On rocky lntert~dal shores in Oregon USA the ascoglossan (= sacoglossan) sea slug Plaada dendnhca A & H consumed the green seaweed Cod~um fragile (Sur ) Har but not the eplphytic red alga Ceram~um cod~cola J Ag attached to C frag~le F~eld stud~es conducted from 1986 to 1992 suggested that herb~vore grazing negatively affected the seaweed Dunng spnng and summer P dendnhca attacked ca 70 % of the large C fraglle (thall~ > l 0 cm) at Boller Bay, Oregon Ascoglossans were most abundant on the axillary region of branches Grazing by the he1 b~vore weakened branches, resulting In branch loss C fraglle thall~ w~t h the epiphyte C cod~cola were attacked more frequently than those w~thout ep~phytes In a 1 mo f~eld expenment P dendnt~ca settlement and survivorsh~p were greater on thalh wlth C cod~cola than on thalli w~t h epiphytes expenmentally chpped Recently settled ascoglossan recruts associated with ep~phyte cover more frequently than did larger ascoglossans The sea slug-seaweed-ep~phyte associahon represents an unusual form of herbivore-plant mteract~on In that (1) herbivores do not consume epiphytes but (2) epiphytes enhance herbivore attack of seaweed

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.

Four common species of temperate NE Pacific ascoglossan (= sacoglossan) sea slugs were examined in 1990 and 1991 to determine the nature and effectiveness of their defensive responses and the potential ecological importance of predation. The 3 small, cryptic species (Placida dendritica, StiLiger fuscovittatus, Alderia modesta) responded to mechanical stimulation by waving and autotomizing cerata (finger-like projections on dorsal surfaces), secreting viscous white fluid, and reducing body surface pH. These responses, however, were ineffective deterrents against common intertidal predatory fishes and crabs: the predators readily consumed the cryptic herbivores in short-term laboratory experiments. Furthermore, predators s~gnificantly reduced densities of A. modesta on algal mats of Vauchena sp. in a 12 d field experiment conducted in Yaquina Bay, Oregon, USA. Thus, small, cryptic ascoglossans were subject to intense predation pressure In contrast, the large, conspicuous Aplysiopsis enterornolphae had more intense defensive responses (e.g. ejection of viscous fluid, acidic secretions), and few predators consumed the ascoglossan.