But even in fish it has its price
Have you ever had this feeling that you were really hungry or thirsty and needed to eat or drink something, but you were too exhausted (or let’s put it bluntly: too lazy) to get up and go to the shop? Getting up, putting on your coat, walking to the shop, etc., all that takes time and would, of course, also have required extra energy. The decision as to whether to stay at home or to go out procuring food is one that animals have to make all the time .
Imagine the predatory pike or some other fish in a lake. It is cooler closer to the bottom of the lake and consequently, digestion is slower; ingested food will, therefore, last longer and the fish needs to search for food items less frequently. However, there is much more food available near the surface rather than at the bottom and therefore it could be a better place to be near the surface. But then the fish would have to eat more, swim around more, expend more energy, because ingested food won’t last so long. So, what to do?
Bony fishes, generally, have a gas-filled swimbladder in their bodies. This organ enables a fish to remain motionlessly, neutrally buoyant, in the water column, thus allowing it to save energy. That sounds great, but since gases are more compressible than water, a fish with a swim or gas bladder is somewhat limited when it comes to exploring and exploiting its environment through vertical migrations. If the fish were to swim up into shallow water, the gas in the swimbladder would expand (species of the carp fish family under these circumstances “burp air” that can be seen as little bubbles coming from their mouths, but fishes of other families are unable to do that). If the fish swims down into greater depths, the volume of the gas in the swimbladder will shrink: it becomes compressed and more gas, usually via the blood will have to be liberated into the buoyancy organ. And that takes time.
As an adaptation to more efficiently (and faster) chase prey throughout the vast vertical expanse of ocean water, tuna and mackerel species have forsaken their buoyancy aid and through evolutionary pressure lost their swim bladders. You could say, they have become “liberated” from the constraints a swimbladder with its gas inside represents. The downside is that they are now in this predicament that without a swimbladder allowing them to rest neutrally buoyant at a given depth of water, they will henceforth always sink unless they keep swimming. Sharks are in the same position, but their ancestors never possessed a swimbladder and so there was no question for them to “give up” something. To avoid sinking into the dark and cold abyss fish without a swimbladder simply have to keep swimming.
Swimming requires muscles to work and for that energy is required, which needs to be obtained in the form of food, which these liberated fish can now hunt, unencumbered by swimbladder constraints throughout their three-dimensional world. It seems a vicious cycle though: the lack of the swimbladder gives these fish the freedom to move up and down the water column with ease, but at the same time forces them to swim forever and to seek prey non-stop to support their continual swimming activity. We therefore conclude that even in the world of fish, there’s always a downside and every bit of progress has its price.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2018.
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