If you’ve ever gone fishing or seen fish swimming in an aquarium, you may have wondered how they’re able to float and dart around so effortlessly underwater. Unlike humans, fish have a special organ that allows them to regulate their buoyancy and depth in water – the swim bladder.
If you’re short on time, here’s a quick answer to your question: Not all fish have swim bladders. While most bony fish (teleosts) possess swim bladders, there are exceptions including flatfish, eels, and some bottom-dwelling species. Cartilaginous fish like sharks do not have swim bladders at all.
In this comprehensive article, we’ll explore what exactly swim bladders are, what they do for fish, the different types of swim bladders, which fish species lack them entirely, and how fish without swim bladders are still able to control their buoyancy.
What Exactly Is a Swim Bladder?
The swim bladder, also known as the gas bladder, is a gas-filled internal organ found in many bony fish species. It serves a variety of important functions related to fish buoyancy and health.
A Gas-Filled Internal Organ
The swim bladder is an elastic, gas-filled sac located in the dorsal portion of the fish’s body cavity, between the digestive organs and the spinal column. It contains gases that are secreted by special gland cells and is similar in structure to a lung.
The most common gas found in swim bladders is oxygen.
By adjusting the amount of gas in the swim bladder, a fish can control its buoyancy in the water. If a fish wants to dive deeper, it can contract muscles around the swim bladder to decrease its volume and reduce buoyancy. To move up in the water, the muscles relax to increase bladder volume.
This important function allows fish to stay suspended at their desired depth without wasting energy swimming.
Located in the Body Cavity
The swim bladder’s location within the body cavity differs among fish species. In physostome species, it is connected to the esophagus via a pneumatic duct, while in physoclists, the duct disappears as the fish matures.
In fish like salmon and trout, the swim bladder extends along much of the body length. In others, like cod, it is more compact and oval-shaped.
The presence of the swim bladder does not compromise space for other organs since it shrinks and expands as needed. It is located alongside vital organs including the stomach, intestine, liver, and kidneys.
A sturdy membrane called the peritoneum surrounds the swim bladder and protects these organs during changes in bladder volume.
Varies in Size and Shape
Swim bladders display an impressive diversity in size and proportions. Relative to overall body size, some of the largest bladders are found in anglerfish and opah. Very small bladders occur in flatfish like halibut that spend most of their time resting on the seafloor.
In terms of shape, tubular designs are common, as are more round or oval forms. Some bladders are simple sacs, while others have complex features like gas glands, encapsulated compartments, and even maze-like channels.
The unique swim bladder anatomy in each species is adapted for its particular lifestyle and buoyancy needs.
While swim bladders are a defining feature of most bony fishes, there are some exceptions. Sharks, rays, and lampreys lack this special organ entirely and must swim continuously to avoid sinking. In contrast, bony fishes with swim bladders can effortlessly maintain neutral buoyancy, conserving energy as they rest or lie in wait for prey.
The Main Functions of the Swim Bladder
Buoyancy Control
The swim bladder, also known as the air bladder, is a gas-filled organ that helps many bony fish control their buoyancy. It acts like a balloon inside the fish, filling with gas to make the fish more buoyant and deflating to make it sink.
Proper inflation of the swim bladder allows the fish to ascend, descend, and hover in the water column with minimal effort. This is crucial for fish living at different depths and moving between shallow and deep waters. The swim bladder makes swimming more energy efficient for fish.
Many fish species have a duct called the pneumatic duct that connects the swim bladder to the gut. Fish can adjust the amount of gas in their swim bladder by gulping air at the surface and releasing it through the duct into the bladder.
Other species lack a pneumatic duct and must adjust the contents of the bladder using special gas gland cells. These cells extract dissolved gases from the blood to inflate the bladder.
The swim bladder evolved independently in many different groups of bony fish. It is found in most species apart from flatfish like flounder and sole. Some fish even have multiple swim bladders. The size and location of the bladder varies between species.
Salmonids like salmon and trout have small bladders located far back near the kidneys. Meanwhile, goldfish have two large sac-like bladders on either side of their spine.
Sound Transmission
The swim bladder also plays a key role in hearing and sound production for many species. It acts to transmit and amplify sound waves. Vibrations in the bladder caused by incoming sound waves are detected by the inner ear. Some fish use their swim bladder to produce sounds for communication.
They vibrate muscles near the bladder to make drumming or croaking noises.
Otophysan fish like goldfish and carp have a specialized connection between their swim bladder and inner ear. Small bones called Weberian ossicles connect the two organs and enable excellent transmission of sound waves.
This adaptation allows otophysans to hear higher frequency sounds and better detect noise from predators or prey.
Other fish like herring and cod use their swim bladders to produce deeper drumming noises for communication. They rapidly contract muscles against the bladder wall to make loud pulsating sounds. Some species’ calls can be heard up to several kilometres away by other fish.
The Two Main Types of Swim Bladders
Physoclistous (Closed) Swim Bladders
The first main type is the physoclistous (closed) swim bladder. This type of bladder does not connect directly to a fish’s digestive tract or have a duct allowing water to enter or exit. Instead, it relies on a complicated gas gland or rete mirabile to adjust the levels of gas and maintain buoyancy control.
