Graduate Thesis Or Dissertation
 

Marine herbivore-plant interactions : the feeding ecology of the sea slug Placida dendritica

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  • The feeding ecology of a common temperate mesoherbivore, the oligophagous sea slug Placida dendritica (Gastropoda: Opisthobranchia: Ascoglossa), was examined from April 1985 to June 1989. Along the central coast of Oregon, Placida consumed three host species: the low intertidal green algae Codium setchellii, C. fragile, and Bryopsis corticulans. Individual slugs tended to specialize on a single algal host species with limited capacity to change host species. In laboratory experiments, many individuals died in the presence of unfamiliar host species that sympatric conspecifics consumed. This rigid diet specificity was not modifiable through hunger level, algal condition, or intraspecific interaction. Placida populations, therefore, were functionally subdivided into sympatric subpopulations. Placida formed feeding congregations on Codium spp. Members of groups composed of similar-sized slugs grew significantly faster than solitary individuals. In mixed-sized congregations, however, trophic benefits were not shared equally among all members. Small slugs always benefited from the presence of conspecifics; large slugs, however, benefited only if conspecifics were large. The mechanisms of intraspecific facilitation involved both behavioral stimulation of feeding and modification of algal food quality. Placida was numerically and functionally the major herbivore of C. setchellii. The alga had a partial refuge from the slug in high sand or wave disturbance habitats. Placida's attack was concentrated on algal thalli whose anti-herbivore defenses were probably compromised by stress: thalli in desiccation-prone microhabitats and thalli with existing grazing damage. Field transplant experiments indicated that Placida's herbivory may restrict the between-habitat distribution of C. setchellii along the central coast of Oregon.
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