MORTALITY-RATES OF JUVENILE STARFISH ACANTHASTER PLANCI AND NARDOA SPP MEASURED ON THE GREAT-BARRIER-REEF, AUSTRALIA AND IN OKINAWA, JAPAN

Acanthaster planci (L.) and Nardoa novaecaladoniae (Perrier, 1875) are two coral reef asteroids having planktotrophic and lecithotrophic lar val development, respectively. Comparative sizes at metamorphosis are 0.5 to 0.7 mm for A. planci and 1.2 to 1.6 mm for N. novaecaladonine. Mortality rates of small juveniles (one month old) of each species wer e measured experimentally in the field on the Great Barrier Reef, Aust ralia. Mortality rates of N. novaecaladoniae were low (1.5 %.d(-1)) co mpared to 7.8 %.d(-1) for A. planci. Survival of the two species was s imilar between habitats. However, mortality rates of A. planci were hi ghly variable both within-sites and between-sites within-habitats (for e reef 15 m depth, reef flat 2 m and back reef lagoon 12 m). There was no apparent effect of density of A. planci on mortality rates. Mortal ity is thought to be principally due to predation by infauna which are abundant in the coral reef rubble. A study of survival rates of newly metamorphosed Nardoa sp. (1.0 to 1.2 mm) in Okinawa, Japan, found ver y low mortality rates of just 0.2 %.d(-1). The abundance of potential predators among the rubble infauna was very low on the Okinawan reef c ompared to the Great Barrier Reef. These studies provide evidence of t he importance of predation as a determinant of survival rates of small starfish and that a reproductive strategy providing for a large size at settlement facilitates greater survivorship.