Species with physoclistous bladders include ray-finned fishes like damselfish, cichlids, wrasses, gobies, and seahorses.
Because physoclistous bladders are closed off, the gas inside must match surrounding water pressure to prevent organ rupture or collapse. This gives most physoclistous species limited vertical mobility compared to those with open bladders. However, the composition of gas inside can change over time.
This enables some long-term buoyancy adjustments, just at a slower pace. For example, the plainfin midshipman fills its bladder almost solely with oxygen during summer mating season to become more buoyant and guard its nest.
Physostomous (Open) Swim Bladders
In contrast, the second main category is the physostomous (open) swim bladder, which connects directly to a fish’s esophagus or intestine via a pneumatic duct. This allows gas exchange with the environment, giving species precise control over short-term depth changes.
Flounders, cod, salmon, catfish, and more sport physostomous bladders.
The duct’s permeability means water must actively be prevented from infiltrating the bladder. Special gland cells extract oxygen from artery blood to counter water diffusion. This keeps the bladder inflated with gas while allowing rapid volume adjustments.
Some fish can even achieve neutral buoyancy this way, suspending themselves effortlessly in water columns.
Physostomous bladders come with tradeoffs, however. Species must expend energy maintaining gas balance and water outflow. And invasive parasites sometimes enter the bladder through the duct, hampering function.
Still, for active fish making frequent depth changes, the quick volume control offers key advantages.
Fish Without Swim Bladders
Flatfish
Flatfish like flounder, sole, and halibut lack swim bladders. As bottom-dwellers, they spend most of their time resting on the seafloor rather than swimming actively in the water column. Their flattened body shape is an adaptation that helps them camouflage themselves by lying flat on the bottom.
According to research from the University of Washington (source), around 800 of the approximately 30,000 known fish species lack swim bladders. Flatfish make up a significant portion of these bladderless fish.
Eels
Eels are another group of common fish that manage just fine without a swim bladder. For example, moray eels and garden eels have very reduced or completely absent bladders. Their long, slender shape makes it easy for them to navigate holes and crevices in coral reefs.
One study found the swim bladder only makes up 0.2% of an eel’s body cavity since so much space is occupied by their lengthy gastrointestinal tract (source). Eels instead control their buoyancy through lipid deposits and by altering their body posture.
Sharks and Rays
Sharks, rays, and their relatives the chimaeras lack swim bladders as well. Their large livers play an important role in maintaining near neutral buoyancy in seawater. The balance between tissue and water density controls their ability to hover in place.
For example, the liver in basking sharks makes up 25-30% of their total body weight (source). Pretty impressive for an organ best known for detoxification!
Other Bottom-Dwellers
Catfish, loaches, gobies, and other fish that spend most of their time scavenging along the bottom also frequently lack swim bladders. Like flatfish, their lifestyle simply doesn’t require strong swimming capabilities or precise depth control.
For instance, loaches like weatherfish and kuhli loaches live in small streams, ponds, and rice paddies. They occasionally gulp air from the surface instead of relying on a swim bladder to maintain buoyancy.
How Fish Without Swim Bladders Control Buoyancy
Regulating Gas in the Gut
Fish that lack swim bladders have evolved clever ways to control their buoyancy in the water column. Many regulate the amount of gas, especially oxygen and carbon dioxide, in their guts. By gulping air at the surface and holding it in their stomachs, the fish can increase buoyancy.
Alternatively, they can decrease buoyancy by releasing the gas from their digestive systems. Species like cod and cusk actively gulp air into special internal sacs connected to the gut, providing more lift than just stomach air alone.
The ingenious designs of their gas bladders maximize surface area, increasing the potential for gas exchange.
Adjusting Levels of Lipids and Water
Other fish alter the amount of lipids and water in their bodies to change density and control buoyancy. Many species will metabolize fat reserves when food is scarce, becoming denser and less buoyant. Cartilaginous fish like sharks and rays rely heavily on large livers rich in low density oils to maintain lift.
The livers act like hydrostatic balloons. Some fish can also regulate ions and water in special cells and tissues to adjust their density relative to seawater. For example, coelacanths use a fatty oil-filled sac called the notochord to control up and down movements in the water.
Using Fins for Control
Fish lacking swim bladders actively use their fins to maintain depth. Species like tuna and mackerel have high aspect ratio tail fins that generate lift as they swim. The fins act like wings, counteracting the weight of the fish.
Many bottom dwellers like flatfish rely on wide pectoral fins to produce forces needed to hover above the seabed. Fins help compensate for the lack of gas filled organs. According to a 2020 study, the wing-like fins of ocean sunfish produce substantial lift, enabling the heavy fish to traverse vertical distances with minimal effort.
Conclusion
While the vast majority of bony fish possess swim bladders to help control their buoyancy, there are some exceptions among bottom-dwellers and eel-like species. Cartilaginous fish lack swim bladders entirely.
For those without this special organ, alternative methods like lipid regulation and fin movements allow fish to navigate smoothly through the water column.
The swim bladder remains a key adaptation that has allowed most teleost fish to thrive in aquatic environments. This gas-filled organ’s ability to alter buoyancy via gas exchange helps fish minimize energy expenditure while swimming and resting, avoid predation, and even aids in sound transmission for communication.
