Fish Buoyancy – How Our Finny Friends Stay Neutral (Unless They Don’t)

Swim bladder at work. A glasseye snapper changes depth on the reef at Bonaire.

ONE THING IS CLEAR – FISH HAVE BETTER BUOYANCY THAN YOU.  And, they don’t have to press any buttons.

Many bony fishes have built-in versions of the buoyancy-compensators that divers use to control their position in the water versus changing ambient pressures. In these fishes’ case, it’s an internal gas-filled sac called a swim bladder that automatically works to counteract the ambient pressures applied by the waters surrounding it and keep the fish at neutral buoyancy.

Otherwise, the fishes – heavier than the water they displace – would inevitably find themselves destined for a journey to the sea bottom.

As the pressures change with ascending or descending, the swim bladder absorbs or eliminates gas – oxygen drawn from the fish’s blood – to keep the swim bladder at an appropriate volume. And, to keep the fish at neutral buoyancy.

Red-lip blennies are among bottom-dwellers that lack swim bladders.


  • Sharks and rays don’t have swim bladders but compensate with fin anatomy and oils in their livers. By and large, they have to keep swimming to avoid sinking in the water column.
  • Tunas, those magnificent, planet-roaming bony fishes, have poorly developed swim bladders and also rely on oily livers, stiff fins and constant swimming to control their positions in the water column.
  • Many bottom-dwelling bony fish like red-lip blennies, gobies, scorpionfish and lizardfish have evolved to lose their swim bladders and compensate by…mostly dwelling on the seafloor.


It’s believed swim bladders are an adaptation handed down from early fish who needed lungs to breath air when their shallow habitats dried up. From this, some fish – including herrings, gars, trouts, carps, catfishes and lungfish – evolved “open” swim bladders that are connected through their throats to their mouths.

They obtain oxygen for their bodies by filtering water through their gills but they adjust their “open” swim bladders by rising to the surface and gulping in air – and burping it out when necessary during ascensions. It’s notable that most of the fish in this group are freshwater and shallow-water denizens.


 Internal organs of a closed-bladder fish, via Wikimedia Commons under an Attribution-Share Alike 3.0 Unported license.

Over time, many fish – like angels, grunts, groupers, drums and parrots – developed “closed” swim bladders that control the gasses within the bladder by diffusing oxygen in and out of the hemoglobin in their blood. Since they’re constantly absorbing oxygen into the blood through their gills, more oxygen is always available.

Anatomically, the bladder is located just above a fish’s stomach and intestines, ensuring that its center of gravity is below the gas-filled organ, letting them remain stable and right-side up.

The diffusion process is a chemical response to the ambient pressures they experience, not a conscious action on their part.

A pair of juvenile spotted drums are able to ceaselessly patrol their little part of the reef thanks to closed swim bladders.

Acquiring buoyancy capabilities meant that closed-bladder fishes were freed to give up tail power for more evolved fins, more deft control and immense maneuverability. “Ray-finned” fishes developed fins supported by light, bony spines, or rays, that are each fixed into individual sockets and managed by their own muscles to provide the flexibility to, say, raise, lower or tilt them.

Swim bladders let fish like this bluestriped grunt hover in safety in the cover of a rope sponge.

With these flexible fins they can hover in the current to capture plankton or hide within a sea plume, graze slowly on algae, back-peddle, and pursue lives in deeper and differing depths of the water column.


Sharks and rays lack any kind of swim bladder but do have large livers – up to 25 percent of their body weight, in some sharks’ cases – stocked with low-density oils that, lighter than the waters they swim through, provide them with a degree of buoyancy.  They’re helped by skeletons made of cartilage, a substance less heavy than bone.  Still, they’re negatively buoyant and naturally inclined to sink.

Mainly, what sharks have are stiff pectoral fins that slope down and back and provide them with lift as they swim, like airplane wings – hydrodynamic lift rather than aerodynamic. A shark’s asymmetrical tail fin also contributes to lift when it’s lower blade pushes against the water unevenly, placing more pressure on the shark’s underside.

But this means that sharks have to keep swimming to maintain that lift. They’re helped, mostly, by torpedo-shaped bodies that facilitate fast swimming.  They rely on side-to-side movement of their caudal, or tail, fins for locomotion. Dorsal and anal fins help stabilize the shark in the water and can be moved to help turn.

Sharks, like this reef shark photographed in Belize, need the lift provided by constant swimming and stiff pectoral fins to maintain position in the water column. Asymmetrical tails provide propulsion and also additional lift.


If this doesn’t sound positive for the shark, in reality they have great maneuverability in the water. Although, in fact, unlike many bony fishes of the reef, they can’t stop.  They can veer and circle round but they cannot just stop.

Rays have taken the pectoral fin/hydrodynamic lift concept so far as to develop their pectoral fins into broad wings.  Eagle and manta rays’ wings let them soar like…well, eagles…flapping and gliding. But eventually, unless they continue to flap to maintain lift, they are going to sink. Stingrays have the same lift factors but do spend much of their time close to or on the seafloor.


Many of the same sharkish issues – swim bladder deficiency, stiff pectorals, need to constantly swim for lift – appear to apply to tunas but nailing down the details is frustrating. With regard to both soaring tunas and the non-soaring bottom-dwellers discussed below, the sources are more anxious to talk about how swim bladders work than to do more than mention, in passing, the fishes’ swim bladders don’t work for at all, being underdeveloped or absent.


Lizardfish are among the many species without swim bladders that rely on ambush hunting from a still position on the reef or sea bottom, sometimes burying themselves in the sand. Relatively stiff fins enable them to prop themselves up while lying in wait.

Conceivably, always having neutral buoyancy might be a disadvantage to bottom-dwelling species of fish when it comes to hiding on the seafloor, lying in wait to ambush some tasty prey that might pass by. Whether they lost their bladders because they stayed on the bottom or they stayed on the bottom because they lost their bladders, or both together, is up for grabs.

In any event, a lot of bottom-dwellers, including many if not all gobies, blennies, scorpionfish and lizardfish seem to lack them. One suspects flounders and frogfish do as well (see, lack of source details, above).

Because they’re tied to the seafloor, or coralhead, or wherever, bottom-dwellers adopt either of several strategies to earn their livings. Scorpionfish, flounders and frogfish become masters of camouflage, lizardfish often bury themselves in the sand to bolt out after prey, gobies perhaps become cleaners and thus circulate with amnesty.


Generally, it’s bad idea to touch things that just lie stationary on the bottom.

PRINCIPAL SOURCES:  Watching Fishes, Roberta Wilson and James Q. Wilson; Marine Biology, Fourth Edition, Peter Castro, Michael Huber;  Encyclopedia of Fishes, John Paxton and William Eschmeyer;  “Swim bladders diffuse gas: ray-finned fish,” Encyclopedia of Life; “How Do Fish Rise and Sink in the Water?”; “Swim bladder helps maintain buoyancy,”; “Why Do Dead Fish Float?,” LiveScience; “Fish anatomy: The Swim Bladder,” EarthLife.Net; “Swim Bladder versus Oily Liver,” Catalina Island Marine Institute; “Shark Anatomy from the Outside In,”; “How Sharks Work,”; “Shark Anatomy,”  TheShark