Reef fish cope with low oxygen.

Despite their seemingly idyllic surroundings, coral reef fish show unexpected toughness, according to a new analysis offish respiration.

In the first tests of low-oxygen tolerance among reef fish, all 31 species captured off the coast of Australia did surprisingly well, says G6ran E. Nilsson of the University of Oslo. The reef fish matched the performance of the epaulette shark, famed for surviving oxygen drops in tide pools cut off from the surf, Nilsson and Sara Ostlund-Nilsson report in an upcoming Proceedings of the Royal Society of London B.

When respiration physiologists look for adaptation to low oxygen, they poke around in still waters, such as swamps and tide pools, says Nilsson. But coral reefs? "It's not something that most people have bothered about," he says. Plenty of churning there should mix the seawater well enough to avoid oxygen drops.

Nilsson didn't think to look for low-oxygen tolerance at reefs until he began to observe mouth brooding in cardinalfish around Lizard Island on Australia's Great Barrier Reef. A male of these small reef fish takes a female's egg mass, sometimes a quarter of his body weight, into his mouth for a week or two, until the eggs hatch. Since the egg mass might inhibit water flow through the mouth to the gills, Nilsson wondered whether these fish have a special capacity to tolerate low oxygen.

To test the cardinalfish, the researchers sealed them temporarily in containers with seawater and a sensor for oxygen concentration. Many organisms keep consuming the available oxygen at a steady rate until there's so little left that they no longer metabolize normally. The cardinalfish surprised Nilsson by breathing steadily until the oxygen concentration had dropped to 20 percent of its normal concentration.

At first, Nilsson says, he thought he'd found especially low oxygen tolerance in cardinalfish. For comparison, he decided to test reef fish that don't carry broods in their mouths. Surprisingly, he says, all the species he caught showed low-oxygen tolerance.

Data by other researchers show that some freshwater fish such as trout proved only half as tolerant of low oxygen as the reef fish did. The most tolerant of swamp fish require only a little less oxygen than the most tolerant of the reef fish do. Nilsson notes that no one has made comparable measurements on ocean fish away from reefs.

Perhaps reef fish hide in low-oxygen culde-sacs of coral, Nilsson speculates. In lab measurements, he found that those pockets can have oxygen concentrations below 20 percent of saturation.

Physiologist Jeff Graham of the Scripps Institution of Oceanography in La Jolla, Calif., says that plenty of questions remain. He'd like to see comparisons between fish that hide in reef nooks and those that don't.