This guide is designed for identification “in the field” where you might be looking at live crabs by eye or through binoculars or from photographs. I will generally try to avoid characters that will require you to physically catch the crab, although I may mention a few for secondary verification. It does not include the more strict taxonomist-style characters that may only be visible under a microscope or via dissection. It is also assumed that the individuals are living, as death (and even capture) can cause dramatic color change.
This is a guide to the fiddler crabs of Fiji (the eastern most part of Melanesia) and Polynesia, including Tuvalu, Wallis and Futuna, Tonga, Samoa, American Samoa, the Cook Islands, and French Polynesia (but excluding New Zealand, Hawaii, Easter Island, and the Pitcairn Islands, none of which have fiddler crabs). Only five species reach this far into the Pacific and most of these islands only have three:
Note: This guide is not complete. While most are described, a few of the species are obscure enough that I do not currently have enough information to reliably describe their appearance or how they differ from some of the other overlapping species.