When identifying fiddler crabs, it is important to use conservative features such as the presence of groves on the nipper, rather than features like colours which vary widely between individuals or populations.
|Uca elegans from Gove, Northern Territory|
|Uca signata at Cairns Airport mangrove board walk ~ 2000 km along the coast from Gove|
From the front, the crabs are quite different or at least the nipper is. Nipper shape is variable and crabs that have regrown lost nippers grow replacement nippers that different in shape and strength to originals. Features that are used for keying crabs include the groves on the nipper (Chelae), presence of enlarged teeth in the gape of the nipper and the size and shape of the 'cutting surface' which is where the fingers of the nipper come together like scissor blades. Other features on the crab's body and minor nipper can also be used but are not clearly visible in these photos.
On the basis of the differences in the features of the nipper, I regard these crabs as different species. However, I am going to confirm my identifications carefully.