The ovulid gastropod Cyphorna gibbosum IS a widespread and common predator of gorgonian octocorals ('sea whips') on Caribbean coral reefs. C. gibbosum is not randomly distributed, but tends to be clumped into small groups. To examine possible causes of this phenomenon, field and laboratory experiments were performed with C. gibbosum and Plexaura hornornalla, a common Caribbean gorgonian. These experiments suggest that clumping in C. gibbosum is produced by mucous trail following. Gregariousness may have evolved in C. gibbosum to increase the protection afforded by distastefulness and aposematism, although other explanations cannot be excluded. Octocoral-gastropod interactions possess several important parallels to terrestrial plant-herbivore systen~s. Further comparison of these systems may provide new insights into predator-prey coevolution.

The ovulid gastropod Cyphoma gibbosum Linnaeus is a specialist consumer of gorgonian octocorals. Although the direct effects of its feeding do not appear to be great, the indirect consequences may be substantial. An 8 wk study of the gorgonian Plexaura hornornalla Esper in Curaqao found that C. g~bbosum occurred primarily on the lower 30 cm of colonies, and that there were significant correlations between C glbbosum density and the extent of colony fouling on the lower 30 cm of colonies. This correlation occurred whether one measured in~tial fouling, final fouhng, or change in fouling during the 8 wk. There were no significant correlations of C. gibbosum density and fouling for colony portions more than 30 cm above the holdfast. These findings and direct observations in the field suggest that feeding by C. gibbosum increases fouling of gorgonians by exposing the axial skeleton and providing settling sites for fouling propagules. Because fecundity of P. homornalla is reduced when fouhng separates colonies into a series of smaller aggregates, the indirect effects of C. g~b b o s u m grazing could strongly affect the fitness of P. hornornalla and other gorgonians.