June 2, 1998 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--are not related to crabs on the other side of the world but have evolved from one common Jamaican marine ancestor very recently--only 4 million years ago."
The research team includes Hedges, Christoph D. Schubart, of the University of Southwestern Louisiana, and Rudolf Diesel, of Bielefeld University in Germany. Schubart collected 22 crab species from Jamaica and surrounding areas, including Venezuela and Panama, then brought them to Penn State for genetic research in Hedges's lab. "We sequenced two genes from each of these species, which gave us a little over a thousand base pairs--enough to say statistically that all the terrestrial Jamaican crabs form a single group," Hedges says.
The genes used in the study have accumulated mutations at a fairly constant rate relative to one another during their evolution, so the researchers could use the changes like the ticking of a molecular clock to trace the history of each species back to its origin. "The clock-like mutations in the gene sequences start ticking away as soon as a new species evolves, so the molecular clock takes you back to the actual time of origin of the species," Hedges explains.
The scientists calibrated this molecular clock to an evolutionary event well established by geological studies, the closing of the Panama land bridge between North and South America 3.1 million years ago that separated species of marine crabs into two breeding groups--those living on the Caribbean side of Panama and those living on the Pacific side. Using this date as a secure calibration point--and the mutation rate for the two genes from related crabs on either side of Panama as a timing device--the researchers were able to determine how long ago all of their 22 crab species originated.
"We found that these Jamaican land crabs began evolving only 4 million years ago, so their evolution has been quite rapid," Hedges says. "This date makes sense because it corresponds to a time in Jamaica's geologic history when the land had risen far enough out of the sea to provide new ecological niches for the ancestral marine crab that began evolving strategies for living entirely on the land," he explains.
The scientists also determined how closely each of the 22 species is related to the others by comparing the molecular sequences that make up each of the two genes and then determining which are most similar. "We found that the closest relative of the Jamaican terrestrial crabs is a Jamaican marine crab," Hedges says.
"Jamaican land crabs look and act very different from Jamaican marine crabs, yet they have been evolving separately for the same amount of time as the marine crabs we used for our calibration on either side of Panama, which have remained almost identical," Hedges says. "Such rapid adaptation to a new ecological niche and rapid radiation of new species is not common in nature, but it apparently has occurred much more quickly than we had thought possible in these Jamaican terrestrial crabs."
This research was sponsored by the German Science Foundation and the U. S. National Science Foundation.
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