Under Pressure: Buoyancy Control and Adaptations to Life in the Deep

On land, we can jump, duck, and cartwheel, but, for the most part, we live in two dimensions. Underwater, it’s a different story. Organisms float up, hover, and sink to the sea floor. This is called 'buoyancy,’ which is an important part of aquatic life.

A positively buoyant animal will float, while one that is negatively buoyant will sink. When we see a fish swimming midwater or a jelly drifting in the current, they have achieved a neutral buoyancy. To understand how buoyancy impacts aquatic life, we first have to understand the effects of pressure.

Under Pressure

When divers at the aquarium enter our habitats, they wear a buoyancy control device [or “BCD”] on their shoulders like a backpack. As the name suggests, a BCD helps divers control their buoyancy. It attaches to their air tank, and the diver can inflate or deflate this device underwater using a low-pressure inflator [LPI] hose.

As gas descends in water, increasing pressure causes the volume of a gas to decrease. This affects both the air in the BCD and another important tool divers use to control buoyancy, their lungs. After a diver deflates their BCD in order to descend, they will add just a little bit of air to achieve a neutral buoyancy. By doing this, divers are able to rise with a breath in and sink with a breath out.

The deepest dive at the aquarium is the Pacific Kelp habitat, which is only around 10 meters deep—a shallow depth for recreational divers. At further depths, the risks associated with expanding air—or decompression, commonly known as “the bends"—are" heightened, requiring divers to take precautions like midwater safety stops.

Built-In Buoyancy

While humans put on equipment to hover underwater, aquatic animals have buoyancy control devices built into their bodies.

Bony fish use a swim bladder to control their position in the water. The swim bladder is an organ that expands and contracts depending on the volume of gas inside. This allows fish to maintain their position in the water column.

However, if a fish swims too close to the surface, they risk their swim bladder rapidly expanding. Unlike divers with their LPI controls, most bony fish do not have a built-in purge device.

Some, such as amberjacks, are adapted to purge through a valve in their gills, enabling them to make rapid ascents. Similarly, if a fish swims too deep out of its range, its swim bladder will shrink or even collapse, causing them to become negatively buoyant and sink. This is why most species of fish do not make deep dives.

Built Different

However, not all bony fish use swim bladders to regulate their buoyancy. The ocean sunfish, or Mola mola , for example, has skin that is neutrally buoyant at any depth. As opposed to a swim bladder, this adaptation allows the ocean sunfish to carry out its hunting pattern of diving from the surface to catch prey at depth.

Similarly, to the Mola mola , cartilaginous fish, including elasmobranchs like sharks and rays, also use skin with dermal denticles alongside their cartilaginous skeletons to regulate buoyancy. Unlike a bony skeleton, cartilage is less dense and therefore more buoyant.

Alongside these features, cartilaginous fish also have an oil-filled liver that helps these fish maintain position in the water column, similar to a bony fish’s swim bladder. Because oil is less dense than water, it is positively buoyant, helping the animal float.

Picture salad dressing with the oil floating on top. Like the swim bladder, cartilaginous fish's buoyancy is typically within a certain range of depth. Most sharks, such as the aquarium’s lemon sharks, inhabit shallow waters close to the surface. Others, especially older species like the six-gill shark, specialize in deep sea environments.

Whale of a Time

Cartilaginous fish are not the only organisms to use oils to counteract negative buoyancy. Spermaceti is a fatty compound found most abundantly in the deep-dive sperm whale. It is held in a caudal organ in the whale's head, allowing the animal to be positively buoyant at the surface. For the sperm whale, this oil solidifies due to extraordinarily cold temperatures as they descend to depths of around 600 meters to hunt for food.

Among other species of whale, buoyancy control differs and, in many cases, is still somewhat mysterious. The humpback whale, for instance, is positively buoyant at the surface and is hypothesized to counteract positive buoyancy by vigorously swimming downwards during dives until reaching a depth of neutral buoyancy. Another common hypothesis is that whales collapse their lungs while making dives, absorbing more oxygen and nitrogen in the bones and muscles.

This theory is supported by the recent findings of whale carcasses showing signs of decompression sickness (DCS), an illness previously thought to be confined to human divers.

What is Decompression Sickness?

Human SCUBA divers can get DCS by ascending too fast, allowing nitrogen bubbles in the body tissue to expand rapidly. However, this is because SCUBA divers breathe compressed air—free divers, who breathe air at the surface like whales, turtles, and other deep diving animals, don’t get DCS.

This means that when whales dive after a lung collapse, their bodies continue to absorb dissolved nitrogen in their connective tissues, putting them at risk of DCS if they make an uncontrolled ascent. Whales had no observed struggles with such ascents until recently, however it’s possible that due to interaction with human shipping vessels, which often startle and confuse the animals into quickly ascending, this may be changing.

Until recently, it was thought that whales were immune to DCS. As the life of these deep-dwelling and deep-dive animals poses many unknowns, it is still unclear how human activity at sea affects the unique environment. This is why it is important for us to stay up to date with our oceans, making sure we show respect to our animals, even those thousands of meters below us.

Ocean Adventures Await!

Ready to meet some beautifully buoyant fish? Visit Ripley’s Aquarium of Canada and see the amazing world of the deep blue up close!