while on taveuni last year, i noticed little yellow things on a beach. everytime i tried to get closer, they would just disappear in the sand.
it took some time sitting really really still, but eventually even the close slowly ones came out again and continued with their odd behaviour: they waved with one claw, and they all did it synchronously.
i later found out that these crabs are called fiddler crabs (uca spp., in german: winkerkrabben, “waving crabs”).
the males in the uca-genus have asymmetrical claws – one is tiny, the other one incredibly oversize (up to half of their body mass!). they wave with the big one to get females interested (i’m think i’m gonna try that too :-)).
according to michael rosenberg of www.fiddlercrab.info, the crab is almost certainly a lemon-yellow clawed fiddler crab (uca perplexa).