Recruitment, ecology, growth and behavior of juvenile Acanthaster planci (L.) (Echinodermata: Asteroidea)

On Suva Reef in the Fiji Islands the refuge substratum of early juvenile A. planet «20 mm) was found to be subtidal coralline algae (Porolithon onkodes) on the windward reef front. Highly cryptic juveniles (10-30 mm maximum diameter) were found under coral blocks and rubble in the boulder zone of exposed reef fronts, on the dead bases of Acropora spp. in more sheltered areas, and in interstices on the reef crest and on the fore reef slope to depths of8 m. Recruitment, monitored over 9 years, was very low in most years (in 1985 the density of8-month-oldjuveniles was 0.004 m-2 in the boulder zone). A massive recruitment occurred in 1984 (density of 7-month-old juveniles was 8.3 m-2 in the boulder zone). Monitoring of this cohort over 2 years indicated that growth was sigmoidal. The maximum diameter increased by 2.6, 16.7 and 5.3 mm/month in the algal-feeding, early coral-feeding and adult phases, respectively. An epidemic caused a high mortality between 10-16 months. The diet switched from coralline algae to scleractinian corals at age 13-15 months but behavior remained cryptic. At about 20 months feeding switched from night to day and aggregative behavior began. The onset of sexual maturity commenced at 23 months. Between 26 and 28 months the population migrated into the deeper waters of the reefslope. Mortality between ages 8 and 23 months was very high (ca. 99%) and is attributed to disease, and possibly the effects of rough seas and predation. These findings indicate that recruitment is erratic and that an infestation arises from a single massive settlement. Large feeding aggregations of the coral-eating asteroid Acanthaster planci have caused widespread damage to Pacific coral reefs during the past 25 years (Endean and Stablum, 1973; Birkeland, 1982; Moran, 1986). Infestations of adult starfish characteristically appear suddenly on a coral reef, consume much of the living coral, and then depart. Where the starfish come from has never been established. The paucity of information on the early life history is perhaps the main reason why the Acanthaster phenomenon is so poorly understood (Moran, 1986). Although A. planci has been raised in the laboratory and the larval stages and juvenile growth and behavior in captivity are well known (Henderson and Lucas, 1971; Yamaguchi, 1973; 1974; Lucas, 1982; 1984) there have been very few reports of early juveniles (< 20-mm diameter) in nature. The young of many coral reef echinoderms are rarely seen (Yamaguchi, 1973) and those of A. planci have remained elusive despite the concerted efforts of researchers. Early juveniles have been reported by Pearson and Endean (1969) in the basal branches of Acropora spp. in shallow water on the reef edge of Green Island and nearby Fitzroy Island on the Great Barrier Reef. Juveniles of 50-100 mm have occasionally been located in similar habitats (Endean and Stablum, 1973; Laxton, 1974; Moran et al., 1985). In this study we describe in detail the habitat of the juveniles and monitor recruitment over a period of 9 years. The growth, behavior and general ecology of a single cohort is traced over almost 2 years.