A remarkable hot-spot of fiddler crab coexistence created by random assortment of species.

Small-scale distribution of fiddler crabs was investigated at a site on Pulau Kaledupa, Indonesia (5°29.8'S,123°45.4'E), where all eleven species known from the 1740 km coastline of the province of Sulawesi Tenggara inhabit one small 0.15 ha intertidal mudflat, there coexisting with four species of ecologically-equivalent macrophthalmid crabs. Fiddler crab abundance across the site was clumped, but their species density was randomly distributed. The mudflat site exhibits not only unusually high sympatry but also extreme fiddler crab syntopy, with up to seven species occurring together within areas of just 2 m2. The various fiddler species tended to partition the site in respect of the presence/absence of shade (mostly human created), but otherwise the unprecedented degree of syntopy was precisely that expected by chance if, granted their overall frequencies of occurrence, the available species were distributed independently of each other. Such random assortment indicates minimal ecological interaction between species and little subdivision of the habitat. At a mean 11 individuals m-2 (maximum 27 m-2), observed population density was very low, most likely under top-down control including as a result of human disturbance, the mudflat being in a busy village serving as a ferry port, local market and administrative centre. Overall, the level of sympatry seems largely a consequence of biogeography and that of syntopy one of low crab density permitting random species assortment.