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ABSTRACT1. Spirobranchus giganteus is a ubiquitous serpulid polychaete that is an obligate associate of living coral. At Sodwana Bay, South Africa, it is a conspicuous associate of Acropora clathrata plates.2. This study explores the relationship between S. giganteus abundance and factors hypothesized to represent stress to the corals, in particular coral bleaching, diving intensity and exposure to destructive wave action as a function of depth. In addition, plate size was investigated to increase understanding of habitat selection by S. giganteus.3. There was no significant correlation between diving intensity and S. giganteus abundance on the three reefs studied, suggesting either that the current level of diving intensity causes no stress or that S. giganteus does not indicate stress.4. Depth influenced the size distribution of A. clathrata plates, with the larger plates being at greatest depths (>0.4 m 2 at depths >14 m). Extremely high S. giganteus densities (most plates with number of individuals >50 m À2; maximum number of individuals 413.63 m À2 at 9 m depth on one reef) were recorded at depths >14 m, with density increasing overall with depth.6. The high densities of S. giganteus on A. clathrata plates are possibly because the plates are a physically elevated platform above the reef, where factors such as predation, competition and smothering by sand are far less important than on the reef surface. In turn, upwelling of nutrients and dependent plankton may encourage S. giganteus to maintain high population levels.7. Climatic or other stresses are not considered currently to be responsible for these high S. giganteus densities. Rather, the worm data reflect a healthy upwelling, which may change as the coral reef becomes stressed with global climate change.

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