Fish Swim Bladder and Buoyancy

How the swim bladder keeps bony fish neutrally buoyant: physostomous and physoclistous types, the gas gland and rete mirabile, buoyancy control, and the main categories of swim bladder disorders.

What the swim bladder is

The swim bladder, also called the gas bladder or air bladder, is an internal gas-filled organ of bony fish. Its main job is to adjust buoyancy so a fish can hold its depth without swimming to generate lift, which costs more energy. It usually consists of two gas-filled sacs high in the body cavity, though a few primitive species have a single sac. Its flexible walls expand and contract with surrounding pressure. The walls carry few blood vessels and are lined with guanine crystals that make them nearly gas-tight. The swim bladder is evolutionarily related to the lungs of tetrapods and lungfish. It develops as an outgrowth of the foregut, budding from the upper side, while lungs bud from the lower side.

Not every fish has one. Sharks and rays have no swim bladder and stay up by swimming to create lift, by storing low-density lipids, or both. Some bottom-dwelling bony fish lose the swim bladder during embryonic development. In carp, catfish and their relatives, the swim bladder is linked to the inner ear by a chain of small bones, the Weberian ossicles, which improves hearing. In some species, such as the red-bellied piranha, the swim bladder also acts as a resonator when sound is produced.

Physostomous and physoclistous fish

Physostomes keep a pneumatic duct between the swim bladder and the gut. They can fill the bladder by gulping air at the surface and let gas out the same way, which also allows them to rise quickly without the bladder overexpanding. Examples include bichirs, gars, many carps, trouts, herrings, catfish and eels. Some physostomes can also use the bladder as a lung when oxygen in the water is very low.
Physoclists have no connection to the gut as adults. The duct exists only in the embryo or very early larva, for the first filling, and then disappears. Afterwards, gas is added by a gas gland and removed through a vascular area called the oval (ovale). The physoclistous condition is considered to have evolved from the physostomous one. Eels are an exception: they are anatomically physostomous but their bladder functions more like that of a physoclist.

The first filling happens early in life. Physoclist larvae have to reach the surface to take their first gulp of air before the duct closes. Among physostomes, salmonids fill the whole bladder by swallowing air after the yolk sac is absorbed, while cyprinids fill the posterior chamber first. In cyprinids the pneumatic duct connects only the posterior chamber to the oesophagus.

How the gas gland fills the bladder

In physoclists, gas is secreted into the bladder, mostly as oxygen. The gas gland releases lactic acid and produces carbon dioxide, which acidifies the blood passing through a capillary bundle called the rete mirabile. Acidified haemoglobin releases its oxygen (the Root effect), and part of that oxygen diffuses into the bladder. As the blood leaves, it runs back through the rete in the opposite direction. There most of the excess oxygen and carbon dioxide diffuse across into the incoming arterial blood. This countercurrent multiplication loop concentrates gas until very high pressures are reached, enough to keep gas in the bladders of deep-sea fishes at pressures of hundreds of bars. Other gases also come out of solution into the bladder. To lower buoyancy, the fish lets gas diffuse back into the blood at the oval.

Buoyancy control and its limits

Because the bladder expands and shrinks with pressure, any change of depth changes buoyancy, and the fish has to correct it by adding or releasing gas. Teleosts are thought to lack a sense of absolute water pressure, but they may detect depth through the rate at which bladder volume changes. The physoclist design has one key weakness: the fish cannot rise fast, because gas cannot be vented quickly enough and the bladder can burst. Physostomes can burp gas out, but they then need to regain gas before going deeper again. Some vertically migrating fishes replace gas with low-density wax esters, which do not expand and contract with pressure.

Rapid pressure drops cause barotrauma. In laboratory decompression trials that simulated turbine passage, the most common injury was a ruptured swim bladder, often with free gas in the body cavity. Other injuries included internal bleeding, bulging eyes and gas bubbles in blood vessels, gills and fins. Physoclists are usually considered more vulnerable, but in those trials young physostomous cyprinids with two-chambered bladders, such as roach (Rutilus rutilus) and common nase (Chondrostoma nasus), were the most sensitive. Their front chamber has no direct vent and tended to rupture.

Swim bladder disorders as categories

Buoyancy disorders are common in ornamental fish, especially goldfish. Short, rounded body shapes, such as those of some fancy goldfish varieties, appear to predispose fish to them. Onset is usually sudden. Affected fish lie on the bottom (negative buoyancy), float at the surface (positive buoyancy), list to one side, hang head-down or turn upside down. The abdomen may be swollen, often unevenly. Fish stuck at the surface may get dried-out skin, and fish stuck on the bottom may develop pressure sores. Veterinary literature groups the causes roughly as follows.

Overinflation: linked to chronic bacterial or granulomatous disease, gas supersaturation (see the gas bubble disease guide) and kidney tumours.
Fluid in the bladder: transudate or exudate, often with bacterial infection. In physostomes, opportunistic bacteria can reach the bladder through the pneumatic duct, and poor water quality favours this. Inflammation of the swim bladder is called aerocystitis.
Displacement, torsion or compression: the rear chamber can be pushed aside by masses such as kidney tumours, polycystic kidneys or gonadal tumours.
Collapse, rupture or herniation, including injury from rapid pressure change.
Deformity and developmental problems: malformed or extra bladders, which have been linked to genetic factors and selective breeding in fancy cyprinids.
Parasitic disease: swim bladder inflammation of common carp (Cyprinus carpio) fingerlings is associated with the myxozoan Sphaerospora dykovae.
Problems outside the bladder: gas in the intestine (intestinal tympany) can mimic a bladder disorder.

In one veterinary case series of nine adult koi with buoyancy loss, imaging found fluid in the bladder in six fish. Bacteria were cultured from bladder fluid in six of the seven fish tested, and structural anomalies or a tumour were found in four.