Dynamics of algal secondary metabolites in two species of sea hare

Abstract

The function of acquired algal secondary metabolites in sea hares is the subject of debate, in part because the dynamics/processing of metabolites by sea hares is poorly understood. This study investigates the dynamics of red algal secondary metabolites in two sea hares, Aplysia parvula and Aplysia dactylomela. Secondary metabolite levels were quantified for the dietary red algae Laurencia obtusa and Delisea pulchra and for sea hares collected from these seaweeds in the field. The patterns and dynamics of algal secondary metabolites were further investigated in the laboratory by quantitative analysis of secondary metabolites in sea hares grown on diets of L. obtusa, D. pulchra, or the green alga Ulva sp. Sea hares accumulated the most abundant metabolites from each red alga, the terpene palisadin A from L. obtusa, and the halogenated furanone 3 from D. pulchra, and stored a greater proportion of these metabolites than other algal metabolites. A. parvula accumulated D. pulchra metabolites at much higher levels than L. obtusa metabolites. A. dactylomela accumulated similar concentrations of L. obtusa metabolites to A. parvula. The loss of L. obtusa metabolites by A. dactylomela matched that expected for dilution of metabolites via growth of the sea hares. However, the loss of L. obtusa metabolites by A. parvula was faster than predicted for growth alone, suggesting that metabolites were actively metabolized or excreted. Data for the loss of D. pulchra metabolites by A. parvula was equivocal. The secretions of A. parvula fed D. pulchra or L. obtusa were analyzed for the presence of algal secondary metabolites to investigate one possible path of excretion. L. obtusa secondary metabolites were detected in the mucous and opaline secretions of A. parvula, but D. pulchra metabolites were not detected in any secretions. The deployment of L. obtusa secondary metabolites in secretions by A. parvula may explain the more rapid rate of loss of these compounds and is consistent with a possible defensive role for acquired metabolites.