Underwater urbanites: sponge-dwelling snapping shrimps are the only known marine animals to live in colonies that resemble the societies of bees and wasps

In the past couple of decades biologists have documented eusociality in a growing list of-animal species besides the social insects. Those species include the naked mole rat, a burrow-dwelling mammal that lives in East Africa; certain aphids; a group of inconspicuous gall-froming insects called thrips [see "Altruism in the Outback," by Bernard J. Crespi, November 2001]; an Australian "ambrosia" beetle; and, now, sponge-dwelling snapping shrimps.

As the number of such examples grows, so does the opportunity to identify the evolutionary drivers of advanced social life. A good way to start is to look for commonalties among disparate eusocial animals. In 1991 the evolutionary biologist Richard D. Alexander of the University of Michigan in Ann Arbor and his colleagues compared naked mole rats and termites. Both of these eusocial species are genetically diploid--that is, their offspring carry two sets of chromosomes, one set from each parent. In that regard, they are just like most animals other than the hymenopterans.

Alexander and his coworkers then proposed that most cases of eusociality in diploid animals would arise when three conditions are satisfied: the animal undergoes a gradual metamorphosis sometime during its life cycle; the offspring received extensive parental care; and the individual animals occupy, in the words of Alexander and his colleagues, "long-lasting, expansible niches (nests or microhabitats) safe from predation and rich with food that does hot require exiting the safety of the niche to obtain it." Those three conditions, they contended, promote sustained interaction among close relatives. Furthermore, when the three conditions are satisfied, nonbreeders can increase their genetic contribution to future generations indirectly, either by helping close relatives to breed or by defending the communal nest.

Sponge-dwelling shrimps are also diploid, and so they provide an independent test of the hypothesis put forward by Alexander and his team. And sure enough, the eusocial species of Synalpheus are among the few crustaceans that undergo gradual metamorphosis (other examples are sow bugs and sand fleas), and among the even fewer in which offspring remain with their parents for a goodly amount of time. Moreover, the quoted passage is an almost perfect description of life within the long-lasting niche of a sponge's canals. The lives of snapping shrimps seem to offer dramatic support for the contention that those three conditions lead to eusociality.

But according to Alexander and his colleagues, there is a fourth condition that strongly drives eusociality: enemy pressure. Here, too, evidence from synalpheids supports their analysis. The occupied territory in an individual sponge is usually filled to capacity. But the canals of a living sponge are too narrow for most predators of the shrimps. What kind of enemy could the shrimps be defending against?

The answer appears to be other synalpheids. The host sponge provides such a scarce and valuable resource-combining abundant food, living space, and safety from predators--that settling and keeping it is a matter of life and death for the sponge dwellers. My associates and I have witnessed defenders among captive colonies in the laboratory fighting to the death to repel intruders--clear evidence of the high stakes of territorial defense [see photograph above].