Sea Slug's Shopping Habits Dictated By Hunger, Scientists Report

When hungry, the slugs (Pleurobranchaea californica) may ravenously attack even dangerous prey. With a full stomach, however, they actually turn away from and avoid potential food, scientists report in the March 28 issue of the Proceedings of the National Academy of Sciences. Such avoidance behavior is important for marine snails, because any time spent eating puts them at risk for being prey themselves.

The research was designed to study the mechanisms of decision-making, said Rhanor Gillette, a professor of physiology and neuroscience at the University of Illinois. Foraging animals and shopping humans should make decisions that produce the greatest benefit at the least cost. In this case, Gillette's team asked if degrees of appetite affected the readiness of snails to attack or avoid available prey. Responses were measured by the concentrations of food chemicals at which they would bite or turn away.

"What we've found in studying this very simple sea slug, with a very simple body form and a very simple brain, is that its behavior is organized hedonically, much like ours," he said. "If an animal's internal state changes, its responses to food and pain stimuli change, too. It is as if they make decisions based on a sliding scale of pleasure and pain. This is surprising for a simple invertebrate. Previously such behavior was thought to be exclusive to higher vertebrates."

Hungry snails tempted with the betaine -- a chemical found in most marine invertebrates that stimulates predators -- were quicker to strike than less hungry snails. Higher concentrations of betaine eventually induced biting by the satiated snails, but in general the less hungry snails withdrew their heads, turned and moved away from the food source.

Hungry snails also were more likely to try to attack a noxious acidic stimulus, researchers found. However, satiated snails avoided the noxious stimulus, and even hungry snails with previous exposure were more likely to avoid it. "This could reflect the need of the starving sea slug to pay a higher cost for a meal, if it had to overcome the defenses of prey unwilling to be eaten," Gillette said.

(To see a snail learning to avoid noxious prey, go to http://www.life.uiuc.edu/slugcity/movies.html. Click on "One Trial Learning.")

"We may have been looking at a very fundamental structural organization that will be found in the behavior of most foraging animals," he said. "Animals tend to make wise decisions when they forage, and they do so whether or not they have lots of brain power."