This study provides the first evaluation of abundance and diversity of polychaete annelid asseniblages of coral reefs. Quantitative analyses of the polychaete fauna of truncated coral reef limestone platforms off Sumatra and Thailand revealed a total of 76 species (13 reported for the first time from the tropical Indian Ocean) and average population densities of 49,000/m2. The number of species in a single sample ranged from 16 to 32, and species diversity ( H ) ranged from 1.9 to 2.5. Three species of Syllidae, Palola siciliensis, and Dodecaceria lad& occurred in all samples. The syllids dominated numerically in all samples but were quite small, mainly 2-10 mm long and 0.01-0.04 nig dry weight. Samples from the same station were essentially identical in species composition and relative abundance. Similarity decreased with increasing distance between stations. With respect to number of species, population density, and dominance of syllids, the assemblages studied resemble those associated with limestone substrates in marine caves in the Mediterranean.

This study increases the number of species of marine polychaete annelids known t o occur at Easter Island from 11 t o 60, of which 43 are identified t o species. Nearly goo/, of these occur in the tropical Indo-West Pacific region, but 60(1/n are of circumtropical distribution. The strong Indo-West Pacific zoogeographic affinity is consistent with other taxa of Easter Island inshore marinc animals for which data are available, the fishes, corals, and gastropod molluscs.Polychaete species composition and abundance were determined in substrate samples from two types of microhabitat in a tidepool. A sample of 290 ml of sand and 70 ml of algae contained 1,779 specimens of 41 species of polychaetes (species diversity H' = 2.57). A basalt boulder of surface area 340 cinZ contained a polychaete assemblage of 864 specimens of 35 species (H' = 2.49).Population density of the latter sample is equivalent t o 25,400 polychaetes/m2. Of the 49 species present in the two samples, 34 were identified t o species. Although 27 species (55%) were common to both samples, similarity as measured by C,, which weighs differences in relative abundance, was only 24%. More than half of all polychaetes present in the two samples belonged t o the family Syllidae; they comprised 65% of the individuals in the sand-algae sample and 350/, in the boulder sample. Spirorbinae predominated numerically in the boulder sample (40%). Chaetopteridae and Spionidae (8%) were thc next most abundant families in t h e sand-algae sample.