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The Swim Bladder

By Mark E. Evans

Hypothetically speaking, what exclamation would you use, if you inadvertently dropped a netted fish onto the floor? (An occurrence I’ve witnessed a few times at one of my local fish stores). Would you perhaps be classical and swear to King Neptune for his gosh darn mischievous ways, or would you be more reasonable and curse Sir Isaac Newton for his fancy laws of gravity? Or would you just be modern, and use any number of profane words or expressions that you might have picked up at work or at home? Or would you simply realize that mistakes happen and that not all lessons are painfully obvious? Because if you dig a little deeper into any event that occurs in the aquarium hobby, you are bound to be led in another direction, toward an equally intriguing destination. After all, every misstep is a learning opportunity when you’re a fishkeeper. So, a fish falling onto the floor is only the beginning of this story. It is gravity that is the causal factor here, the common thread that runs throughout this article.

Generalized position of swim bladder and intestine in perch-like fish. Swim Bladder = Blue; Intestine = Gray

If you search any of the more popular aquarium forums on the internet, you are sure to notice that any symptoms that deal with abnormal swimming in aquarium fish are usually attributed to a malady of the swim bladder. There are so many examples of the swim bladder being discussed on web bulletin boards that one is forced ask if one organ can indeed cause so much suffering for fish and aquarists alike. Anything from a goldfish swimming at a weird angle, tumbling around the tank like an acrobat, to a betta sitting listlessly on the bottom of a bowl like a contemplative Buddhist is blamed on a bad swim bladder. Even a Tetra found to be swimming in circles (chasing its own tail so to speak) was said to be suffering from swim bladder troubles. The multitude of Q&A on this one subject is large to say the least. But all the information out there begs for further investigation. And as responsible fish keepers we should be open to new information. Especially, since few things in life or the aquarium hobby are as simple as we might hope them to be. This point was forcefully brought to my attention when I once witnessed a relative of mine suffering from a nasty case of the flu. She was unable to stand, and when she tried to get up from her sick bed she was afflicted with a faulty equilibrium. Her sense of balance was off. The world was all topsy turvy to her because the influenza virus had infected her inner ear, the region of the our brains which helps us to keep our balance. Not coincidently, fish also have a sensory system in their brains that provides the same function for them. The ear ossicles, as they are called, consist of fluid filled canals that mimic the various axis that exist in our three dimensional world. Not unlike the principal of a carpenter’s level. And if you’re a human being or fish, and you suddenly find yourself upside down, then you feel that you are in fact upside down. It’s all about a sense of equilibrium, and it’s all driven by the force of gravity. So, given these simple facts, it might be that a fish supposedly suffering from a swim bladder infection may instead be afflicted with a pathogen like a virus, a bacterium or a parasite, and as a result the fish may have a faulty equilibrium.

Figure 2 - Open swim bladder configuration connected to intestine for gas exchange thru esophagus. Swim Bladder = Blue; Intestine = Gray

But then again, if it swims like a guppy and looks like a guppy, it just might be a guppy. Sometimes the most obvious conclusion is correct. After all the swim bladder is affected by gravity too (it has to be able to counteract its effects), and it can become diseased, and aquarists do experience fish swimming at odd angles. But sadly here there is one problem with our investigation into the swim bladder. Not all fish have the exact same anatomy. The term fish isn’t even a scientific one. Not all the internal organs are present in all species. Not all swim bladders are identical, and some fish species don’t even have a swim bladder. But generally, there are two types to be found in fish that do posses this fascinating organ, open and closed. The open swim bladder is connected to the esophagus (the canal which connects the throat to the stomach) and this gives fish the ability to gulp air and so adjust their buoyancy. The air is stored in the swim bladder in order to counteract the force of gravity and to give the fish neutral buoyancy, or something to close to neutral buoyancy. Many fish will sink if they stop swimming because they have less than neutral buoyancy. It is the movement and shape of their fins that keeps them from stalling in the water and slowly sinking to the bottom. Many sharks have this problem. They have to keep moving or they stall and sink. The natural world is full of similar imperfections.

