Cold-Blooded Critters Reveal Their Secret

Scientists think they may have solved a mystery as old as the first polar sea expeditions: Why do cold-water animals grow so much larger than their warm-water cousins? The answer may have little to do with the temperature of the water, researchers report in tomorrow's issue of Nature, but everything to do with the amount of oxygen it contains.

Nature's giants are often found near the poles. For warm-blooded animals this makes sense: The bigger you are, the better you retain body heat--that's why polar bears and walruses are big. But scientists have long been scratching their heads over the question of why cold-blooded sea creatures, whose body temperature equals that of their surroundings, tend to be bigger in colder areas, too. To address the problem, ecologist Gauthier Chapelle of the Institut Royal des Sciences Naturelles de Bruxelless and physiologist Lloyd Peck of the British Antarctic Survey compiled statistics--such as size, water temperature, and oxygen concentration--for a single evolutionary group of animals that dwells in habitats from the tropics to the Antarctic: the shrimplike crustaceans known as amphipods.

Their survey turned up data on 1853 species of bottom-dwelling amphipods at 12 scattered sites, both in oceans and fresh water. They found species to be larger at colder sites, but there was a hitch: The freshwater amphipods tended to be larger than the ones exposed to similar temperatures in the ocean. Thus temperature alone did not determine size. But oxygen did. Both heat and salinity diminish oxygen's solubility, so colder waters have higher oxygen concentrations. When the researchers graphed species size against water oxygen content, amphipods from all sites fell on the same line. Oxygen-rich blood can travel farther from the gills before all the oxygen is depleted, and so it can support a bigger body, the researchers say.

But there's a flaw in this argument, says physiologist James Childress of the University of California, Santa Barbara: Oxygen is not particularly abundant in the deep sea, yet some of the amphipods known to be living there dwarf the polar species. Oceanographer Les Watling of the University of Maine, however, says something special in the blood circulation of those deep-sea giants could differ from other amphipods, and that's what scientists ought to study next. Meanwhile, he says Chapelle and Peck's graphs are compelling: "They seem to have explained the size conundrum."