The world's most land-loving crab, a thin and delicate Jamaican species that spends its entire life in a tree, made a surprisingly rapid evolutionary transformation from its large and rugged ocean-dwelling ancestors, according to genetic research to be published in the May 28 issue of the journal Nature by an international team of biologists.
"These very unusual crabs, which are the most terrestrial of any in the world, live in little pockets of rainwater inside bromeliad plants, which grow on the branches of tropical trees," says S. Blair Hedges, an evolutionary biologist at Penn State and a member of the research team. The tiny bromeliad crabs are less than an inch long and are thin enough to squeeze between the leaves at the base of the bromeliad plant, where rainwater collects. The researchers say these crabs are by far the most attentive mothers of all known crab species and the only ones known that actively feed and care for larvae and juveniles during the several months they spend in their rainwater nursery. "The mother crab manipulates water quality by removing debris, by circulating the water to add oxygen to it, and by carrying empty snail shells into the water to buffer the pH levels and add calcium," Hedges says.
Because the bromeliad crab looks and behaves so differently from its
ocean-dwelling neighbors, scientists thought the two species must have required
a long time to evolve from their last common ancestor--on the order of 50
million years or so. Other scientists thought the tiny crab might, instead,
have somehow immigrated from Southeast Asia or Indonesia, where there are some
freshwater species that also care for their young, although not to the unusual
degree of the bromeliad crab. "We decided to find out how the Jamaican land
crabs are related to other species and when they came to the island by looking
at their genes," Hedges says. "We found that the bromeliad crab--and also the
eight other species of Jamaican land crabs-
Contact: Barbara K. Kennedy