The “Sally Lightfoot” crab or Karkasay (Grapsus tenuicrustatus).

I have previously posted on the importance of hermit crabs in the island ecosystem (see post of 16th July 2008) and the work we have undertaken to look at the occurrence and populations of the three species that occur on the island.

Denis, of course, has other crabs of both land and coastal species and in this post I will focus on the “Sally Lightfoot”, known locally as “Karkasay” (Grapsus tenuicrustatus) which is a crab of rocky shores. It is interesting that the species occurs at all on Denis as the island is a sand cay without rock. Denis was formed during the last glaciation when sand piled up on a patch reef that today forms the reef flats around the north, east and south east coast of the island and can be considered, in some respects, as the “backbone” of the island.

The “rocks” that are to be found around Denis and that form the habitat for G. tenuicrustatus are in fact made of sand! A large part of Denis’ surface area is today actually made up of sandstone. Sandstone that was formed from the action of acids, leached from millennia of sea-bird droppings, acting to bind the coral sand particles together. There are in fact two layers of sandstone on the on the island suggesting that it may have been submersed at some point in its history before re-emerging above the waves.

This sandstone is not particularly strong – it being possible to break fragments of it by hand – it does nevertheless form a solid structure against wave action and can be found forming extensive wave-smoothed tablets along the east, south and south west coasts of the island and distinct outcrops at the southern point of Denis. This therefore is what provides the habitat for a rocky shore crab on an otherwise sand cay island.

G. tenuicrustatus is a herbivore that grazes on the algae that grows on rocks on the coast. Its sleek highly streamlined form, necessary for it to cling to rocks as waves break, gives it an almost alien appearance when seen in profile. Research elsewhere (Martinez 2001) has shown that a very minor shift in stance of the crab greatly reduces its drag in water and enables it to withstand 200% faster flow of water without being dislodged.

The Karkasay on Denis are alert and wary of intrusion, very swiftly retreating into crevices or if necessary jumping into the sea when someone approaches. Clearly this is a predator avoidance technique and though I have yet to sea anything predate them I imagine visiting herons and the ever present conger eels pose a threat - not to mention of course man, seeing as Karkasay make a good curry!

I was able to get some good shots of the crabs on the south and southwest coast (see sidebar). Of particular interest was that the crabs did not retract their eyes at the moment a wave hit, further evidence of their adaptation to the coastal environment.

Sometimes what appears to be a pink-red corpse of a crab can be found intact on the rocks – this is in fact an exoskeleton shed by a growing crab.

There are several other species of crab to be found on Denis and I aim to cover them in future posts…


Ref: Martinez, M.M. (2001). Running in the surf: hydrodynamics of the shore crab Grapsus tenuicrustatus. Journal of Experimental Biology, 204, 3097-3212, (2001).