Lobster hygiene: healthy animals quick to spot another's ills.

Caribbean spiny lobsters somehow detect and shun potential roommates carrying a virus, even before the infected lobster can pass along the disease, a research team reports.

These lobsters, Panulirus argus, often share hiding places. Yet underwater surveys on the Florida coast found most sick lobsters alone in dens, report Mark Butler of Old Dominion University in Norfolk, Va., and his colleagues. Lab tests showed that healthy lobsters avoid shelters already inhabited by an infected animal, even one in the early stages of illness, he and his colleagues report in the May 25 Nature.

This is the first evidence that healthy animals in the wild quarantine infected members of their own species, Butler says.

Although spiny lobsters don't have the big, meaty claws of North Atlantic lobsters, they're the main edible lobster throughout much of the world. The Caribbean spiny species ranges from Bermuda to Brazil.

Unlike the North Atlantic species, the Caribbean spiny lobsters are social. In autumn, certain populations march single file along the ocean bottom for several kilometers to reach deeper water less troubled by winter storms. Lobsters sharing a den join in its defense by poking their long, stiff antennae menacingly at intruders.

"They'll drive the antennae into the flesh of fish--or of researchers," says Butler. Collecting the lobsters is 'like trying to grab a porcupine out of a crevice," he notes.

The new study grew out of the curiosity of coauthor Donald Behringer, also of Old Dominion University. In 1999, he noticed that troubled-looking young lobsters, with discolored shells and lethargic attitudes, usually lived alone. The researchers eventually established that these loners carried a virus, which they named PaV1. It's the first virus known to strike lobsters.

The disease caused by PaV1 is most evident in the young. It spreads through physical contact, but the smallest lobsters can catch it from virus-tainted seawater. When scientists put lobsters together in an experiment, more than 60 percent of the lobsters with infected den mates died within 80 days.

In an underwater survey of juveniles, more than 56 percent of the healthy animals shared a hiding place, but only 7 percent of the infected ones did.

In the recent lab tests, infected animals showed no preference between a hideout containing a healthy lobster or a sick one. But uninfected lobsters were choosier. More than 60 percent shied away from a shelter occupied by an animal that had been inoculated with the virus only 4 weeks earlier. That's before researchers could see any symptoms, says Butler.

At 6 weeks after inoculation, infected lobsters were shunned by all the uninfected ones.

That's good timing, says Butler. Lab tests found no virus transmission 6 weeks after inoculation. At 8 weeks, though, PaV1 spreads readily.

These observations may explain why the vires' transmission in the wild is low and isn't affected by lobster density, says Butler.

Ecologist Hamish McCallum of the University of Queensland in St. Lucia, Australia, welcomes the finding as "really interesting" and plausible.

Parasites manipulate their hosts' behavior to favor transmission. "One would therefore imagine the opposite possibility," McCallum says, "that hosts should modify their behavior to decrease transmission."