Swim bladder

The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ in bony fish that functions to modulate buoyancy, and thus allowing the fish to stay at desired water depth without having to maintain lift via swimming, which expends more energy.^[1] Also, the dorsal position of the swim bladder means that the expansion of the bladder moves the center of mass downwards, allowing it to act as a stabilizing apparatus. Additionally, the swim bladder functions as a resonating chamber to produce or receive sound.

The swim bladder is evolutionarily homologous to the lungs of tetrapods and lungfish, and some ray-finned fish such as bowfins have also evolved similar respiratory functions in their swim bladders. Charles Darwin remarked upon this in On the Origin of Species,^[2] and reasoned that the lung in air-breathing vertebrates had derived from a more primitive swim bladder as a specialized form of enteral respiration.

Some species, such as mostly bottom dwellers like the weather fish and redlip blenny,^[3] have secondarily lost the swim bladder during the embryonic stage. Other fish, like the opah and the pomfret, use their pectoral fins to swim and balance the weight of the head to keep a horizontal position. The normally bottom-dwelling sea robin can use their pectoral fins to produce lift while swimming like cartilaginous fish do.

The gas/tissue interface at the swim bladder produces a strong reflection of sound, which is used by sonar equipment to find fish.

Cartilaginous fish such as sharks and rays do not have swim bladders,^[4] as they belong to a completely different evolutionary clade. Without swim bladders to modular buoyancy, most cartilaginous fish can only control depth by actively swimming, which produce dynamic lift; others store up lipids with specific density less than that of seawater to produce a neutral or near-neutral buoyancy, which cannot be readily changed with depth.