THE EVOLUTION OF ENDEMISM IN INSULAR PACIFIC FAUNAS: CORAL-DWELLING STOMATOPODS

Coral-dwelling stomatopods, as well as other groups of marine invertebrates, are dwarfed on Pacific atolls compared to high island or mainland faunas. Particularly on atolls, typical body size diminishes progressively within lineages and among assemblages of species as one moves eastward from the Austral-Asian continental region of highest diversity toward the mid-Pacific, which is a region of high endemism. Populations on atolls are particularly diminutive. Lower productivity in the Central Pacific than on the Indo-West Pacific continental shelf and lower productivity on atolls compared to high islands may explain this pattern. In the mid-Pacific, however, smaller body sizes result in fewer, smaller, less well-dispersing offspring, trapping the lineage as a peripheral endemic. Dispersal from the large species on the Austral-Asian continental area appears to dampen differentiation in the adjacent West Pacific region, which is characterized by low endemism. The East Indies "center of origin" and the "peripheral speciation" models are not mutually exclusive, however, since the Austral-Asian continental seas are inhabited by both large and small species, and endemism among small lineages is even higher in the Indo-West Pacific continental region than in the peripheral areas. Here, biotic factors associated with competition for refuges may have selected for small species that can exploit the much more numerous small refuges in the reef, while the pugnacious larger species follow a riskier and energetically expensive path, fighting viciously for increasingly rare large holes in the calcareous substrate in order to reap the temitorial and reproductive benefits that result from large body size.