Interactions between Acanthaster planci (Echinodermata, Asteroidea) and Scleractinian Corals at Kona, Hawai'i

Annual assessments of reef communities at Kona, Hawai'i, from 1985 to 1995 determined that the corallivorous sea star Acanthaster planci preferred as prey the smaller colonies of Pocillopora meandrina over the far more abundant Porites compressa and P. lobata. This finding came from three distinct settings: a Reef-Top habitat, where large (> 15 em) colonies of Pocillopora meandrina were dominant; a Reef-Face/Boulder habitat, where living scleractinians-mainly an encrusting form of Porites lobata-covered <5% of the substrate; and a CoralRich habitat, where living scleractinians-mainly Porites compressa and a massive form of P. lobata-covered >95% of the substrate. Although a corallivore, A. planci was most numerous on the reef face and adjacent boulders, where corals were fewest. There it fed mainly on colonies of P. meandrina, even though this species represented < 1% of the sparse coral coverage. Virtually all P. meandrina colonies in that habitat were < 10 em in diameter, and all seen eaten by A. planci there were <5 em. The sea star was less numerous where corals were most abundant, and there it fed on species essentially in accordance with their relative abundance in the environment. A strong inverse relationship in occurrence between the sea star and small P. meandrina colonies on the reef face and adjacent boulders indicated that this favored prey was a limited resource and that sea-star predation prevented it from becoming established in that habitat. PREDATION BY THE Indo-Pacific asteroid Acanthaster planci (L.) on scleractinian corals has been extensively documented, with attention directed mainly at large aggregations that have done great damage to established colonies (review by Moran 1986, Birkeland and Lucas 1990). Most of this information, however, has come from the western Pacific. Our experience has been that interactions between A. planci and reef corals in Hawai'i vary somewhat from the standard perception. For example, we have not seen aggregations of more than 30 to 40 during > 30 yr of study on Hawaiian reefs, and these have been rare. Although during 1970 Branham et al. (1971) observed an aggregation of about 20,000 on a reefoff the Hawaiian island of Moloka'i, the vast majority seen by us have been solitary. Furthermore, although the bleachedwhite skeletons typical of corals recently digested by Acanthaster have been common, we I Manuscript accepted 13 May 1996. 2 Southwest Fisheries Science Center, Tiburon Laboratory, 3150 Paradise Drive, Tiburon, California 94920. have never seen the extensive reef destruction attributed to this sea star elsewhere (e.g., by Chesher 1969). Our understanding of relations between A. planci and scleractinians in Hawai'i developed from study of one reef system over 11 yr. This occurred along the southern edge of Kealakekua Bay, on the island of Hawai'i (Figure 1), where we monitored the reef communities during September of each year from 1985 to 1995. This report describes relationships between A. planci and corals on those reefs and discusses implications.