| Coral crabs belonging to the family Tetraliidae Castro, Ng and Ahyong, 2004 include the genera Tetralia Dana, 1851 and Tetraloides Galil, 1986. The systematics and natural history of the twelve living species of these small, obligate, mutualistic symbionts are reviewed and include species descriptions with illustrations and photographs. A mechanism for the maintenance of species diversity and potential mode of speciation in Tetralia is also presented: agonistic interaction between conspecifics within coral heads provides a relative advantage in recruitment for rare species yielding negative frequency dependent selection that allows co-occurring species to persist. Defended "reproductive space" (whole coral colonies) is larger than defended "resource space" permitting heterospecifics, but not conspecifics to recruit and mature, sustaining a higher diversity of these ecologically similar species. Mitochondrial sequencing, surveys of coral-head occupancy, and agonistic behavior experiments support this model. Because tetraliid crabs use unique color patterns to identify species, this mechanism may also facilitate speciation as initially rare color polymorphs in a population would have a selective advantage. The frequency dependent mechanism explicated here requires further confirmation, however, such a mechanism appears likely to contribute to or sustain species diversity in flocks where species ranges overlap, and defense of mating opportunity has primacy over competition for resources. |
