Two-toned Fiddler Crabs

Making a guide to crabs is hard as many species are quite variable.  This post shows some of the geographic and single site variation present in Uca vomeris, the two-toned fiddler crab.  It is said this species get more colourful as one goes north so lets check that first.  The following photos show a large male captured on the mud flats at Lota in Brisbane, which is at latitude 27 degrees South.


In Cairns (17 degrees South), Uca vomeris can be seen on the small sand flats at the mouth of Barr Creek or Thomatis Creek.  Larger males have an almost white finger on their nipper and a partially or mostly blue back.
However many males have nippers with pink fingers (photo below).  I am not sure why their is so much colour variation and at first thought that there was more than one species present.  Scientific papers have been written on this subject and I must read them, but for now, I still just figuring out which species is which.

Uca vomeris often shares its habitat with other species that can look quite similar, especially in the juvenile stage.  The photo above is Uca elegans, which was engaged in threat displays with the individual below, which is a Uca vomeris.

Gove in the Northern Territory is at 12 degrees South and I captured only one individual Uca vomeris as this species is quite scare and also the ground at Buffalo Creek is very hard.  When taking the photo, I was stuck by how intense the colours were in comparison to Cairns crabs.

Within Site Variation

These individuals from Cairns were captured on the same day and within metres of each other.  Some of the variation is size related with white being more common on larger individuals and beige mottling being more frequent on intermediate individuals.

In fiddler crab habitat, all sizes of fiddler crab are present at the same time from almost microscopic individuals to monsters with nippers over 50 mm long.  Colours and nipper shapes vary widely with size. The individual below was the smallest specimen I found on the day and had an 11 mm nipper and was clearly a juvenile, however that did not stop him from having an affair with a slightly older lady.  To make sure that he mates with the right species and there were 2 other very similar species on the same flat, his genitals have to be the match hers, like a key to a lock.  In fact fiddler crab classification is largely based on gonopores (i.e. genitals) shape.

Not one to be excited by gonopores, I made a chart of variations hoping to get my eye in for the species present.  Part of the chart is shown below.  Even with practice it is hard to assign a species to some individual crabs.   Keying them out is the only reliable way of identifying them.

Postscript.

I have just found out that this issue has been researched by experimental biologists.  The title of the paper is 'The variable colours of the fiddler crab Uca vomeris and their relation to background and predation' and it was published in 2006 in 'The Journal of Experimental Biology 209, 4140-4153.  Fortunately you can download the text for free by clicking on the title above.