the swim bladder.
On the other hand, the physoclistous swim bladder is completely closed off from the swim
bladder. These fish are able to control gas exchange or the amount of gas in their swim bladder
through the capillaries that are found where the membrane of the swim bladder is very thin.
When a fish is too buoyant, and starts to float upward, gas diffuses out of the swim bladder into
the blood. The gas in the blood is removed at the gill filaments and expelled. Conversely, if a
fish starts to sink, air enters the swim bladder via a gas gland. . Sometimes if a fish is caught and
quickly brought to the surface from deep water, their swim bladder may expand so fast that it
inflates into their mouth or even burst which usually will kill the fish, unfortunately