London (18 May, 2011)(pti)

Land before humans came to the Hawaiian Islands around 1,000 years ago, there ruled land crabs which roamed great distance, a new study has claimed.

Fossils of the crabs, which were as large as a human fist, were found in caves at altitudes of 1,000 metres, highly unusual for crabs, several miles from the coast.

The researchers, who identified the species by comparing it with living relatives on other Pacific islands, believe that early settlers brought animals such as pigs and rats to the islands, causing their extinction.

They described 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,"Gustav Paulay of the Florida Museum of Natural History in Gainesville told BBC News.

"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 said.

Land crabs were particularly vulnerable to contact with animals associated with human spread, and there are documented cases on islands elsewhere where they have fared very badly when rats have been introduced.

Writing in the journal PLoS One, the researchers said settlement of the Hawaiian islands brought huge changes to the ecology, with many native species including birds and amphibians swiftly becoming extinct.

The same fate appears to have befallen this once mighty land crab, dubbed Geograpsus severnsi, they said.

"When you look at the islands of the Pacific, things don't get there easily, so land animals are scarce - and the biggest things on some of them are crabs,"said Prof Paulay.

"They control the ecosystems on atolls to an incredible extent. This particular species would have been important as a predator and as an omnivore-- it would probably have had a big impact on the insect population, on land snails and also maybe on nesting birds."

The fossils have been known for many years from various sites around the Hawaiian islands, including caves at altitudes highly unusual for crabs, several kilometres from the coast.

Many crabs cannot live in these conditions as they need regular immersion in salt water. But G severnsi appears to have evolved the capacity to roam further afield than most, including closely related species found on other Pacific islands, the researcher said.

---30---

RELATED: Study: Hawaiian Islands Feudal system in 1400 AD