One of the world's rarest fish sometimes goes without oxygen for surprisingly long periods of time, even when plenty is available. The oxygen-fast is triggered by ethanol, whether produced by the fish, or added by researchers. Such odd behavior might be one reason why the Devils Hole pupfish is so rare, and explaining its origins could tell us something very important about metabolism in animals generally.
Cyprinodon diabolis originates in a single limestone cavern in the Mojave Desert. Its only other habitats are artificial tanks where insurance populations are kept lest something go wrong at Devils Hole itself. "One possible cause for the small population size of C. diabolis is limited food availability," said Dr. Frank van Breukelen from the University of Nevada, Las Vegas, in a statement. Consequently, van Breukelen set out to investigate how many fish the cavern's food supply could support, but found something far more interesting.
Food requirements depend on metabolism, so van Breukelen set out to measure how much oxygen fish in the insurance populations consume at temperatures ranging from 25 to 38°C (77 to 100°F). Fish raised at 28°C (82°F) behave normally, consuming 300 microlitres of oxygen an hour. However, the Devils Hole spring is a stable 33°C (91°F), and when van Breukelen and his team tested fish raised at this temperature they found something totally unexpected.
Many of these warm water fish didn't consume oxygen at all for periods as long as 149 minutes. "My initial reaction was that there was a malfunction of the electrode," van Breukelen said. However, testing conducted on 295 animals from multiple insurance populations (more than twice the surviving wild population) led to publication in the Journal of Experimental Biology. Some warm water Devils Hole pupfish reject oxygen, even when their water contains plenty. Only one fish raised at lower temperatures behaved the same way.
Instead, these small fish rely on anaerobic processes. These produce only one-fifteenth as much of the energy molecule Adenosine triphosphate (ATP) per glucose molecule of aerobic metabolism, an apparently huge sacrifice. Having established the fish don't switch to save energy through lower metabolism, van Breukelen noticed the warm water fish produce more than seven times the ethanol of their cooler cousins. He wondered if this could be connected to what he termed "paradoxical anaerobic metabolism."
After what must have been an interesting ethics committee application, van Breukelen put ethanol in the water, and found the drunken fish were more likely to switch off their oxygen consumption at either temperature, even at alcohol concentrations of 0.01 percent.
So why do the fish think alcohol and air don't go? The authors are unsure, but suspect the answer lies in avoiding the build up of the waste products of oxygen consumption.
While van Breukelen has found a related species of pupfish that also doesn't consume oxygen when raised at high temperatures, the findings will give a boost to arguments that C. diabolis is too important for its native habitat to be threatened by competing demands on local groundwater.
Main image credit: Olin Feuerbacher/Flickr CC 2.0