Host-Plant Change in Marine Specialist Herbivores: Ascoglossan Sea Slugs on Introduced Macroalgae

In the British Isles, oligophagous marine herbivores, particularly the ascoglossan (= sacoglossan) sea slug Elysia viridis, associate with the introduced green macroalga Codium fragile ssp. tomentosoides. Slugs prefer to associate with and consume the introduced C. fragile to the native C. tomentosum. Our investigation of adult, larval, and juvenile E. viridis focused on whether this association is attributable (1) to a host switch or (2) to an expansion from native hosts to the introduced C. fragile ssp. tomentosoides. Growth rates and maximum body sizes of E. viridis on introduced hosts were greater than on natives. Although the native Cladophora rupestris induced a high rate of slug metamorphosis, recently metamorphosed juvenile E. viridis (from Codium fragile-feeding parents) were generally not able to feed or grow on the native alga. In contrast, juveniles from Cladophora-feeding parents could eat Cladophora, although their performance was highly variable. Small, postlarval slugs (