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Newborn Fish Are Lousy SwimmersAnd now scientists have found out why |
By Vlad Tarko, Senior Editor, Sci-Tech News
5th of April 2006, 14:20 GMT
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The last animal you would expect to be afraid of water is, of course, the fish. But as a matter of fact, fishes are not the natural born swimmers you would expect them to be. The newborn fishes are such lousy swimmers that 99 percent of all fish mortality occurs in their larval stage, when they're just out of the spawn. Although they manage to move around a little, most of them (although not all species) totally lack the swimming skills of adult fish.
The technique fishes use to gain speed, called burst-and-coast, has two phases: In the burst phase the fish is actively swimming, moving its tail horizontally, in an undulatory fashion. Then, in the coast phase, it keeps its body straight and rigid. This way, the water wave produced by the tail in the burst phase is not allowed to advance toward the head and thus it only goes backwards pushing the entire fish forward. There is "a very small or even zero amplitude of undulation in the head region and a large undulation in the immediate neighborhood of the tail," write Mark Grosenbaugh, Scott Gallager, and Houshuo Jiang from Wood Hole Oceanographic Institute, who studied the motion of fish in detail.
One could imagine various reasons for why the larvae are not very skilled in this swimming technique. For example, you could imagine they cannot hold their body straight and rigid during the coast phase and thus they simply engulf themselves in water waves without advancing too much. Or you could imagine they are too small, so perhaps water is like a sort of dense syrup to them preventing them to get enough momentum during the burst phase.
However, scientists from the Wageningen University in The Netherlands have now looked closer to zebrafish larvae and concluded that the larvae have poorly developed side fins (used for equilibrium) and an ineffective swim bladder (the organ that helps fish control their buoyancy). Apparently, they simply can't keep their bodies horizontal. This is why they are such terrible swimmers.
Ulrike Muller explained why the other explanations don't hold: "Momentum can explain some of the poor swimming in larvae, but not all, and the difference in coasting ability cannot be explained by differences in body length either." In other words, there is a fairly large diversity of larvae, and based on these explanations you are lead to expect that some of them would be better swimmers than they actually are.
However, according to researchers, their finding probably also applies to other species as well: "Many fish larvae hatch without fully formed pectoral fins and all hatch without a swim bladder, so similar problems could occur for them."
Image credit: Woods Hole Oceanographic Institution
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