A Little Bit More
The density of water doesn’t change much at all as one descends, but water pressure does (about 1 atmosphere for every 9 meters [30ft] of depth). If a fish were unable to change the volume of gas in its gasbladder, it would be neutrally buoyant at only one depth. As a fish descends, however, its relative density increases as the water pressure tends to compress its gasbladder. To maintain neutral buoyancy, its gasbladder must counteract this compression and expand with gas. Conversely, as a fish ascends, its relative density decreases requiring that its gasbladder deflate to maintain neutral buoyancy. So the gasbladder is able to keep a fish at neutral buoyancy regardless of whether it is on the surface or deep in the ocean depths by increasing or decreasing its gas volume. Most fish accomplish this by exchanging gases within their gas bladder with their bloodstream but a few, like the Carp and goldfish, are able to gulp air and pass it directly into their gasbladders.
Neutral buoyancy is an important adaptation for fishes. Energy conservation is always an important survival goal and those fish that can remain at a given depth without any energy requirements will be able to conserve their energy much better than those that can not. SCUBA divers regularly wear weight belts to neutralize their own buoyancy and thus reduce the “work” needed to navigate underwater. Not all fishes possess a gas bladder but most of the ray-finned fishes do. The cartilaginous fishes, like sharks and rays do not have a gas bladder and either have anatomical adaptations to help them maintain level swimming or an oily liver that contributes to neutral buoyancy.
Activities
Science – Stay Afloat
Objectives: A clearer understanding of neutral buoyancy
Materials: none
Probably, most of us have tried the “dead-man’s float” a time or two and experienced buoyancy in one way or another. For some, floating effortlessly at the surface is easy, while for others, it isn’t. Since muscle is denser than fat, individuals who are more muscular will be less buoyant than those who are not. Our own buoyancy can be easily demonstrated simply by floating in the water with full lungs after which the air can be released. When you do, there should be an immediate change in your buoyancy. Not everyone will sink completely but everyone will certainly float more deeply in the water with most of their lungs emptied.
Here are a few questions you might want to discuss with your students. I’ve answered them for you but you may have better answers than I’ve provided.
1. If humans can float, why can’t fish float without gasbladders? Humans have lungs that are filled with lighter-than-water air, fish do not. Without a gasbladder to maintain neutral buoyancy, fish are really very dense. A fish’s muscle mass and bones are the main factors working against buoyancy. If you were able to remove ALL of the air from your lungs (with normal exhalation we retain about 20% of our lung’s volume even when we exhale everything possible), you’d sink right to the bottom too.
2. Fish are often seen floating dead at the surface. Does this mean something has happened to their gasbladder? Dead fish are being decomposed by bacteria. As the bacteria feed upon the dead flesh, they produce waste products in the form of a gas that is trapped within the body and gradually, as this builds, it decreases the density of the fish causing it to float to the surface. Once enough of the fish’s organic material has been decomposed, these gases can escape and the fish once again sinks to the bottom.
3. Why do some people float more easily than others? Muscle is denser than fat so an individual who is more muscled, will tend to float deeper in the water than a person who is not so muscular. Since women tend to have less muscle and more fat in their bodies, women “tend” to float more readily than men.
Key Concepts
Structures and Functions, Adaptations and Diversity
Questions
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