When crayfish battle for dominance, the fighting can be vicious. One combatant typically tries to pin another, expose its soft abdomen, and disembowel it with ripping, lobsterlike claws. The top crawdad gets first dibs on food and the best rock crevice to hide in, but it also gets to watch its chitinous back constantly. One of its main defensive techniques is the tail flip--an explosive and reflexive swish of the tail that powers the crayfish through the water, away from any usurper that might be sneaking up behind it. Dominant crayfish, it turns out, are much better at tail flipping than their subordinates.
But according to neurobiologist Donald Edwards at Georgia State University, great tail-flipping crustaceans are not born that way, they’re made by experience. Edwards and his graduate student Shih-Rung Yeh have discovered changes in a crayfish’s nervous system that correlate with changes in its social status--up or down--and that give rise to changes in tail-flipping ability. Essentially the nervous system of the crayfish is being changed to adapt it to its new circumstances, says Edwards. That’s the exciting bit, and it hasn’t been reported before. This is the first time, he explains, that social interactions may have been shown to have a direct effect on the neurophysiology of an animal.
Crayfish have perhaps the best understood neural circuitry of any animal, which is why Edwards and Yeh have been studying them for years now. The tail flip is triggered, says Edwards, when hairs on a crayfish’s tail become bent--as they might when one crayfish sneaks up behind another. The bent hairs stimulate nerve cells in the animal’s abdomen that control the tail. To avoid unnecessary flips--say, when the animal brushes up against a rock--the nerve cells have a stimulus threshold below which they won’t fire. The reaction threshold is raised or lowered by a chemical, serotonin, that hooks up with receptor molecules on each nerve cell’s surface. Serotonin is known to modulate aggressive and depressive behavior in a host of animals, from crustaceans to humans.
And in crayfish, Edwards and Yeh have found, serotonin seems to be what makes dominant animals aggressive and subordinates depressive--at least when it comes to tail flipping. The researchers paired two crayfish in an aquarium and allowed the animals to fight for dominance. After 12 days, the researchers killed the animals, removed their abdominal nerves, and applied serotonin to the nerve ends. Then they stimulated the nerve ends with an electrode to mimic the bending of hairs on a crayfish’s tail and measured the electrical pulse transmitted through the nerves. In the dominant animals the response goes way up, says Edwards, while in the subordinate animals the response goes way down. Serotonin evidently lowers the stimulus threshold needed for triggering a tail flip in dominant crayfish but raises it in subordinates.
These results might at first seem to show that dominants are simply blessed with an innately aggressive response to serotonin, while subordinates are not. But when Edwards and Yeh dissected dominants that had been bumped from the top spot, they found that serotonin dulled the once- dominant animals’ tail-flip response even as it had once enhanced it. That indicated to Edwards and Yeh that when a crayfish changes its status, its nervous system somehow also changes, altering the animal’s response to serotonin.
How could the same chemical have such radically different effects on crayfish? Many animals, including humans, are known to have different kinds of receptors on their nerve cells that bind to serotonin molecules. Indeed, Edwards and Yeh have recently found evidence that crayfish have at least two kinds of receptors--one that responds to serotonin by enhancing the nerve response that triggers tail flipping, the other by inhibiting it. It may be that the enhancing kind takes over in dominants while the inhibiting kind takes over in subordinates. Exactly how these different receptors get turned on and off is Edwards and Yeh’s next research target. The thrust of our research, Edwards says, is going to be to try to understand how the nervous system adapts to changes in dominance status. That should be applicable broadly across the animal kingdom, including to humans.