Identifying Fiddler Crabs

A person who had published a scientific paper on the colours of fiddler crabs suggested that I had misidentified Uca elegans in a previous post and that the species shown was Uca signata.  This shows that colours are not a good way of identifying crabs.  The only sure way is to submit voucher specimens to a museum or other authority that maintains specimen collections.  As a museum director once told me, it is important to collect common species as they may not stay that way.  Indeed common species are usually under represented in collections.  However submitting specimens is not very convenient and I key them out using keys published by scientists, in this case a book titled  'A revision of the fiddler crabs of Australia' by R.W. George and Diana S. Jones (1982).

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

The crabs are surprisingly similar from the back.  Whilst the carapace in the top image shows some mottling, mottling is common in juvenile crabs and is lost as the crabs mature and it not an important identification character.

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.