Larval supply, settlement and recruitment In echinoderms

2Over the past 2 decades, numerous studies have addressed settlement and recruitment patterns of echinoderms, particularly species (mainly echinoids and asteroids) of ecological or economic importance. However, the processes which regulate the transitions from early life stages through to adulthood, and thereby determine the demography, distribution and abundance of echinoderm populations, remain poorly understood. The supply of planktonic larvae of echinoderms to benthic populations is regulated by a complex interaction of biotic and abiotic factors such as hydrodynamics, sea temperature, predation and starvation. Echinoderm larvae have been induced to settle on various substrata in laboratory studies and specific chemical inducers have been isolated in some cases. However the importance of settlement induction and substratum preferences in determining settlement patterns in natural habitats is not clear. A salient feature of most echinoderm populations is a high degree of spatial and temporal variability in settlement and recruitment. Settlement variation exists at spatial scales ranging from metres between adjacent habitats to hundreds of kilometres between regions. Seasonal settlement is common among echinoderm species but interannual variation in settlement and/or recruitment is often high, and several years may elapse between successful recruitment events. Some of this variability is likely attributable to measurement inaccuracies associated with sampling individuals which are small, cryptic and transient, and procedural inconsistencies among different studies. A variety of post-settlement, processes including predation, migration, disease and starvation may alter observed patterns of settlement and play an important role in regulating recruitment rates and patterns. This review attempts to consolidate our understanding of the importance of early life history events in determining population structure and dynamics of echinoderms in order to identify deficiencies and suggest fruitful avenues for future research.