Foraging Patterns of Cyphoma gibbosum on Octocorals: the Roles of Host Choice and Feeding Preference

¹ Department of Biological Sciences, State University of New York at Buffalo, Buffalo, New York 14260
² Department of Biological Sciences, State University of New York at Buffalo, Buffalo, New York 14260 and Division of Biology and Living Resources, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Cswy., Miami, Florida 33149
³ Division of Biology and Living Resources, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Cswy., Miami, Florida 33149

The distributions of small, relatively slow moving grazers are often portrayed as expressions of feeding preference. However, more careful analysis of such distribution patterns in concert with measurements of feeding suggests that the distribution patterns of the gorgonian-eating snail Cyphoma gibbosum may not reflect the influence of feeding preference alone. C. gibbosum is common at many sites in the San Bias Islands, Panama, occurring at densities ranging from 2-30 snails/100 m². The movements and feeding of 244 labelled snails were followed at three sites in the San Bias during the summers of 1984-1986. Relative to gorgonian colony abundance C. gibbosum were preferentially found on colonies of Pseudopterogorgia spp., Pseudoplexaura spp., and Plexaura homomalla. Snails exhibited the same preferences in their movements between colonies, and often remained longer on those same gorgonian species.

The prolonged occupancy on colonies of some species was the dominant factor controlling the amount of tissue consumed from any one species. Thus preferences for different host gorgonians also established total feeding. Preferences loosely correlated with the organic content of the different gorgonian species (% ash-free dry weight), but do not explain the observed preference for P. homomalla colonies. Host preferences were not reflected in feeding rates. Feeding rates on Pseudopterogorgia colonies were lower than on other frequently occupied species. The weak correspondence between feeding rates and occupancy preferences suggests that factors in addition to feeding such as social interactions and predator avoidance may play an important role in establishing host preferences.