Fossils have been found at altitudes of 1,000m (3,000ft)Â - unusual for a crab.
Writing in the journal PLoS One, researchers say they identified the species by comparing it with living relatives on other Pacific islands.
Early settlers brought animals such as pigs and rats - wiping the crabs out.
The researchers describe this as the first documented extinction of a crab in the human era.
"They'd already vanished from the islands by the time the Europeans got there - nobody's ever seen one alive," said Gustav Paulay, curator of the Florida Museum of Natural History in Gainesville.
"There are stratigraphic sequences (layers of rock) that have been studied mostly for bird extinctions, and we can date when Polynesian settlers came in, partly by occurrence of Polynesian rats: and when that happens, you don't see any more crabs," he told BBC News. Read the full report here.
Source: by Richard Black, Environment correspondent, BBC News, U.K.
