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 central Indian Ocean, including the Maldives and the British Indian Ocean Territory (Chagos Archipelago). The central Indian Ocean is largely bereft of islands and only one fiddler crab is currently known to be found in this region, so identification is easy.
Gelasimus tetragonon is a wide-spread species that is something of an oceanic island specialist, although it is occasionally found on continental shores. It usually has a predominantly blue and black carapace with bright red legs (which occasionally may be darker) and gray eyestalks. The large claw of Gelasimus tetragonon is usually orange, with a noticeably darker red spot near the base of the pollex, and a white dactyl. Gelasimus tetragonon frequently has brown spots on the top part of the hand of the claw.
Gelasimus tetragonon has a lot of additional variability. In some places the carapace can lighten so that there is almost no blue, just a cream or pale orange with black markings. The pattern of the colors on the carapace can vary from stripes or blotches to tiny spots.
