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 Andaman and Nicobar Islands. There is a bit of uncertainty as to the fiddler crab species present on these islands as they sit near an intersection of several different faunal regimes. The literature includes the names of many species that have been found here, but in many cases it is unclear if some of these may be based on out of date nomenclature or confusion among species.
Based on iNaturalist observations, four species that are definitely present in these islands are Austruca annulipes, Gelasimus tetragonon, Tubuca alcocki, and Tubuca coarctata. Interestingly, the latter is not reported in the literature; it may be a new arrival or has been reported under other names. Beyond these four species, there are a number of additional species that should be viewed as potentially present.
At least one of the Gelasimus (Gelasimus) species is definitely present (there are multiple observations on iNaturalist), but at this time it is not clear to me if it is Gelasimus hesperiae (which is found throughout the western half of the Indian Ocean), Gelasimus vocans (which is common in the western Pacific Ocean and may reach into the eastern Indian Ocean), or both. Both have been reported for these islands in the literature, but there is enough ambiguity in distinguishing the two, that we are left uncertain which of these species is present at this location.
Austruca bengali has been reported at these islands (there are currently no iNaturalist observations of this species from here), but a few years ago Austruca bengali was split into two species: Austruca variegata, which is found on the west and northern side of the Bay of Bengal and Austruca bengali which is found on the southeast side of the Bay of Bengal. It is most likely that the species found on these islands is Austruca bengali given how much closer they are to its known range versus that of Austruca variegata, but we do not know for certain which (if not both) of these species is present here.
Finally, a variety of names have been used for species on the islands that are either well out of range or nomenclaturallly out of date, but could reasonably apply to either Tubuca paradussumieri or Tubuca alcocki. LIke the Gelasimus species mentioned above, one of these—Tubuca alcocki—is generally found in the western half of the Indian Ocan, while the other—Tubuca paradussumieri—is generally found in the western Pacific Ocean but likely extends into the eastern edge of the Indian Ocean. While we know for certain that Tubuca alcocki is found here, it is less clear if the varous literature references to similar looking species are Tubuca alcocki or Tubuca paradussumieri, as I am generally under the belief that these two species are frequently confused with each other. With these islands sitting in an area that could reasonably be considered near the border of their distributions, it is possible that Tubuca paradussumieri is also found here, or that all of the references to paradussumieri-like species are actually Tubuca alcocki.
I am going to hold off on a guide to the species of these islands until we resolve some of the confusion over which species are present.