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Sleep similarity. The behavior and brain activity of sleeping crayfish is surprisingly similar to that of sleeping mammals.

Crayfish Catch a Nap

Crayfish catch z's much like mammals do, a new study finds. When the crustaceans crash out, their brains emit slow waves of electricity surprisingly similar to those seen in the brains of slumbering mammals.

In terms of behavior, sleep is characterized by an animal's indifference to the outside world and its need to compensate for doze deficits. In mammals and other vertebrates, sleep also evokes slow, regular waves of electrical activity in the brain. However, although studies in fruit flies and bees hint at proper sleep, slow waves have not been found in these animals. Now, in a paper published online this week in the Proceedings of the National Academy of Sciences, researchers report that crayfish exhibit slow waves, as well as the behavioral signs of sleep.

Fidel Ramón of the National Autonomous University of Mexico and his colleagues first watched crayfish (Procambarus clarkia) around the clock. They noticed that when the animals struck a particular pose, lying motionless on one side and holding on to the tank with just one appendage, they also seemed practically comatose. These groggy animals were less responsive to being shaken by a motor--the equivalent of a cell phone on vibrate--compared to animals strutting around the bottom of aquaria. When the researchers flashed regularly timed lights, like strobes at a disco, the nappers, unlike alert crayfish, didn't seem to notice when the scientists skipped a flash. Finally, the researchers found that sleep-deprived crayfish made up for lost shut-eye the next night, a telltale sign of a regular snoozer.

Throughout the study, electrodes measured the electrical activity in crayfish brains. Like most invertebrates, awake crayfish exhibited noisy, spiky patterns of electrical activity. But those spikes were replaced by slow waves in animals in the sleeping posture. These results, say the researchers, suggest that sleep might be fundamental to all animals.

"The study provides important evidence for the universality of sleep," agrees Ralph Greenspan, a neurobiologist at The Neurosciences Institute in San Diego, California. However, he notes, the nature of electrical activity during sleep seems to differ between animals. In mammals, slow waves originate in the cerebral cortex, but are absent in reptiles and amphibians which apparently still sleep despite not having a cortex. Where and how crayfish make these waves is truly mysterious, Greenspan says.