In sponge-covered reefs off the coast of Belize, a Virginia scientist has discovered "hives" of tiny shrimp living cooperatively like honeybees and wasps, providing the first known example of such a highly evolved social system in a marine animal.
The colonies of pink and orange "snapping shrimp," so called because of the clicking noises they make with their front claws, have a single, large queen that does all the reproducing, hundreds of genetically identical shrimp siblings that share resources and a specialized class of soldiers defending the community against invaders.The bizarre finding has sent ripples of excitement through the ranks of evolutionary ecologists, who for a century have wrestled with a dilemma first noted by Charles Darwin: How can evolution, with its maxim of "survival of the fittest," lead to species in which the vast majority of individuals have given up their ability to reproduce?
Scientists facing this conundrum have presumed that under certain circumstances, survival of a species' genetic heritage is best ensured by having most individuals forgo reproduction and instead care for their siblings - which are, after all, as closely related to one another genetically as parents are to their children. The new work with snapping shrimp, an organism very different from the handful of other species known to use this strategy, helps to clarify what are the environmental and biological conditions that are likely to favor the evolution of this specialized social system.
"It's totally wonderful," said Jon Seger, an evolutionary ecologist at the University of Utah. "This is the same excitement you'd get from discovering new life on another planet."
The discovery was made by J. Emmett Duffy, an ecologist at Virginia's College of William and Mary School of Marine Science in Gloucester Point, who descibed his work in the journal Nature. Duffy studied the shrimp species Synalpheus regalis, which lives in shallow Caribbean reefs. The shrimp, which measure less than a half-inch in length, spend their lives inside living sponges - labyrinthine environments that can house 300 or more shrimp.
By dissecting 30 such sponges, Duffy found that each shrimp colony had a single egg-laying female, easily identified by her ripe ovaries and clutches of eggs, which stick to the mother until they hatch into crawling larvae. Several generations of larvae, which grow bigger with age, were present in each colony. And when Duffy performed a test resembling DNA fingerprinting on more than 600 shrimp youngsters of various generations, he found that all appeared to be the offspring of a single set of parents.
Finally, Duffy made the surprising discovery of job specialization within the colonies. Snapping shrimp brandish their oversized front claws and snap them shut when confronted by intruders. Laboratory experiments in which Duffy exposed colony members to shrimp of a different species resulted in "intense battle," Duffy reported, waged mostly by the larger colony members - soldiers of sorts. In two of 10 contests, pairs of soldiers cooperated to drag the vanquished intruder from the colony.
"Because larger individuals, most of whom are unlikely ever to reproduce, perform most of the colony defense, and thereby benefit juveniles, this nest defense amounts to cooperative brood care," wrote Duffy.
Cooperative care of the young is one of three requirements that ecologists use to declare that a species is "eusocial," the most complex form of social organization among animals. The other two requirements are multiple generations living together and reproductive specialization, in which only one or a few individuals do all the procreating for a colony.
Duffy's shrimp apparently meet all those requirements, an evolutionary achievement otherwise found almost exclusively in insects such as bees, wasps, and termites and a few other animals such as naked mole rats, a peculiar East African mammal that lives in underground colonies.
"This strongly supports the idea that similar sorts of ecological and evolutionary pressures yield similar types of social systems across the animal world," said Paul Sherman, a professor of animal behavior at Cornell University who studies naked mole rats. "When ecological factors like predation or weather or habitat make it dangerous to disperse, groups form and the young stay at home."