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 parlance of behavioral ecology, the genetic social properties of S. regalis and its direct-developing, colonial cousins make the species "eusocial." The term means that most inhabitants of the colony, rather than engage in reproduction themselves, help raise and defend the offspring of a lucky few.

In spite of the seeming evolutionary advantages of a common defense, eusocial animals present one of the most enduring paradoxes in nature: If adaptive evolution proceeds via the differential survival and reproduction of individuals, how can a species arise in which most individuals never breed at all? Darwin

himself was famously troubled by the dilemma; in Origin of Species he writes that the phenomenon of sterile workers among the social insects posed the "one special difficulty, which at first appeared to me insuperable, and actually fatal to my whole theory." The extreme case of the sterile worker, however, is just one example of a broader question: How does evolutionary theory account for the occurrence of altruism--behavior that does hot benefit an individual creature but is beneficial to others of its species--in a Darwinian world?

Darwin correctly anticipated that the key to the paradox of eusociality is the close genetic relatedness between an insect colony's breeders and its sterile workers. But the full explanation did hot emerge until the 1960s, when the late English evolutionary biologist Williams D. Hamilton first put forward his ingenious ideas. As Hamilton pointed out, among species in the order Hymenoptera, a complex mechanism of sex determination known as haplodiploidy gives rise to "supersisters": all the female offspring of a colony's queen share 75 percent of their genes, rather than the 50 percent shared by full sisters in most other animals. In other words, in colonies of-social insects the sterile workers, which are almost exclusively female, are more closely related to their sisters than they would be to their own offspring (if they had any).

Hamilton suggested that the differential relatedness between sisters and offspring might explain the high frequency of eusociality among hymenopterans. In theory a female social insect gets a larger genetic payoff from raising a sister than she does from raising a daughter. Hence a worker female is better able to transmit her own genes to future generations indirectly, by ensuring that the queen mother of the colony produces more "super-sisters," than she can by breeding herself. The social structure of the colony thus emerges from the genetic self-interest of its constituent workers.

The close genetic relatedness among colony members in S. regalis is consistent with the usual pattern of eusociality based on kinship. But genetics is only part of the story. Many animals live in kind groups; few, however, have taken family life to the extremes that the eusocial animals have. Colonies of bees and snapping shrimps have hundreds of members, but the number or breeders in each colony hovers around one. As Hamilton recognized, the other, interlocking part of the story behind what is often called "animal altruism"--the foundation of advanced sociality--is ecology.