Fish with the second type of swim bladder, the closed type, do not require gulps of air from the atmosphere to adjust their buoyancy. Their anatomy is slightly different. In the closed type the swim bladder is not linked to the esophagus. Instead, it is connected to the bloodstream by a a gas gland which allows oxygen, carbon dioxide, and nitrogen to enter or exit the swim bladder. As a result fish that posses this type of swim bladder don’t require access to the surface and therefore can enjoy a wider range of habitat such as the deep sea. Although, there is a downside to this arrangement; adjustments to the gas level in the swim bladder takes longer than in the previously mentioned open type. This is illustrated by the fact that when some deep water fish are brought to the surface too quickly they expand and burst forth their internal organs because they cannot compensate for the sudden change in pressure. Certainly, in aquarium fish it is not unreasonable to consider the possibility that there are diseases that might affect the gas exchange between the blood and the swim bladder.

Closed swim bladder configuration connected to circulatory system for gas exchange thru blood. Swim Bladder = Blue; Intestine = Gray; Circulatory System = Red

The location of the swim bladder in most fish may also present a clue as to another possible cause of buoyancy problems. The swim bladder organ is not unlike the main ballast tank of a submarine, which is situated near and above the center of gravity of the vessel for obvious reasons. (Otherwise a submarine or a fish would find it hard achieve a level position in the water). In fish the gas filled swim bladder is located above the liver and intestine which in most fish is near the center of the body. Now, if one considers the numerous internal ailments that occur in aquarium fish, such as parasites, bacterial or viral infections, or just overfeeding, then one can see the possible difficulties. An enlarged intestine or liver pressing against the gas filled swim bladder might disable its ability to function properly. And a fish with negative buoyancy might lay listlessly on the bottom. 

At this point you may be thinking that this discussion of fish physiology and anatomy is interesting, but are there any recommended treatments for fish with swimming or swim bladder disorders. Unfortunately, fish disease books are vague on swim bladder disorders. Even the more scientific ones provide scant information on practical treatments for the home aquarium. But I think the behavior of the ubiquitous goldfish gives us some clue as to what we should do as aquarists in order to prevent swim bladder disorders in our own fish. Goldfish are known for sifting through gravel in search of food. They suck it into their mouths and then spit it out. It is an entertaining behavior to watch. But it is not inconceivable that if gravel in an aquarium is dirty with detritus and goldfish sift through such gravel, the goldfish may be exposed to pathogens that could infect their swim bladder. Remember, the esophagus is connected to the swim bladder in some fish. And how common is it to see an aquarium overcrowded with goldfish? Such an environment is often home to poor water quality and an unhealthy level of detritus, the exact ingredients needed to give a poor goldfish an internal infection. But, of course, there are other hypotheses as to the cause. One implies a connection between the shape of the various goldfish varieties and abnormal swim bladders. And there is another very popular one that blames swim bladder disease on fish eating too much dry flake food. But there are many unknowns in all this fishy speculation. And not surprisingly, most of the veterinary work done on fish is aimed at fish farming not the aquarium hobby. No, it is up to us as responsible aquarists to keep our own fish healthy, and not to expect a quick remedy from an off the shelf remedy. The best medicine is preventative. And keeping a clean aquarium is paramount. Not allowing detritus to build up to unmanageable levels in the gravel or on the bottom of the tank is particularly important, especially if you keep fish that inhabit the bottom or fish that sift through the gravel for food. Detritus is home to all kinds of bacteria, beneficial and potentially harmful. And a bacterial bloom is a sign that either an aquarium filter is overwhelmed by fish waste or perhaps a tank is overstocked. Keeping an aquarium free of metabolic waste is just a good idea in preventing diseases that affect the swim bladder or swimming behavior. Another is not allowing drastic changes in pH or other water chemistry parameters such the level of nitrogen waste or conductivity. Water pH, nitrogenous waste, and dissolved solids (salts) affect the blood directly. (There is only a thin layer of cells in a fish’s gills that separate the blood from the external environment). And if you remember, the closed type of swim bladder is controlled through a gas gland; the gases dissolve through the gland and to and from the blood. If blood chemistry is abnormal because of poor water quality, the swim bladder may be affected.

From the complexity of fish anatomy and physiology, we can conclude that not all the questions asked in the aquarium hobby have simple easy-to-digest answers. A particular disorder may have many different causes. And similar symptoms may be caused by different diseases. And to add to the complexity, not all fish have the same anatomy or physiology. They are quite a diverse group of creatures. But we can take great pleasure in getting to know some of the species by keeping them in our homes and by providing them with a clean microcosm in which to live. However, we should not forget that we can learn a few lessons from them. Such as a few facts about fish biology. And if you think about it, all we have to do is watch our fish swim around a beautiful aquarium from the comfort of our living room. It’s the beauty of the natural world from an easy chair. And it’s better than TV!


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