The estuarine crab Hemigrapsus crenulatus (H. Milne Edwards, 1837) is a native of New Zealand, but it is also known from the coast of Chile some 8900 km eastwards across the Pacific. Mary Rathbun (1898) was the first person to call crabs from the Chilean coast, H. crenulatus, saying that they were conspecific with crabs described by Milne Edwards (1837). However, Bennett (1964) took the advice of H. Balss that the Chilean species should be known as Hemigrapsus granarius (Nicolet, 1849). No type specimens are available for either of these species. We have used mitochondrial 16S rRNA and COI sequences derived from fresh specimens to find out who was correct. There are few if any consistent morphological differences between populations from either side of the South Pacific and very few molecular differences (only 3 fixed nucleotide differences out of 1190 bp total sequenced – 0.25%). None of these differences exceed variation within either population, so we must declare Miss Rathbun the winner! The molecular data suggest that there is only one species, Hemigrapsus crenulatus, and that the two populations have been separated only recently in the last few thousand years, perhaps during the last glacial epoch. How this species colonized the coast of South America remains an open question. It does not even occur on off-shore islands near the New Zealand mainland so it probably needed assistance to cross the Pacific.
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