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 think lungs first appeared in ancient fish that needed to breathe air when there was less oxygen in the water. Over time, some fish changed these lungs into swim bladders. Swim bladders help fish stay at one depth in the water.

The West African lungfish possesses a lung homologous to swim bladders

Some fish have changed their swim bladders to hold special oils instead of gas. This 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 well if the sound is the right frequency. This is because the swim bladder acts like a tiny drum that vibrates at certain frequencies. Scientists use this sound bouncing to learn how many fish are in an area. This helps protect fish and the environment.

The best frequency depends on the size of the swim bladder and other details about the fish. Knowing this helps scientists guess fish numbers better with 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 saw 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 find food. 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.

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

People have used swim bladders for different reasons. They were once used to make items for special events.

Fish maw

Main article: Fish maw

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

Because many people want this food, a small porpoise called the vaquita is in danger. Vaquita live only in Mexico's Gulf of California. They often get caught in nets meant for catching totoaba, another fish with valuable swim bladders.

Isinglass

Main article: Isinglass

Swim bladders are 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 in aquariums. When a fish has this issue, it might float upside down, with its nose pointing down and tail up. Sometimes, 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 make loud sounds that may hurt fish with gas bladders. Some fish cannot let air out of their gas bladders quickly enough, which can make them more likely to be harmed. While these fish might be injured, it usually does not kill them. Scientists use special tools to study how these sounds affect fish.

Similar structures in other organisms

Siphonophores have a special organ that helps jellyfish-like groups stay on the water's surface, with their tentacles hanging down below. This organ works differently from the one in fish.