Warm blood for cold water – evolution of warm-blooded fishes – 1993 – The Year in Science
Clarence V. Reynolds
STAYING WARM CAN BE HARD WORK FOR US LANDLUBBERS. BUT IT CAN BE EVEN HARDER FOR SEA DWELLERS: BESIDES PUTTING UP WITH ALL THAT COLD WATER, FISH ALSO LOSE LARGE AMOUNTS OF BODY HEAT FROM THEIR GILLS WHILE EXTRACTING OXYGEN FROM WATER. THAT’S WHY MOST OF THE WORLD’S 30,000 SPECIES OF FISH EVOLVED TO BE COLD-BLOODED; IT’S SIMPLY LESS WORK. SO BIOLOGISTS HAVE LONG puzzled over the reason a few dozen fish stubbornly evolved to be warm-blooded despite the high metabolic cost. Last April, University of Chicago animal physiologist Barbara Block announced that she may have figured it out.
For years, most researchers have accepted one of two explanations for why some fish evolved to be warm-blooded. One theory was the concept of “niche expansion,” the idea that warm-blooded fish could swim in colder waters and thus expand their hunting territory.
The competing “aerobic capacity” theory argued that water temperature wasn’t the point: warm-bloodedness simply improved muscle power, allowing the fish to chase prey farther and faster. Block’s work now lends support to the niche expansion theory.
Block and graduate student John Finnerty used genetic analysis to construct a family tree of 29 species of scombroid–a suborder of large, bony fishes that includes both warm-blooded and cold-blooded members–and three other distantly related fish. The two researchers studied the same gene in all the fish (the gene for a protein called cytochrome b) and looked for small differences in the gene’s DNA sequence among the various species. From those differences they could infer how closely related the species were. They assumed that the warm-blooded minority of scombroids–the bluefin tuna, the butterfly mackerel, and the swordfish–would be quite closely related: that they would share a recent common ancestor that was also warm-blooded. To the researchers’ surprise, it turned out that the three species are not closely related at all. They evolved their warm-bloodedness independently.
If warm-bloodedness keeps popping up in fish evolution, it must be a big advantage. But what exactly is the advantage? Although the bluefin tuna, the butterfly mackerel, and the swordfish don’t share a recent ancestor, they do have something in common–they can all tolerate cold water.
Given that they evolved warm-bloodedness independently, it seems unlikely they would share both warm-bloodedness and cold tolerance by coincidence. Block and Finnerty conclude that the fish became warm-blooded as a way of expanding their niche into colder water and not just as a way of swimming faster.
The particular style of warm-bloodedness in the swordfish and the butterfly mackerel supports this conclusion. Neither species warms its entire body; both warm only their brain and eyes. They do this, say Block, with a muscle that contains specialized “heater cells” and that sits just below the brain “like a 200-watt bulb.” Warming just the eyes and brain, the muscle does nothing to allow the fish to swin faster. What it does do is protect the nerve functions that are most vulnerable to temperature changes and most crucial to finding prey in dim, chilly waters. The “space heater” muscle allows swordfish, for instance, to chase squid to depths of 1,600 feet, where the temperature drops to near freezing.
On the other hand, bluefin tuna warm their entire bodies, as mammals do. Unlike other fish, the bluefin’s tiny blood vessels are intermingled–the cool, arterial blood that’s carrying oxygen from the gill is heated by passing close to veins carrying warm, oxygen-depleted blood. The blood is warmed not by a specialized heater muscle but by the red muscle that powers sustained swimming in tuna as in most fish. In the bluefin this muscle runs down the center of the body. That well-insulated location, along with the tuna’s heat-exchanging blood vessels, helps the fish conserve metabolic heat before it is lost through the gills. A bluefin tuna is able to maintain temperatures that are up to 25 degrees higher than the water surrounding it.
Like whales, says Block, the tuna can spawn in the warm waters of the tropics, then swim north to the Arctic, where the water is cold, rich in oxygen and nutrients, and abounding with prey.
In all three species, clearly, the metabolic gamble has paid off: gains from cold-water forays offset the cost of being warm-blooded and give the fish an advantage over their cold-blooded competitors. For a fish in search of a meal, says Block, “it’s a useful thing to be able to go where you want to go.”
COPYRIGHT 1994 Discover
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