A shrimpy find: communal crustaceans - Synalpheus regalis is the only marine animal that is eusocial - Brief Article

Science News, June 8, 1996 by Tina Adler

What lives in a cooperative colony, has a queen that bears all of the young, and workers that defend the home nest from intruders? Oh, and is related to a popular dinner delicacy? A snapping shrimp (Synalpheus regalis), of course!

S. regalis and probably other members of the Synalpheus genus have the same community spirit that characterizes the lives of other so-called eusocial creatures-bees, ants, termites, and naked mole rats, J. Emmett Duffy of the College of William and Mary's Virginia Institute of Marine Science in Gloucester Point reports in the June 6 Nature.

The shrimp is the first marine animal known to be eusocial. Duffy's study shows that "in all important respects, the social organization of S. regalis resembles that of many eusocial terrestrial animals," Jon Seger of the University of Utah in Salt Lake City and Nancy A. Moran of the University of Arizona in Tucson assert in an accompanying commentary. Researchers know little about the 100 or so other species in the genus, Duffy says, but divers who explore Caribbean coral reefs are familiar with the loud crackling sound-similar to that of frying bacon-produced by snapping shrimp. He analyzed more than 30 shrimp colonies inhabiting sponges in the coral reefs of Carrie Bow Cay, Belize. Of these colonies, 17 had an intact female whose eggs he could count. The sponges ranged in size from a tennis ball to a football, and each held one shrimp colony.

A sponge's resident shrimp have strong genetic similarities, according to Duffy's analysis of soluble proteins found in the creatures. Indeed, the data suggest "that most colony members (there may be more than 300) are offspring of the queen, and possibly of a single male."

In addition to the genetic data, other evidence suggests that a single female produces all of the young. For example, the colonies of larger, older queens have more members than those of smaller, younger queens. If females besides the queen were contributing offspring, the numbers in the nests might be closer, he says.

Like other eusocial species, S. regalis faces considerable competition for housing and will fiercely defend its nests, he reports. Duffy set up eight small colonies in the laboratory and provided each with a female, eight of her large male offspring, and eight of her juveniles. The next morning, he introduced at separate times into each new colony a member of its original group and a member of a different species of snapping shrimp. Within a few hours, colony members had killed the foreign intruder. In contrast, they welcomed their former colony mate into the new nest. Because of their close genetic relationship to the colony's breeders, eusocial animals need only care for and defend the colony in order to ensure that their own genes get passed along, researchers believe.

The snapping shrimp now joins the mole rat and termite on the short list of eusocial creatures that are diploids, which inherit one set of chromosomes from each parent. Diploids are less closely related to each other than ants and bees, whose males inherit only one set, from the female.