QUANTITATIVE VARIATION OF SECONDARY METABOLITES IN THE SEA HARE APLYSIA-PARVULA AND ITS HOST-PLANT, DELISEA-PULCHRA

We measured quantitative variation of structurally similar halogenated furanones in the sea hare Aplysia parvula (Opisthobranchia; Anaspidea ) and in its host alga Delisea pulchra (Rhodophyta; Bonnemaisonales). Mean total levels of furanones from D. pulchra in A. parvula were 13.3 % of the dry weight of the sea hares, with one metabolite comprising o n average 86% of the total metabolite load of the sea hares. Levels of furanones in the sea hares were highest in the digestive gland but we re also found in other tissues, including the skin in at least mg g(-1 ) (dry wt) levels. Metabolite levels in the skin of the sea hares did not differ from those in D. pulchra (typically between 5 and 10 mg g(- 1) dry wt). Variation of metabolites in D. pulchra on the scale of bot h meters and kilometers was low and only minor variation in levels of individual metabolites was observed. There was significant variation i n levels of metabolites within plants, with concentrations generally h igher at the distal end of the thallus. The large differences in metab olite concentrations on a scale of millimetres found in some other red algae were not observed in D. pulchra. Metabolites occurred in signif icantly different relative amounts in A. parvula versus D. pulchra, wi th concentrations of individual metabolites in A. parvula ranging betw een 0 and 83 times the concentrations found in host algae. These resul ts show that the sea hares differentially bioaccumulate algal metaboli tes. However, only 1 metabolite was enhanced in concentration in the s ea hares relative to the alga. This metabolite was the most abundant c ompound in the animals, and has been previously shown to be effective as a predator deterrent. A second metabolite, which was not effective as a predator deterrent in A. pasvula, decreased in relative concentra tion in the animals. in contrast to recent suggestions in the literatu re for sequestered algal metabolites in sea hares, our results indicat e that the distribution and level of D. pulchra metabolites in A. parv ula are consistent with a role as acquired chemical defences against p redators.