Fast Fish... It's in Their Blood

It has long been known by scientists that some species of fish possess a unique physiological characteristic - a web of arteries and veins lying very close together - that enables them to raise their internal temperatures higher than that of the water surrounding them. Now, new research shows that warm-blooded fishes swim faster, and farther, than their cold-blooded counterparts.

This includes some sharks and tunas, which are able to to warm their core amidst cooler waters - a process called endothermy. And thanks to their warm-blooded ways, it turns out these species can swim 2.5 times faster than those whose body temperature doesn't change. In addition, these species can also swim twice as far - ranges comparable to those of warm-blooded animals such as penguins and other marine mammals.

"The cost of moving faster and farther is high so there has to be an ecological reason that outweighs the physiological expenditure," research biologist Jenn Caselle, from the University of California, Santa Barbara, said in a statement. "These endothermic fishes are putting a lot more energy into each unit of movement than their cold-blooded counterparts are.

"In fact, the estimated cost of transport is twice as high, but in return they're getting benefits from that increased swimming speed and wider range of migration," she added. "We hypothesize these gains allow these endotherms to be more efficient hunters and to span larger areas in their migration, which probably provides feeding and reproduction benefits."

To find out more, the research team used existing data, as well as attached sensors to several sharks in different locations around the world. According to their analysis, warmer "red" muscle endothermy allows certain fish to swim faster and gives them greater endurance. This, in turn, lets these fishes swim long distances relatively quickly.

The researchers speculate that, as a result, the fish can take advantage of seasonally variable food sources. (Scroll to read on...)

The not so cold-blooded creatures discovered during this study include four shark species - salmon, porbeagle, white and shortfin mako - and five species of tuna - yellow fin, southern bluefin, Atlantic bluefin, Pacific bluefin, and albacore.

Interestingly, one endothermic species in particular, the white shark, has a migration range greater than that of the humpback whale.

So if being endothermic is so beneficial to these species, allowing them to swim faster and farther in search of food, why exactly then didn't all fish evolve this ability? The research team hopes that this study may help scientists better answer that question.

Endothermy evolved independently in these distinctly different groups of fishes. The two taxonomic groups diverged more than 450 million years ago, and their common ancestor was most likely cold-blooded.

"The mechanisms of convergent evolution aren't always the same, although in this case they pretty much are," Caselle said. "There are only a limited number of ways a fish can rewire.

"This research begins to shed light on possible reasons why these endothermic fish evolved in this way," Caselle concluded.

The researchers' findings appear in the journal Proceedings of the National Academy of Sciences.

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