Nine species of fiddler crab (genus Uca) have been found coexisting within an area of only 20 x 25m in the Wakatobi Marine National Park, Indonesia. Fiddler crabs are gregarious animals that are often found living in groups of hundreds or thousands; however this high level of sympatry has never before been recorded in this genus. This research aims to explain how the remarkable level of sympatry is achieved and also, by extension, to contribute to a general understanding of how ecologically equivalent species manage to coexist.
Quantitative sampling reveals that although all species may occur on the same shore, populations of each species remain discrete. The main factors controlling species distribution appear to be shore height, sediment type and habitat structure. Sediment and mouthpart analysis suggests feeding preferences differ between species allowing for niche separation. This is supported by behavioural observations, showing limited competition between species despite territory overlaps.
For an environment to sufficiently sustain a population the number of coexisting species cannot exceed the amount of resources present and species cannot be too similar in their resource utilization. So far, this research has supported this statement, in that the species present are not out-competing each other and due to the tidal cycle in the adjacent mangrove there is a constant supply of nutrients.