FACTORS ASSOCIATED WITH OLIGOPHAGY IN 2 SPECIES OF SEA HARES (MOLLUSCA, ANASPIDEA)

Unlike most marine invertebrate herbivores, many sea hares (Genus Aply sia) are relatively specialised feeders. In order to assess the factor s associated with narrow diet breadth of Aplysia parvula (Guilding) Mo rch and A. juliana (Quoy & Gaimard) near Sydney, N.S.W., occurrence of Aplysia spp. on host plants, algal abundance and nutritional quality, growth and consumption of different diets by Aplysia spp. and deterre nce of predators by sequestered algal metabolites were measured. Both species only occurred on, and only ate, one (A. juliana) or a few (A. parvula) species of algae. Aplysia juliana only occurred on and only a te the green alga Ulva lactuca (Linnaeus), which was locally abundant and lacks secondary metabolites. The major host-plants for A. parvula, the red algae Delisea pulchra (Greville) and Laurencia obtusa (Hudson ), were low in abundance, but were both rich in secondary chemistry. T he nutritional value (% nitrogen content and % organic matter) of diet ary algae for the sea hares was not higher than in co-occurring non-di etary algae. The growth and conversion efficiency of A. parvula varied strongly with diet and also varied depending on the host plant from w hich the animals were collected. Surprisingly, A. parvula did not grow when fed only D. pulchra-a main host plant-in the laboratory or field . Aplysia parvula sequesters secondary metabolites from D. pulchra, an d the metabolite sequestered in highest concentrations by the animals significantly deterred feeding by fishes in field assays. In contrast, a second metabolite which is sequestered at low concentrations did no t deter feeding by fishes. These experiments suggest that deterrence o r avoidance of predators strongly affects food choice by A. parvula an d further suggest that accumulation of D. pulchra metabolites by A. pa rvula has a significant cost to these animals. The factors affecting d iet breadth for A. juliana are less clear. This species may gain a spa tial escape from predation by specialising on the intertidal alga U. l actuca.