How do animals dive?
BCD’s of the natural world
Written by Roy Kittrell 18 March 2024.
Our other resident diver and article writer, Julie Cole, with her husband Anthony, on our trip to Bali in Sept ’23
The Buoyancy Control Device, otherwise known as the BCD, is one of the most fundamental pieces of equipment for divers, allowing them to establish neutral buoyancy at different depths in the water and at the surface. As you will all know, by filling the BCD with air from our tank, we can adjust its volume to compensate for the increasing and decreasing outside pressure allowing us to remain neutrally buoyant.
However, animals (at least the pelagic, or free-swimming ones!) that live in the ocean also have to wrestle with this fundamental problem of maintaining buoyancy. But without a tank and a BCD, how exactly do they do this? Let’s look at some of the different strategies used in the underwater world!
Fish
Many bony fish (for example in the class Actinopterygii) move around up and down through the water column, and in doing so will experience differences to their buoyancy. They compensate for this by having an entire organ dedicated to maintaining this buoyancy, called a swim bladder (see diagram). 
This ancient adaptation first appeared in bony fish some 400 million years ago when they branched off from cartilaginous fish in the class Chondrichthyes. The organ is filled with gas, and expands or contracts to alter the volume of air inside to change the buoyancy of the fish. Many fish have swim bladders that are angled in a way that the center of mass always points down helping to keep it level in the water column, which is very handy!
Some fish have also found ways to use their swim bladder for other things too. In some freshwater species, the swim bladder is connected to the ear, and allows the fish to have greater hearing sensitivity. Piranhas have been found to use the swim bladder to amplify their clicking sounds for communication and hunting. In areas where the water is low in oxygen, some fish have even found ways to use their swim bladder to ‘breathe’ air, in fact one of my favorite fish, the gigantic (and delicious!) Arapaima gigas found in the Amazon, has this ability.
There is a downside to having an internal BCD in the form of a swim bladder though. Sometimes the organ can be infected or have some other medical problem, and the fish is no longer able to maintain its buoyancy, floating to the surface and floating around upside down unable to move… much like some of the more inexperienced divers have been known to do!
The Remora: A Fish with no natural BCD
So, if fish went to great trouble to evolve an entire gas filled organ to help them regulate their diving, it was fascinating to find out there was a kind of fish that decided to do without it entirely- the Remora!
You will see these fish in the ocean attaching themselves to much larger pelagic creatures, like this turtle in the picture, or on the underside of larger animals like the whale shark or a manta ray. This is because they have a modified dorsal fin which suctions onto the larger animal, and they let the larger animal decide where to go. Because they don’t have to deal with the actual effort of diving, they no longer have their own BCD and their buoyancy is negligible… amazing!
Also; don’t worry about the larger fish. The remoras help them out by eating parasites and loose bits of skin
Turtles and other Reptiles
Reptiles that inhabit the ocean, such as the beloved sea turtles, have the challenge of breathing air and then diving underwater, so all of the air they contain to maintain their buoyancy is contained within their lungs. Sea turtles such as the green turtle (Chelonia mydas) stay relatively shallow in the reef, and from what I have been able to gather online regulate their buoyancy when they take in air at the surface, taking in more or less as required, then diving down with their powerful fins. As the air compresses the deeper, they go, they can find a level where they are neutrally buoyant and use less energy to conserve their oxygen, or breathe out air as needed. This is something they are very good at- some sea turtles can stay underwater as long as 7hrs!
The current diving world champion turtles, the mighty Leatherback Turtle, are unusual in that they do not exhale before diving down to depths of as much as 1200m to reach their food sources. Once there, researchers found they were not swimming as hard and were able to ‘glide’ underwater, indicating they had reached neutral buoyancy.
Sea Snakes, another reptile found in the oceans who also have to come to the surface to breathe, have adopted much the same strategy as the turtles, taking in lungful of air and using their powerful tails to descend until they become neutrally buoyant, the deepest being around 30m. They have also adapted lungs and bodies that use pulmonary shunting which are thought to aid with both O2 consumption and buoyancy.
Cephalopods
While most cephalopods and other mollusks are soft-bodied and primarily consist of water, and therefore do not require an internal BCD like other animals, there is one interesting example of a cephalopod that does: the Paper Nautilus!
Paper nautiluses are fascinating to me for many reasons, and it is a long-held life goal for me to photograph one someday, but perhaps the most amazing thing about them is that the females create spiral shells which they use as egg cases when breeding. These shells look very similar to the now extinct Ammonite branch of the cephalopod family, and is a fascinating example of convergent evolution.
This shell is hollow and the female lives inside of it, and she comes to the surface of the ocean to let a small amount of air inside the shell, which she then compresses or decompresses with her body to control her buoyancy. She might even come up to the surface again on different nights to maintain this little air pocket… an amazing buoyancy control!
Whales and other Mammals
Whales, a type of mammal (one that I would dearly love to see in the wild someday!) that breathes at the surface, uses its lungs to compress air at various depths to control buoyancy. They are generally positively buoyant closer to the surface, and kick down powerfully to go to deeper depths for 60-90mins at a time. As most divers will know, you ascend quicker the closer to the surface you are because the air in your BCD expands, and this is also the case with whales. However, from what I have been able to read on the internet they actually use this to their advantage by ‘gliding’ back to the surface, gaining speed from the added ascent and using that momentum to go back down, so they move along the ocean in a sort of undulating rhythm. Maybe I should try this on my next diving trip with Flow!
Sources:
S. Fossette, A. C. Gleiss, A. E. Myers, S. Garner, N. Liebsch, N. M. Whitney, G. C. Hays, R. P. Wilson, M. E. Lutcavage. Behaviour and buoyancy regulation in the deepest-diving reptile: the leatherback turtle. Journal of Experimental Biology, 2010; 213 (23): 4074 DOI: 10.1242/jeb.048207
Subsurface Buoyancy Regulation by the Sea Snake Pelamis platurus
J. B. Graham, J. H. Gee, J. Motta and I. Rubinoff
Physiological Zoology
Vol. 60, No. 2 (Mar. – Apr., 1987), pp. 251-261
Buoyant Balaenids: The Ups and Downs of Buoyancy in Right Whales
Douglas P. Nowacek, Mark P. Johnson, Peter L. Tyack, Kenneth A. Shorter, William A. McLellan and D. Ann Pabst
Proceedings: Biological Sciences
Vol. 268, No. 1478 (Sep. 7, 2001), pp. 1811-1816 (6 pages)
Published By: Royal Society
-Roy Kittrell is an avid naturalist and underwater photographer, his work can be found on Instagram @roythedivebro