Dynamics of algal secondary metabolites in two species of sea hare

The function of acquired algal secondary metabolites in sea hares is the su bject 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 re d algae Laurencia obtusa and Delisea pulchra and for sea hares collected fr om these seaweeds in the field. The patterns and dynamics of algal secondar y metabolites were further investigated in the laboratory by quantitative a nalysis 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 abund ant metabolites from each red alga, the terpene palisadin A from L. obtusa, and the halogenated furanone 3 from D. pulchra, and stored a greater propo rtion of these metabolites than other algal metabolites. A. parvula accumul ated D. pulchra metabolites at much higher levels than L. obtusa metabolite s. A. dactylomela accumulated similar concentrations of L. obtusa metabolit es to A. parvula. The loss of L. obtusa metabolites by A. dactylomela match ed that expected for dilution of metabolites via growth of the sea hares. H owever, the loss of L, obtusa metabolites by A. parvula was faster than pre dicted for growth alone, suggesting that metabolites were actively metaboli zed 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 wer e analyzed for the presence of algal secondary metabolites to investigate o ne possible path of excretion. L. obtusa secondary metabolites were detecte d in the mucous and opaline secretions of A. parvula, but D. pulchra metabo lites were not detected in any secretions. The deployment of L. obtusa seco ndary metabolites in secretions by A. parvula may explain the more rapid ra te of loss of these compounds and is consistent with a possible defensive r ole for acquired metabolites.