DIVERSITY, UTILIZATION OF RESOURCES, AND ADAPTIVE RADIATION IN SPIALLOW‐WATER MARINE INVERTEBRATES OF TROPICAL OCEANIC ISLANDS1

Abstract

Important features of the inshore marine benthic invertebrate fauna of tropical islands are very high species diversity and the geomorphological role of reef‐building and sediment‐generating animals in contributing the structural framework of habitats. The former is due to the adaptive radiation and persistent co‐occurrence of large numbers of species in a number of genera, particularly of molluscs, echinoderms, and crustaceans. Among the molluscs, epifaunal gastropod genera contribute importantly to the high species diversity. On tropical Indo‐West Pacific island shores and coral reefs, 80% of prosobranch gastropods belong to genera having more than three sympatric species; in diverse temperate faunas the corresponding figure is 20%.Features of the environment and of the animals tending to promote both speciation and high faunal diversity are listed: environmental heterogeneity and ecological and behavioral specialization are the most approachable. In a representative gastropod genus (Conus) with available quantitative ecological data, topographically complex, climatically equable subtidal coral reefs support more species, lower population density, and animals of larger size than do intertidal marine benches that are topographically uniform but subject to more severe weather conditions. Recurrent group analysis shows strong affinity among six common species on reefs and three on benches spanning most of the longitudinal extent of the Indo‐West Pacific region, demonstrating wide species distribution and constancy of species composition in these habitat types.Degree of specialization and overlap in resource utilization by co‐occurring congeneric species are discussed with reference to the theory of limiting similarity. Conus species are demonstrated to specialize more to different prey species than to substrate type. A more general hypothesis, that co‐occurring, ecologically similar predator species tend to adopt this strategy, while detritus feeders are more likely to specialize to microhabitat than food type, is only partially supported by the limited data available on other benthic invertebrate taxa.