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

Abstract

Investigated the association of Placida dendritica (Gastropoda: Opisthobranchia) with one of its algal food species, the crustose green alga Codium setchellii, along the central coast of Oregon. 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. dendritica occurred on 6-32% of C.setchellii thalli at three low intertidal sites; mean densities were 7-12 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 numerically dominated in fall and winter. Sea slugs damaged the alga more frequently and consumed it more rapidly than did co-occurring, generalist herbivores. 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. 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. -from Author