Natural history articles about land animals always include their ranges – where the animal can be found. This information is described in two dimensions; think latitude/longitude. However when we consider the aquatic environment, the third dimension, depth, comes into play. Most animals are denser than water so without some adjustment to help buoy them up, they sink. The ability to regulate buoyancy is critical for every animal that submerges in water, and most adaptations involve air and/or fat, both less dense than water, in one way or another.
Fish are the masters at counteracting sinking. Most anglers are familiar with the swim bladder. This bladder, which originated as an offshoot of the esophagus in boney fishes, in some species remains connected to it while in more advanced fish the duct is sealed off. The addition of gas from the bloodstream into the bladder and release of gas from the bladder by reabsorption regulates the buoyancy. It is involuntary; in other words, they do not even have to think about it. But if they are unnaturally relocated from one depth to another, as in the case of deepwater fishing, the swim bladder does not have the time to reabsorb the gases and as it expands with the decreased pressure of shallow water it literally pushes organs out of its way – out of body cavities if necessary. Anglers use a technique to vent fish to release the gas in the bladder in order to safely return them to the water and allow them to sink to their natural habitat at their own pace. A side benefit of the swim bladder is that the flexibility of its tissues enables some fish to create sound by vibrating the walls and others to be able to detect and “hear” the vibrations created by other sources.
Fish that live on the bottom, like flounder and sculpins, have the least need to regulate buoyancy and they tend to have very small gas bladders or none at all. Other boney fish, like tuna, whose predation methods depend on speed and the ability to change depth rapidly, do not even have a gas bladder, swimming constantly to counterbalance their negative buoyancy – an immense energy drain which is no doubt responsible for their large appetites.
None of the elasmobranches, neither sharks nor rays, have swim bladders. Pelagic sharks use the technique of swimming constantly, like the tuna, to provide upward lift. Low-density fats in their greatly enlarged livers sometimes account for up to 25% of their mass and add buoyancy. Th e livers of more benthic species, for instance the nurse shark, make up only about 5% of their body mass adding less buoyancy and allowing for their bottom-dwelling habit.
The necessity for marine mammals, particularly the great whales, to precisely control buoyancy comes from not only their need to descend to the bottom or mid-depths to feed but to return regularly to the surface to breathe. They exploit both their ample lung capacity and fat and in some cases have decreased bone density to help stay afloat. However, in order to dive deep, fi rst they need to overcome their buoyancy. As they dive deeper the increased pressure physically compresses the lungs and the fat and shift s the balance to increase their density toward negative buoyancy. Strong propulsion with tremendous fl ukes, particularly among deep-diving whales, is the key.
Finally there are manatees. The Florida state marine mammal is unlike its very distant relatives, the great whales. They have very dense bones and surprisingly heavy skin. It is thought that rather than functioning to help them sink, the heavy bones act more like ship ballast, assisting in buoyancy and stability, important for this shallow water herbivore. They have the ability to compress the air in their lungs but also to shift its distribution to affect their pitch and roll. And their secret weapon? As it turns out, healthy manatees can also use their muscular diaphragm to control their flatulence to their advantage.
Be careful out there. November is Manatee Awareness Month, so remember to scan the water ahead of your boat and obey the posted manatee speed limits, many of which go into affect this month. If you see harassment or an injured animal, call the FWC Wildlife Alert Hotline at 888-404-FWCC (3922).