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LARVAL DISPERSAL AND SETTLEMENT
29. Where do the larvae of crown-of-thorns starfish settle on reefs? What happens after settlement takes place? This is another aspect of the early life cycle of the crown-of-thorns starfish for which there are very poor biological and ecological data. The only way to determine where larvae of the crown-of-thorns starfish settle is to go and search for them on reefs. This is an almost impossible task given that they are so small (a newly settled starfish is about O.3-0.5 mm in diameter) and reefs are composed of countless places (eg. small cracks and crevices) where they can hide. Since it is thought that small starfish are unlikely to move great distances during their first year of life scientists have turned their attention to trying to find larger juvenile starfish (ie. from 6-12 months of age (10-700 mm)) in order to obtain data on where larvae settle and the critical factors involved in their survival. This also has proved to be a very difficult undertaking and large populations of juveniles have only been found infrequently on reefs; more often a result of good fortune rather than good planning. Information concerning large populations of juvenile crown-of-thorns starfish has come from the Great Barrier Reef and several other places in the Indo-Pacific region. By far the best data come from a population (estimated to comprise many millions of individuals) which was found on a reef near Fiji. Even in this study the starfish could not be recognised individually because they could not be tagged (see 30). Nonetheless, repeated sampling allowed scientists to follow the general change in the average size and density of individuals in the population. This was the first time data of this sort had been obtained. Good correlation was found between these growth data and those obtained for specimens which had been developed in captivity. As a result of this work scientists have a better understanding of the relationship between the age and size of young starfish observed on reefs. The study also was noteworthy in that it recorded a catastrophic decline in the population which was attributed mainly to the effects of a disease (see 32). Most juvenile populations (including the one found in Fiji) have been found in shallow water, on the tops of reefs (eg. reef flats). This has lead to the assumption that the larvae of crown-of-thorns starfish normally settle in shallow water. This may not be true however. If it were then most coral mortality during outbreaks would be expected to occur initially in these areas. Field data from a large number of reefs in the Great Barrier Reef indicate that corals in deeper water (at the base of reefs) normally are the first to be killed during the initial stages of outbreaks (see 24). Given this, it is unlikely that starfish settle in shallow water and migrate to deeper water without leaving evidence of their movement (by way of numerous feeding scars). A satisfactory explanation of this dilemma will not be forthcoming until more extensive field data on the distribution and abundance of juveniles are obtained. Such information would be extremely useful in understanding the mechanisms responsible for settlement of larvae; predicting the occurrence of outbreaks on reefs; controlling outbreaks and developing mathematical models of outbreaking populations.
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