Insular Freshwater Fish Faunas of Micronesia: Patterns of Species Richness and Similarity

Freshwater fishes are an important but relatively little known component of the highly diverse fish fauna of Micronesia. Localities supporting communities of freshwater fishes include large high islands, with considerable habitat complexity, and smaller low islands, such as atolls and raised coral islands, with limited freshwater habitat. Both types of islands may support species with adult life history styles that are (a) amphidromous and catadromous, (b) euryhaline (often estuarine), or (c) marine species which enter freshwater from time to time. We compared patterns of species richness and similarity between Micronesian localities for amphidromous and catadromous, euryhaline and marine species (ACEM) pooled, and for amphidromous and catadromous species (ACFW). Species richness of both ACEM and ACFW fishes was greatest on larger high islands compared to smaller high and all low islands. Cluster analysis of similarity indices for ACEM species between localities revealed two faunal components: high islands and low islands. High islands were further partitioned into a Caroline Islands cluster and a separate Mariana Islands cluster. Cluster analysis of ACFW species was more complex. One cluster consisted of a low island and a small high island, both in the Carolines chain and with limited freshwater habitat. The second cluster was partitioned into high islands and low islands that reflected influences of both size and geographical location.