DIETARY INDUCTION OF OPISTHOBRANCH MORPHOLOGY: PLACIDA DENDRITICA (ALDER & HANCOCK, 1843) ON DIFFERENT GREEN ALGAL HOSTS

The relationship of phenotypic morphological and behaviouralvariation was investigated for the ascoglossan (=sacoglossan)opisthobranch Placida dendritica. Morphological attributes weredocumented for individual slugs collected from three algal hostspecies along the central Oregon coast, USA, the green algaeCodium setchellii, C. fragile, and Bryopsis corticulans. Becauseindividuals on B. corticulans generally were substantially largerthan conspecifics on Codium spp., the species appeared polymorphic.Comparably sized slugs from different host genera also differedsignificantly in morphology: individuals from Codium spp. hadmore numerous, shorter cerata on wider bodies than did conspecificsfrom B. corticulans. Principal components analysis indicatedthat despite considerable overlap of morphological characters,ceratal shape features were diagnostic of algal host. When slugscollected from C. fragile were maintained on C. fragile andB. corticulans for 4 days in the laboratory, the size and shapeof slugs fed different diets diverged, and changes were consistentwith field data. Thus, phenotype was labile and changed as aresult of modified diet. Although a variety of molluscs exhibitdietary induction of morphology, a general awareness of thephenomenon has only recently begun to emerge, despite its importantecological and taxonomic implications. (Received 5 February 1996; accepted 2 July 1996)