Mesoherbivory: the ascoglossan sea slug Placida dendritica may contribute to the restricted distribution of its algal host

Abstract

The influence of small herbivores in marine rocky intertidal communities has been largely ignored. From 1985 to 1990, I investigated the association of the oligophagous ascoglossan (=sacoglos-San) sea slug Placida dendn'tica (Gastropoda: Opisthobranchia) with one of its algal food species, the crustose green alga Codium setchellii, along the central coast of Oregon, USA. The alga has a restricted local distribution. shaded rocky surfaces in low intertidal areas strongly influenced by sand scour and burial. During spring and summer, P. dendn'tica occurred on 6 to 32 % of C. setchellii thalli at 3 low intertidal sites, mean densities were 7 to 12 sea slugs per occupied thallus The sea slug composed a major, often numerically dominant, component of the herbivore fauna on the alga in spring and summer whereas the small snail Lacuna marmorata numencally dominated in fall and winter. Sea slugs damaged the alga more frequently and consumed it more rapidly than did CO-occurring, generalist herbivores (e.g. snails, gammarid amphipods, idoteid isopods). The alga had a small-size refuge from the sea slug but not a low-density refuge: recruiting larvae were extremely effective at locating all transplants, even at low densities. Sea slugs often attacked damaged algal hosts more frequently than undamaged hosts. Because C. setchellii was more tolerant of physical disturbance than was P. dendritica, the alga had partial refuges from sea slugs in areas with high sand or wave disturbance. Under conditions of low sand movement and low wave force, sea slug herbivory may exclude the alga from low intertidal, rocky sites along the Oregon coast. The restricted distribution of C. setchellii, therefore, may reflect spatial variation in effects of disturbance on sea slug recruitment and herbivory.

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

“…The number of slugs on algal hosts is an index of feeding preferences, as has been well demonstrated by Trowbridge (1992a); the presence of slugs on algae, the growth of slugs, and algal mass loss are all correlated. Individual pre-weighed Elysia viridis collected from Codium fragile ssp.…”

Temperature, Herbivory and Epibiont Acquisition as Factors Controlling the Distribution and Ecological Role of an Invasive Seaweed

“…tomentosoides does have a suite of specialized molluscan predators, of which the ascoglossan P. dendritica is most conspicuous (Trowbridge 1992(Trowbridge , 1993(Trowbridge , 1997(Trowbridge , 2002Trowbridge and Todd 2001;Harris and Mathieson 2000). Two species of the ascoglossan Genus Elysia, E. chloroticus and E. viridis are present in the Great Bay Estuary and coastal zone.…”

Interactions between an ascogiossan sea slug and its green algal host: branch loss and role of epiphytes

“…The ascoglossan's suctorial feeding produces damage distinctly different from that of other grazers (Trowbridge 1992a). Codium thalli are composed of interwoven filaments that form an unpigmented medulla and a pigmented cortical region with siphonlike utricles.…”

“…(3) It was noted whether Placida occurred as a solitary individual or a group member closely associated with other conspecifics; feeding aggregations of Placida create extensive damage to Codium spp. (Clark 1975, Trowbridge 1991b, 1992a.…”