Factors associated with oligophagy in two species of sea hares (Mollusca: Anaspidea)

Abstract

Unlike most marine invertebrate herbivores, many sea hares (Genus Aplysia) are relatively specialised feeders. In order to assess the factors associated with narrow diet breadth of Aplysia parvula (Guilding) Mörch 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 deterrence 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 ate 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. The nutritional value (% nitrogen content and % organic matter) of dietary algae for the sea hares was not higher than in co-occurring non-dietary algae. The growth and conversion efficiency of A. parvula varied strongly with diet and also varied depending on the host plant from which 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, and 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 not deter feeding by fishes. These experiments suggest that deterrence or avoidance of predators strongly affects food choice by A. parvula and further suggest that accumulation of D. pulchra metabolites by A. parvula has a significant cost to these animals. The factors affecting diet breadth for A. juliana are less clear. This species may gain a spatial escape from predation by specialising on the intertidal alga U. lactuca.