Effects of algal diet on the performance and susceptibility to predation of the sea hare Aplysia parvula

Abstract

Although sea hares are well known for acquiring algal secondary metabolites from their diet, the effects of diet on fitness and the susceptibility of sea hares to predators are poorly under- stood. We examined the effects of diet on the performance of the sea hare Aplysia parvula, and mea- sured predation by reef fishes on sea hares raised on different seaweeds. Diet-switching, body size, the presence of conspecifics, and the presence of epiphytes on dietary algae were also investigated experimentally. A. parvula were fed the chemically rich red algae Delisea pulchra or Laurencia obtusa in laboratory experiments, and consumption, growth, egg production, conversion efficiency, and survivorship were measured. Sea hares fed L. obtusa consumed more algae, grew faster, laid more eggs, and had higher survivorship compared to sea hares fed D. pulchra. Most other factors investigated—body size, the presence of conspecifics or epiphytes —were in general unimportant relative to the effects of diet. The disparity in fitness of A. parvula fed D. pulchra versus L. obtusa is primarily due to different levels of consumption of each seaweed, as conversion efficiencies were similar. Predation on sea hares raised on different diets and containing different types and levels of acquired secondary metabolites was measured in field experiments in which sea hares were exposed to mixed assemblages of reef fishes. A high proportion of both chemically rich (fed red algae) and chemically poor sea hares (fed the green alga Ulva sp.) were eaten by fishes (with predation rates of 25 to 55% over 1 to 2 h), although this sea hare was relatively unpalatable compared to squid tissue. Juvenile A. parvula were eaten at a significantly greater rate than adults. Sea hare ink did not deter some fishes, which consumed both de-inked and untreated sea hares. Predation by fishes was simi- lar in adjacent habitats, but varied between sites, and was not restricted to A. parvula as reef fishes also ate juvenile A. dactylomela of equivalent size. The benefit A. parvula gain from acquiring algal secondary metabolites is unclear, as these compounds appear ineffective as defences against some fishes. A. parvula may sequester algal secondary metabolites as part of a broader defensive strategy that includes crypsis and escape movements.