Temporal and spatial variability among grazers: variability in the distribution of the gastropod Cyphoma gibbosum on octocorals

Abstract

Censuses of the gorgonian grazing gastropod Cyphoma gibbosum were conducted at 3 sites in the San Blas Islands. Panama. during the summers of 1981 to 1986. Snail density at the sites varied as much as 6-fold between years and sites. The most commonly occupied gorgonians were Pseudopterogorgia spp., Plexaura homomalla and Pseudoplexaura spp. They were occupied by snails in numbers disproportionate to the gorgonian's abundances. C. gibbosum exhibited significant clumping in their distribution on colonies. Most individuals exhibited strong preferences for a single species or in some cases for a single gorgonian colony. Snails observed over multiple years often switched pre-ferences between years. Individual preferences, the movement of different snails through the study sites, and the formation and breakup of aggregations created significant temporal variability in observed preferences. Even greater levels of variation in preferences were present in distribution data collected during single visits to reefs at 17 sites throughout the Caribbean. Preferences at these sites as well as other published accounts only weakly support the preferences observed in the San Blas. Pseudopterogorgia spp. were most often preferred but at only 29% of sites. Data suggest that C. gibbosum distribution patterns are controlled by a combination of species preferences and social behaviors which lead to aggregated distributions. The combination of effects leads to distribution patterns which mask overall preferences and which may bias short-term observations. When examined over longer periods and/or across multiple sites a pattern emerges in which snails are distributed among a combination of actively preferred and randomly chosen species. Thus the distribution pattern is driven both by differences among host species and by stochastic processes. Recognition of these different mechanisms of host choice will be important in predicting the effects of grazers like C. gibbosum on their prey populations.