Post-ingestive consequences of consuming secondary metabolites in sea hares (Gastropoda: Opisthobranchia)

Abstract

Secondary metabolites of plants and animals often deter feeding by potential consumers; whether they also have negative post-ingestive consequences is debated. We fed three secondary metabolites to the sea hare Aplysia juliana and two to A. kurodai, at approximately 1% of the dry mass of their diet. Luffariellolide, a sponge secondary metabolite, and malyngamide B, a cyanophyte metabolite, significantly reduced the growth of Aplysia juliana; pachydictyol A, a brown algal metabolite, had no effect on growth. Previous studies have shown that blood-glucose concentrations of sea hares are affected by many types of stress. Blood-glucose concentrations of Aplysia juliana were affected by secondary metabolites in the diet; however, this effect was not correlated with the toxicity (effect on growth) of the metabolites. Aplysia juliana sequestered pachydictyol A and malyngamide B in the digestive gland at c. 1.5% dry mass, an order of magnitude higher than luffariellolide. Patterns of growth of Aplysia kurodai fed luffariellolide and pachydictyol A were similar to those of Aplysia Juliana, but were not statistically significant. Blood-glucose concentrations of Aplysia kurodai were not significantly affected by secondary metabolites. Aplysia kurodai sequestered pachydictyol A and luffariellolide at similar concentrations, as did Aplysia juliana. We conclude that some secondary metabolites may reduce the growth of sea hares, but that blood-glucose concentrations offer little prospect for use as an indicator of this stress.