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 Malay Archipelago, consiting primarily of Indonesia (excluding Sumatra) and the Philippines, as well as other nations found on islands shared with Indonesia, including Borneo (Brunei and parts of Malaysia), New Guinea (Papua New Guinea), and Timor (Timor-Leste), as well as Palau and most of Melanesia (Solomon Islands, Vanuatu, and New Caledonia, but excluding Fiji). This complex region may have as many as 20 species present:
The complex distribution of islands in the Malay Archipelago makes it the only area where fiddler crabs are largely distributed in two dimensions, as most other coastlines are essentially linear and even the islands of the Caribbean are mostly distributed in a linear chain. Not all of these species are found throughout the entire region; about eight are found universally, another five are reasonably widely distributed, and the remaining seven are more local or restricted to various edges of the region.
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.