Spiny lobsters: there's safety in numbers.

Spiny lobsters are party animals. Juveniles and adults tend to congregate in groups of 10 or more in large crevices within coral reefs or in roomy gaps between the seafloor and the rotting planks of old shipwrecks. But this relatively open, gregarious lifestyle would seem to leave the lobsters more vulnerable to predators than going it alone in a hole sized for one, where larger animals such as sharks would have a tougher time reaching them.

Now, two fisheries ecologists have found evidence that spiny lobsters' socializing may in fact help them elude predators. The new study suggests that the lobsters developed their gregarious habits to avoid ending up as lunch.

David B. Eggleston and Romuald N. Lipcius of the Virginia Institute of Marine Science in Gloucester Point, spent a year and a half studying the shelter preferences of spiny lobsters off the gulf coast of Mexico. They began their experiments to determine whether the artificial dens, called casitas, used by many Caribbean fishermen to attract lobsters for harvest, rendered the lobsters more open to attack by predators, thereby reducing the harvestable number.

In the June ECOLOGY, Eggleston -- who now works at the University of Washington in Seattle -- and Lipcius report the results of two types of experiments on lobster behavior. In the first, they observed lobsters in man-made enclosures; in the second, they monitored the shelter choices of lobsters in the wild.

To conduct the enclosure experiment, Eggleston and Lipcius built a 20-foot-wide pen of wire mesh and placed it offshore in water roughly four feet deep. Into the pen they placed a small, a medium and a large casita constructed of plastic pipes and concrete slabs. The openings of the casitas were scaled to admit only small, medium and large lobsters, respectively.

When Eggleston and Lipcius placed a lone male spiny lobster in the pen, they found that it sought shelter underneath the casita that best fit its body size. But when the researchers put nine male lobsters into the pen together, most of the lobsters congregated beneath the largest casita.

To investigate what would happen in the presence of a potential predator, the researchers repeated the experiment after placing a five-foot-long nurse shark into the pen. This time, they found that the lone lobster crammed itself into the smallest possible casita in order to stay out of harm's way. Similarly, the group of nine lobsters tended to gather in the medium casita rather than the large one.

Eggleston and Lipcius observed similar shelter choices among spiny lobsters in the wild. They conclude that such gregarious behavior helps the lobsters survive.

"This confirms that predation risk is what's driving [the lobsters'] shelter use pattern and suggests their sheltering behavior has evolved as a result of predation pressure," Eggleston asserts.

He speculates that by grouping together, spiny lobsters increase their chances of spotting an approaching predator, while decreasing their likelihood of being the lobster that gets eaten. Moreover, he says, groups of lobsters often cooperate to fend off smaller predators by backing into a circle and using their spiny antennae to swat intruders.

Biologist William G. Lyons of the Florida Marine Research Institute in St. Petersburg says the study "documents... the effectiveness of gregarious behavior." He says there's no other reason why lobsters of the same sex might gather: "There's no Friday night poker game or anything like that."