South Padre Island in Texas has a world-class beach, with lovely fine-grained sand that is wonderful to walk on. The sand grains close up seem to be mostly grains of light or transparent quartz, with just the occasional bit of shell grit. At least, that’s what it looks like to my completely untrained eye.
I personally am not so much interested in the sand, however, as some of the creatures living in it. I have been spending the last few years digging around in the beaches there, studying a small local sand crab, Lepidopa benedicti. Because they are so little known, the species doesn’t have a common name; the entire superfamily is generally called “sand crabs” or “mole crabs.” I got started working with sand crabs for my doctoral research because I was interested in how they are able to dig so quickly and efficiently into the sand, and why they couldn’t walk like other crustaceans (Faulkes & Paul, 1997a, b).
For the last couple of years, I have been studying these crabs more from an ecological viewpoint. During my regular collections, I eventually made a conscious mental note of something that is incredibly obvious when you say it out loud. Some of these crabs are white, while others are almost a battleship gray.
When I looked in the scientific descriptions of these species, the colour of this particular species was never mentioned, and other Lepidopa species were usually described as white. Why are they different colors? We often think of colours as being important signals for animals; think of warning colours, for instance. Sand crabs spend almost all of their time buried in sand. In ten years of digging on this beach, I have seen these sand crabs above the sand twice (weirdly, on the same day!), and even then only for a couple of seconds. The crabs have small, poorly developed eyes. It seemed unlikely that they are using these different colors to signal each other.
Although the eyes of some sand crabs are small, they sometimes take note of their surroundings. Some very nice experiments with sand crabs showed how environmental factors can affect colour. For instance, mole crabs (Hippa) are found on either white coral beaches or black volcanic beaches, and they tend to match their background. What’s even cooler is that you take an animal from a white beach, and put it into black sand, and wait, it will tend to darken up after it molts (picture from Bauchau & Passelecq-Gérìn 1987; see also Wenner 1972).
This doesn't seem to explain the color differences I'm seeing in the local sand crabs, however, because they’re all being collected from one beach, which is a light tan as far as the eye can see.
Another idea that came to mind was that this was related to whether an animal was a male or female. That also turned out not to be the case, as each colour had an even split of sexes.
But two pieces of evidence suggest that these colours are not just incidental. There are more gray sand crabs than white ones. And the gray ones tend to be larger than the white ones, although there is overlap (data from Nasir & Faulkes 2010; carapace drawing by Boyko 2002).
Something that might help sort out this puzzling little question would be to look at other populations. The animals I have collected over the last few years are nowhere near the largest on record for the species. The largest individuals in this species have been found on the Atlantic coast of Florida (Boyko 2002). The average size of sand crabs there is about the size of the biggest individuals I’ve found on South Padre Island. And it makes me wonder, if the Atlantic L. benedicti are that much bigger, are they more likely to be that dark battleship gray?
Bauchau AG, Passelecq-Gérìn E. 1987. Morphological color changes in anomuran decapods of the genus Hippa. Indo-Malayan Zoology 4(1): 135-144.
Boyko CB. 2002. A worldwide revision of the recent and fossil sand crabs of the Albuneidae Stimpson and Blepharipodidae, new family (Crustacea, Decapoda, Anomura, Hippoidea). Bulletin of the American Museum of Natural History 272(1): 1-396. http://dx.doi.org/10.1206/0003-0090(2002)272<0001:AWROTR>2.0.CO;2
Faulkes Z, Paul DH. 1997a. Coordination between the legs and tail during digging and swimming in sand crabs. Journal of Comparative Physiology A 180(2): 161-169. http://dx.doi.org/10.1007/s003590050037
Faulkes Z, Paul DH. 1997b. Digging in sand crabs (Decapoda, Anomura, Hippoidea): interleg coordination. The Journal of Experimental Biology 200(4): 793-805. http://jeb.biologists.org/cgi/content/abstract/200/4/793
Nasir U, Faulkes Z. 2011. Color polymorphism of sand crabs, Lepidopa benedicti (Decapoda, Albuneidae). The Journal of Crustacean Biology 32(2): 240-245. http://dx.doi.org/10.1651/10-3356.1
Wenner AM. 1972. Incremental color change in an anomuran decapod Hippa pacifica Dana. Pacific Science 26: 346-353.
[“Doctor Zen” first got in touch with me in the spring of last year to kindly point me to some research on bubbles behaving like granular materials. I had a look at his personal web page at the University of Texas-Pan American, and discovered his research interest in decapod crustacean behaviour, in particular the unusual crayfish, Marmorkrebs; I was particularly interested in his work on crustaceans digging in sand – the slipper lobster video on his site is strangely compelling. I asked if he would contribute a guest post and he kindly agreed. I hope that readers find this as fascinating as I do – I’m reminded of some of the intriguing evolutionary questions raised by beach mice and the bleached earless lizard. Thanks, Zen, for this – well worth waiting for!].