Swim Bladders in Fish

What is swim bladder?

The swim bladder is also known as the air bladder or gas bladder is an internal organ that is gas-filled that provides the ability to control buoyancy in much bony fish. Due to this, they stay at their depth without active use of energy in swimming.

The swim bladder is possessed by most bony fish.

Structure and Location

Swim bladders can be paired or unpaired gas-filled sacs. The pneumatic duct usually connects to the oesophagus ventrally in dipnoans and chondrostean while dorsally in garfishes, the bowfins, and modern bony fish.

Sometimes this pneumatic duct is linked with the pharynx or stomach. Swim bladders are present above the coelomic lining of the peritoneal cavity, below the dorsal aorta and vertebral column, and lies close to the kidney.

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The walls are made up of elastic tissues and smooth muscles. On the basis of the connection of the pneumatic duct with the gut, fish are divided into two groups:

Physostomous fishes

These types of fishes have open pneumatic duct i.e., linked with the gut such as in dipnoans, chondrostean, holostean, and few teleosts.

Physoclistous fishes

These types of fishes have closed pneumatic duct i.e., it is not linked or any connection with gut such as in catfish, carp, eels, pickerel, herring, and salmon.

Shape of Swim bladder

The shape of the swim bladder varies from species to species of fish. It is usually constricted into anterior and posterior subdivisions.

Gases in Swim bladder

Different fishes contain different gases in their swim bladder. Some fishes have almost 99% pure nitrogen, while some fishes have 87% oxygen. The swim bladder of all fishes contains at least traces of four gases i.e., nitrogen, oxygen, argon, and carbon dioxide.

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Functions of Swim bladder

Some of the important functions of the swim bladder are following:

1. Hydrostatic organ

Teleosts use the swim bladder as their hydrostatic organ. The volume of gas is reflexively controlled in the swim bladder. This changes the specific gravity of the fish and increasing or decreasing its ability to swim.

In some primitive surface-dwelling fishes, gas is taken into the sac through the pneumatic duct. However, in most cases, the gases are supplied by the blood.

A network of small arteries (rete mirabile) called the red gland is present in the lining of the bladder and supplies gas. It may be bubbled through the mouth in physostomous fishes.

2. Reception of sound waves

In some teleosts e.g., carp, catfish, and other Cypriniforms the swim bladder is utilized for the reception of sound waves. It occurs by transferring vibration to the inner ear (membranous labyrinth) either directly or through the chain of bones called Weberian Ossicles.

The Weberian Ossicles are a chain of three modified vertebrae: the scaphium, interclarium, and tripus. The tripus connects the anterior end of the swim bladder with the sinus impar an extension of the perilymph cavity.

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3. Swim bladders as Lungs

In some fishes such as Polypterus, true lungfishes, Protopterus, etc. swim bladder serve as lungs. Neoceratodus has a single swim bladder which is used only as lung when water is deficient in oxygen.

In these fishes, the swim bladder is lined with low septa and may have numerous tiny air sacs. The blood supply to these swim bladders is like the tetrapod lung.

4. Emission of Thumbing sounds

It is observed that in few fishes, when striated muscles attach to swim bladder contract, thumping sound is emitted. They force air back and forth between the chambers isolated by muscle sphincters to produce croaking and grunting sounds.