Swim Bladders

How to be in balance without yoga – use your liver or your bladder!

If you have ever tried to dive underwater, say while snorkeling or swimming, you may have noticed a peculiar thing: it’s hard to submerge after taking a deep breath. But the deeper you go, (or when you exhale), you start to sink and it becomes difficult to get back to the surface. Welcome to the wonderful world of buoyancy, in which the more gases you have inside you, the more you float. So do not plan a bean-heavy meal before your next diving attempt, or you might find yourself with an overly buoyant butt! However, as you go deeper and water pressure increases, the gases in your body are compressed until your bones and other tissues, which are denser than water, cause you to sink. The important message is: without your lungs being filled with air, you sink because your bones and tissues are heavier than water; and even with air in your lungs, you sink once this air has been compressed at greater depths. In short: for humans not to drop to the sea floor, lungs are important! Which sucks if you’re a fish because you probably don’t have lungs.

So really, why do fish not all sink to the bottom? Well, these clever creatures have come up with a few tricks. Some, including sharks and rays, have fatty and oily tissues (typically their liver). Because fat is less dense than water, these tissues counter-balance the weight of the rest of their body (which has a higher relative density than water) – I guess shark doctors are not too concerned about obese liver syndrome, huh?

Other fish, again mostly among the sharks and rays, have to constantly swim, creating uplift with specialized fins similar to how an airplane has to keep moving so as not to crash – I guess shark doctors are very concerned about any lack of exercise? Man, being a shark doctor must be hard, making sure your patients are both constantly on the move AND have a fatty liver. As you can see, both these solutions are feasible but cumbersome.

And indeed, one day in the ancient, ancient past a fish found a way to make all of this less trouble. And its idea is present in most fish today. Including us. Of sorts. Anyways, this solution works in any type of water at any depth and even while sleeping. This fish invented the swim bladder! Swim bladders are the fish equivalent of human lungs (biologists would say: lungs and swim bladders are homologous). This is true both in the descriptive sense – lungs and swim bladders are both gas-filled organs inside the body – and in the evolutionary sense: lungs and swim bladders both evolved from the same organ of the last common ancestor of Osteichthyes (the swim-bladder-possessing fish with true bones) and tetrapods (the lung-bearing, four-limbed organisms to which also humans belong). Aptly, this organ is often referred to as a proto-lung or a proto-swim-bladder. Basically, you are breathing with a bladder as you read these lines. Humbling, isn’t it.

Swim bladders come in two general specifications: either they open to the fish’s gut, or they don’t. Both have their pros and cons: fish whose swim bladder opens to the gut can adjust the amount of air in their swim bladders very quickly, but need to fill their swim bladders from time to time by gulping air from the water’s surface. Obviously, this is rather difficult for deep sea species – these benefit from a closed swim bladder which is filled and deflated via gas exchange with the blood. This system has proven very successful and is found in many species today, even those that do not live at incredible depth, because it makes all of them independent of surface air. However, such a closed bladder does not allow for rapid changes in its content. Here, the above-mentioned changes in water pressure with changing depth kick in: if a fish descents or ascents a certain distance, it is out of balance – its bladder is either too full or too empty. The fish is thus no longer balanced and will either quickly rise to the surface or drop to the floor. Under normal conditions, a fish will avoid this by taking sufficient time during vertical movements to allow enough gas exchange between swim bladder and blood. But when things get hasty, this system cannot keep up! This is also the reason why it is almost impossible to get deep sea species (like anglerfish!) to the surface alive: their bladders expand as you bring them up, making them rise ever faster and eventually killing them. For the same reason, SCUBA divers should never hold their breath while ascending! For rapid ascension the open swim bladders work wonders, because excess gas can just be burped out. So there you have it: to be ‘like a fish in water’ a fish needs its swim bladder.

We close with two little known fun facts about swim bladders and sound: first, swim bladders of many species are involved in acoustic communication, both as a resonating tissue to perceive sounds and as a sack that can be used to produce noises (think bag pipes with ears! What a way to experience the world). Second, because swim bladders are gas filled, they show up on sonar – they reflect sound very differently from water. This is how modern fishing vessels locate large swarms. And it is also why early users of sonar technology thought the ocean floor was moving: in some areas, fish densities are so high that the reflections of their swim bladders look like the bottom of the sea on a sonar scan, but these fish do vertical migrations throughout the day so at midday the ocean appeared ‘deeper’ than at midnight. I guess that was not exactly a selling point for early sonar technology; if it isn’t even capable of reliably detecting something as large and stationary as the sea floor, how can it help hunting down Nazi U-boats? Luckily, fish did not turn into proper axis-powers collaborators and sonar eventually did its job in that area – otherwise I would have to write this whole thing in German. Und das wäre ja doch eher umständlich.