How Sharks Float: The Science of Buoyancy
Introduction
Most fish rely on a swim bladder to float effortlessly in the water column. Sharks, however, lack this organ. So how do these ancient predators avoid sinking? How do sharks float? The answer lies in their unique anatomy and adaptations, especially their enormous, oil-rich livers.
In this installment of Shark Science Explained, we explore how sharks achieve buoyancy, why they’re different from bony fish, and what divers can observe in the water.
Sharks vs. Bony Fish: Different Approaches to Buoyancy
This difference shapes how sharks move and interact with their environment, and it explains why they must keep swimming to maintain position.
The Role of the Shark’s Liver
For large pelagic sharks, such as the tiger shark or whale shark, this adaptation is critical for conserving energy over long migrations.
Hydrodynamic Shape & Lift
In addition to their oily livers, sharks gain buoyancy from:
This combination allows sharks to “glide” effortlessly through the water column, conserving energy while staying mobile.
Why Sharks Sink Without Movement
Because they lack swim bladders, sharks cannot hover motionless in midwater like many fish. If they stop swimming, they gradually sink. Some species, such as nurse sharks, solve this by resting on the seafloor, actively pumping water over their gills while conserving energy.
For highly migratory species, like oceanic whitetips, continuous swimming is part of their survival strategy; it keeps them buoyant, oxygenated, and on the move to find prey.
What Divers Notice Underwater
Divers observing sharks can often see buoyancy adaptations in action:
Conclusion
Sharks float not with gas-filled bladders like most fish, but through a combination of massive oily livers, hydrodynamic design, and continuous swimming. These adaptations make them perfectly suited for life as oceanic wanderers.
For divers, understanding shark buoyancy adds another layer of appreciation. Every graceful glide and effortless turn reflects millions of years of evolution that keep these predators at the top of their game.
FAQs
No. Sharks lack swim bladders and instead rely on their large, oil-filled livers for buoyancy.
Their livers can make up 25–30% of body weight and are filled with oil (squalene) that helps reduce density and provide lift.
Many sharks will sink without movement, though some species, like nurse sharks, rest on the seafloor and actively pump water over their gills.
Pectoral fins generate lift like airplane wings, while their tail provides balance and forward thrust.
