Partial predation on tropical gorgonians by Cyphoma gibbosum (Gastropoda)

Cyphoma gibbosum (Gastropoda, Ovulidae), a generalized predator on tropical gorgonians, forages selectively among gorgonian species on shallow reefs at St. Croix, U.S. Virgin Islands. Mean residence times of marked snails on prey taxa ranged from 1.8 d on Eunicea spp. to 4.7 and 10 d on Gorgonia ventalina and Plexaurella dichotoma respectively. Juveniles were sedentary and remained on single hosts throughout the study period. An analysis of wounds caused by C. gibbosum on gorgonians revealed that colonies of some taxa were consistently stripped of tissue, baring the underlying proteinaceous axis, while others were only superficially grazed. Wounds baring the axial skeleton were considerably more severe than superficial wounds because the bared skeleton could be colonized by algae causing subsequent tissue regeneration to be inhibited. Within all species, wounds were not restricted to any single portion of the colony. However, most wounds occurred within the central regions of colonies and the lowest portions of colonies were seldom damaged. The short residence times and superficial grazing on some taxa such as Eunicea spp. and Muncea spp. may be a consequence of heavy spicular armor over polyps and over the proteinaceous axis in those taxa. Deep wounds to the axial skeleton occurred only on taxa with smaller spicules and a smaller proportion of spicules in the coenenchyme by dry weight. Residence time on a prey species was inversely proportional to both maxlmum spicule length and percent composition of spicules. Short residence times and shallow wounds on colonies of h e a d y splculed species may be a response by C. gibbosum to foraging on suboptimal, heavlly defended resources Despite the short residence times, these potentially suboptimal prey taxa were regularly visited and preyed upon. This suggests that factors other than spicular architecture, such as gorgonian chemistry, may also play a role in the foraging of C. gibbosum.