Note on an invasion of intertidal zoanthid colonies by a chaetopterid polychaete at Park Rynie Beach,KwaZulu-Natal,South Africa

At Park Rynie Beach on the KwaZulu-Natal coast, South Africa, four species of zoanthids were invaded by a sand-tube building polychaete Mesochaetopterus minutus. It is a small polychaete about 15 mm long, which occurs gregariously in dense masses of sandy tubes. M. minutus is an opportunistic species that exploits zoanthid colonies mainly for support and protection from heavy wave action because of the inability of the fragile sandy tube to survive in exposed habitats. Population density studies of the polychaete were undertaken to quantify the degree of invasion of the different zoanthid species. Highest polychaete density was recorded for Zoanthus sansibaricus, the number of polychaetes exceeding 3 500 m^?2, followed by Palythoa nelliae, with a density of 2 200 m^?2. Polychaete densities were comparatively low in Z. durbanensis and Z. natalensis (<1 000 m^?2). No polychaete tubes, however, were seen among the polyps of Isaurus spongiosus.