Studies on the biology of three species of sea urchin (Echinodermata : Echnoidea), on the South African east coast.

Abstract

Ten species of shallow water echinoid are found on the subtropical east coast of South Africa. Although their distributions are patchy, the most common species, Echinometra mathaei, stomopneustes variolaris and Diadema savignyi, are nontheless conspicuous components of intertidal communities on this coast. As little was known about these three species, the overall intention of this study was to provide some fundemental information on their biology and ecology. For the purposes of achieving this aim a life history approach was adopted, where the relative investments by each species in growth, maintenance and reproduction were investigated and compared. These patterns of investment were then related to the habitat occupied by each species, in an attempt to identify the selective forces which may have been implicated in shaping their life histories. It was apparent from the results of investigations conducted between January 1991 and June 1993 that there were distinct differences in the patterns of investment in growth, maintenance and reproduction between the three species. The life history of S. variolaris, which occupied exposed habitats in the lower intertidal, was characterised by a large investment in maintenance, lower reproductive output, slower growth and a longer lifespan, relative to the other two species. In contrast, Q. savignyi, which inhabited less exposed mid-shore pools, had a relatively higher reproductive output, more rapid growth, a smaller investment in maintenance and a shorter lifespan. While selection ln S. variolaris and Q. savignyi appears to favour survival and reproduction respectively, the life history of E. mathaei, a species which also occupies mid-shore pools, was balanced between these two extremes, allocating sufficient resources to maintenance to permit tolerance of harsh physical conditions while still making a moderate investment in reproduction over a lifespan of intermediate duration. The predictions generated by the r-K selection and "bet hedging" theories of life history evolution, were applied in the process of speculating on the selective forces which may have shaped these life histories. However, it was found that neither set of predictions and associated selective forces could adequately explain the observed life histories. Rather, it seemed that the life histories of the three species represented evolved responses to the direct and indirect effects of exposure to wave action and sand movements which dominate the intertidal environment on the South African east coast. In the exposed lower intertidal, unpredictable recruitment, drag and impact forces associated with wave action, which impose limits to body size and necessitate a large investment in maintenance to ensure survival, select for slow growth, low reproductive output and high longevity. In contrast reproduction and growth of species occupying the more sheltered mid-shore pool habitats would be less effected by the demands of maintenance investment or limits to body size. In addition predictable recruitment in the mid-shore, would obviate the need for long life in order to ensure a contribution to future generations.