Swim bladder

The swim bladder is a special gas-filled organ found inside most bony fish. It helps the fish stay at the right depth in the water without having to swim constantly, which saves energy. By changing the amount of gas in the swim bladder, a fish can make itself lighter or heavier to move up or down easily.

The swim bladder of a rudd

Because the swim bladder is positioned on the belly side of the fish, it affects how steady the fish is, but it also helps the fish turn and move quickly. The swim bladder can also act like a sounding board, helping the fish make or hear sounds underwater. This makes it useful for sonar equipment to find fish.

Scientists believe the swim bladder is related to the lungs of land animals and some other fish. Famous scientist Charles Darwin wrote about this idea in his book On the Origin of Species. Not all fish have swim bladders—fish like sharks and rays must swim constantly to stay in one place because they do not have them.

Structure and function

Swim bladder from a bony (teleost) fish

The swim bladder is a special organ in many fish that helps them stay at the right depth in the water. It is usually two gas-filled sacs on the top side of the fish. The walls of the swim bladder are flexible and can change size to keep the fish balanced in the water.

Some fish can fill their swim bladder by gulping air, while others need to visit the surface to fill theirs. This organ also helps fish hear better by carrying sounds to their inner ears. In some fish, like piranhas, it can even help make sounds.

Evolution

The swim bladder in fish is closely related to lungs. Scientists believe that lungs first appeared in ancient fish that needed to breathe air when oxygen levels in water were low. Over time, in some fish, these lungs changed into swim bladders, which help the fish stay at a certain depth in the water.

The West African lungfish possesses a lung homologous to swim bladders

Some fish have adapted their swim bladders to contain special oils instead of gas, which helps them move between shallow and deep water. Sharks and rays do not have swim bladders or lungs; instead, they use special fins to stay up in the water.

Sonar reflectivity

The swim bladder in fish can bounce back sound waves very well if the sound is the right frequency. This happens because the swim bladder acts like a tiny drum that vibrates at certain frequencies. Scientists use this sound bouncing to figure out how many fish are in a certain area, which helps protect fish populations and the environment.

The exact frequency that works best depends on the size of the swim bladder and other details about the fish. Knowing this helps scientists make better guesses about fish numbers using special sound equipment.

Deep scattering layer

Main article: Deep scattering layer

Most mesopelagic fishes are small filter feeders which ascend at night using their swimbladders to feed in the nutrient rich waters of the epipelagic zone. During the day, they return to the dark, cold, oxygen deficient waters of the mesopelagic where they are relatively safe from predators. Lanternfish account for as much as 65 percent of all deep sea fish biomass and are largely responsible for the deep scattering layer of the world's oceans.

During World War II, people using new sonar technology noticed something strange. It looked like a false sea floor 300–500 metres deep during the day, but less deep at night. This was because of millions of tiny deep-sea fish with special air-filled organs called swim bladders that reflected the sonar waves.

These tiny fish, especially lanternfish, move up into shallower water at dusk to feed on tiny floating plants and animals. They return to deeper water during the day for safety. They use their swim bladders to help them move up and down. When they want to go up, they fill the swim bladder with air, and when they want to go back down, they let the air out. This helps them travel through different water temperatures without getting too hot or cold.

Lanternfish are very common in the deep sea and are an important food source for larger animals. Their swim bladders reflect sonar waves, making it look like there is a false bottom on the sonar screen.

Human uses

Historically, people have used swim bladders for different purposes. They were once used to make certain items to meet cultural traditions during special events.

Fish maw

Main article: Fish maw

In some parts of East Asia, swim bladders from big fish are treated as a special food. In Chinese cooking, they are called fish maw and are used in soups and stews.

Because of high demand for this food, a small porpoise called the vaquita is in danger of disappearing forever. Vaquita live only in Mexico's Gulf of California. They often get caught in nets meant for catching totoaba, another fish whose swim bladders are very valuable.

Isinglass

Main article: Isinglass

Swim bladders are also used in food production to get a substance called collagen known as isinglass. Isinglass can be used to make strong glue or to clear up beer.

Swim bladder disease

Swim bladder disease is a common problem for fish kept in aquariums. When a fish has this issue, it might float upside down, with its nose pointing down and tail up. Other times, the fish could float to the top of the tank or sink to the bottom.

Risk of injury

Human activities like pile driving and seismic waves can create loud sounds that hurt fish with gas bladders. Some fish can't release air fast enough from their gas bladders, making them more likely to get hurt. Even though these fish might get injured, it usually doesn't kill them. Scientists use special equipment to study these effects on fish.

Similar structures in other organisms

Siphonophores have a special organ that helps jellyfish-like colonies float on the water's surface, with their tentacles hanging down below. This organ is different from the one found in fish.