Next month, The Crustacean Society will be holding its summer meeting in Hawaii. While nobody would be surprised by the number of crustaceans in the waters around Hawaii, there is something surprising about the islands’ crustaceans.
There are no land crabs.
This is surprising, because land crabs are common on other Pacific islands. Is Hawaii just too young for crabs to have invaded the island and evolve into a terrestrial form?
Hawaii is young geologically, but it’s not that young. There are no land crabs on the island now – but there used to be.
The picture above shows a new species found on Hawaii, Geograpsus severnsi. (Scale bar is 1 cm.) I use the term “new” only in the sense of “new to science.” These are actually fossil remains, dating back to before human colonization of Hawaii a few thousand years ago. The fossils are found inland at quite high elevations, indicating that this was primarily a terrestrial species.
Based on the fossils, Geograpsus severnsi appears to have been an offshoot of Geograpsis grayi, which still survives and is found across much of the western Pacific. Geograpsus severnsi seems to have been abundant on Hawaii before humans arrived, but promptly took a nose dive into oblivion once humans arrived on the scene. Coincidence? While humans may not be directly to blame for the crab going extinct by hunting them to extinction, those early settlers may have brought in rats, dogs, or pigs that could have contributed to the end of this species.
Paulay G, Starmer J. 2011. Evolution, insular restriction, and extinction of Oceanic land crabs, exemplified by the loss of an endemic Geograpsus in the Hawaiian islands. PLoS ONE 6(5): e19916. DOI: 10.1371/journal.pone.0019916