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 the Ryuku Islands of Japan, the long, more-or-less linear chain of islands that runs between the southern edge of the main islands of Japan and Taiwan. There are 10 species found in this island chain:
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.