Beat Goes On: Carp heart keeps pace when fish lacks oxygen

Without oxygen, a mere human dies in minutes, but a Scandinavian fish not only can survive but also maintains a normal heartbeat for days, say researchers.

LOOK MA, NO AIR. A crucian carp can maintain normal heart function and stay active in water with no oxygen, a feat not seen before in a vertebrate. A. Farrell

The crucian carp (Carassius carassius) has long been recognized as a champion survivor, thriving even in shallow ponds that freeze over during long Northern winters. These waters can turn into dead zones as creatures exhaust the oxygen in the water.

What researchers haven’t known, though, is what strategy the carp follows for such a feat, says Jonathan A. W. Stecyk of Simon Fraser University in Burnaby, British Columbia. Turtles, the other vertebrates famed for toughing out times with no oxygen, reduce their heart functions to only about 10 percent of normal.

The new study of the crucian carp, however, shows that its heart rate dips when oxygen drops but rises again to essentially normal rates, Stecyk and his colleagues report in the Oct. 1 Science. “This is the first time we’ve seen in vertebrates that the heart will perform like this,” he says. “I hope it will spur medical research” into protecting human hearts during transplant or malfunction.

Lakes that freeze over in the winter and pile up with snow pose dangers beyond the chill, according to physiological ecologist Gordon Ultsch of the University of Alabama in Tuscaloosa. The snow keeps light from reaching photosynthetic inhabitants, so even they can’t provide any oxygen.

Without oxygen, animals’ metabolic pathways end up with excessive lactic acid. “Turtles can accumulate huge amounts of lactic acid that would stone kill you or me,” says Ultsch. Calcium carbonate from a turtle’s shell neutralizes the excess lactic acid.

To see how crucian carp survive the no-oxygen challenge, Stecyk went to Norway and tested carp in laboratory setups with water at 8°C that contained virtually no oxygen. The fish can derive energy from sugars via a biochemical pathway that doesn’t require oxygen but is far less efficient than the oxygen route.

By monitoring blood flow in 16 carp, Stecyk and his colleagues found that the normal heart rate of about 17 beats per minute dropped for the first 5 hours in the oxygenfree water but then gradually rose to 15. The fish maintained that rate for the rest of the 5-day experiment.

Stecyk and his colleagues also tested the brain’s control of the heart. Injections of a variety of substances that typically disrupt nervous system regulation of the heart had the same effect on fish whether or not they were oxygen deprived. “The heart is still working, and so is the brain,” Stecyk says.

In similar tests of turtles, the brain seems to lose its control of the heart during periods of oxygen deprivation.

Physiologist Don Jackson of Brown University in Providence, R.I., calls the carp findings “significant.” He notes that only certain carp, including goldfish, convert lactic acid to ethanol, which can be released from their gills. He considers that chemistry the “main adaptation” of the fish that makes no-oxygen survival possible.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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