More on littoral crabs... new records for Denis Island.

I posted at length on the 5th of July on the Sally lightfoot crab (Grapsus tenuicrustatus) and was able to get a few good shots of the animals as waves broke over them. G. tenuicrustatus is a common sight on the sandstone formations to be found around the southern and eastern coastline of the island and is indeed the only rocky shore crab previous studies have recorded on the island. I was very interested therefore to find other species present in the southern rocks when I was trying to get photos of G. tenuicrustatus. They were much more shy than the “Lightfoots” and difficult to photo but I was able to get a few snaps sufficient for their identification.

Eriphia smithi – rough red-eyed crab, red-eyed reef crab, pebble crab

E. smithi has an indo-pacific distribution and is reportedly very common on east African coasts. It has various distinctive characteristics which when combined allow for species identification. Most apparent are the bright red eyes on white stalks and this immediately narrows down its identity, this coupled with the numerous spines on carapace edge behind the eyes and the large molariform tooth on the larger claw clearly identify it as E. smithi.

This is an interesting species that grazes on algae during the day but turns predator of crustaceans and molluscs at night. At low tide at night individuals have been reported to roam out up to 50m on to reef flats in search of prey before returning to their crevice hideouts (Vaninni et al 1989). Rock crevices play an important part in their lifecycle for seclusion and mating and individual crabs can remain loyal to a single retreat for 3 weeks or more. The southern reaches of the island seem ideal in this regard with numerous cracks and crevices in the sandstone and large reef flats that are exposed at low tide.

Geograpsus stormi – red nipper.

Now I can’t be sure of this identification as I do not yet have a full dorsal photo of this crab (it moved very rapidly and agilely over the rocks in its search of refuge) but judging from the colour of its legs and eyes and the presence of prominent long bristles on the legs I am fairly certain that it is a specimen of the rocky shore crab G. stormi .

G. stormi has a western Indian ocean to central pacific distribution (Richmond 1997) and has been noted for its consumption of rocky shore carrion. Interestingly Haig (1984) records G. stormi as only occurring on islands that fit the habitat category of “low islands with mangroves” - in this case citing the outer island atolls of Aldabra and Cosmoledo. She does however go on to note that it is likely that G. stormi (inter alia) will be found on islands without mangroves as more “collecting” is done.

References:
1). Cook Islands biodiversity Database (2009). http://cookislands.bishopmuseum.org/species.asp?id=7351
2). Haig, J. (1984). Land and freshwater crabs of the Seychelles and neighbouring islands. In: Stoddardt, D.R. Biogeography and ecology of the Seychelles islands. Junk Publishers, The Hague.
3). Richmond, M.D. (1997). A guide to the seashores of Eastern Africa and the Western Indian Ocean Islands. ISBN 91-630-4594-X.
4).Vaninni, M. et al (1989). Feeding habits of the pebble crab Erithia smithi. Marine Biology 100, (249 -252) 1989.