Disturbance and Monopolization of a Spatial Resource by Zoanthus Sociatus (Coelenterata, Anthozoa)

Zoanthus sociatus is a dominant member of the subtidal Zoanthus zone assemblage at Discovery Bay, Jamaica. The benthic biota at four of five study sites include this zoanthid and several algal genera. Z. sociatus is present but much less abundant than are Z. solanderi and several other cnidarians at a fifth site on the east back reef (Karlson, 1980). Sedimentation data and changes in the percentage of bare substratum suggest that this EBR site is relatively protected from physical disruption caused by storms. Intermediate levels of storm disruption characterize the west back reef; shallow fore reef zones have the highest levels (Woodley et al., 1981).I have conducted a series of substratum disruption experiments at the WBR and at nearby one palm island (1PI) to document the response of this assemblage to disturbance. Recolonization experiments resulted in growth by Z. sociatus at 0.41-1.28 cm/mo in both control and exclosure cages. Lower zoanthid growth rates were typical of clearings in which the sea urchin Diadema antillarum was present, algae absent, and Z. sociatus the only recolonizing species. These zoanthids tended to be well attached to the substratum and not very susceptible to storm damage. Other experiments demonstrate the high regenerative capability of Z. sociatus in response to substratum overturning, shading, transplantation, and predation. Z. sociatus exhibited changes in polyp size (possibly altering susceptibility to storm damage and mode of nutrition), phototropic growth responses, and slow colony degeneration in unfavorable microhabitats.The plasticity of polyps and the regenerative characteristics of zoanthid colonies suggest that disturbance has been an important selective agent over the evolutionary history of Z. sociatus. Its successful exploitation and eventual monopolization of disturbed substrata may result from intermediate levels of disturbance. This is suggested as an alternative to the traditional explanation of competitive dominance at low disturbance levels.