MEASURING SETTLEMENT INTENSITY OF ECHINODERMS ON CORAL-REEFS

A pilot study was conducted at Davies Reef on the central Great Barrie r Reef between 20 November 1991 and 20 January 1992 to assess the suit ability of settlement collectors for measuring larval settlement rates of echinoderms on coral reefs. The collectors were deployed for two m onths during summer when many echinoderm species are known to spawn. A total of 657 newly settled echinoderms were recovered from just 47 co llectors, each having a volume of just 0.005 m(3) but with a utilisabl e surface area of about 1.44 m(2), indicating that competent-to-settle larvae were very abundant during the sampling period. Echinoids (7.8 sampler(-1)) and ophiuroids (5.6 sampler(-1)) were the most abundant g roups on the collectors. Asteroids (0.2 sampler(-1)) and crinoids (0.3 sampler(-1)) were less common. The asteroids were all identified to s pecies and included the crown-of-thorns starfish Acanthaster planci, C horiaster granulatus and Culcita novaeguineae. The newly settled echin oids could not be distinguished from each other but included Echinomet ra mathaei and Mespilia globulus. The abundance of each of the five di fferent classes of echinoderms on the samplers was correlated with the ir abundance on the natural substratum. Significant spatial variabilit y was found in settlement rates of echinoids over 1000s of metres and ophiuroids over 100s of metres, but not over smaller spatial scales. I t is concluded that the collectors can provide a useful tool for monit oring spatial and temporal variability in settlement rates of echinode rms on coral reefs and for testing hypotheses about patterns of larval dispersal.