SECONDARY CHEMISTRY DOES NOT LIMIT DIETARY RANGE OF THE SPECIALIST SEA HARE STYLOCHEILUS-LONGICAUDA (QUOY-ET-GAIMARD 1824)

The sea hare Stylocheilus longicauda (Quoy et Gaimard 1824), (Opisthobranchia: Anaspidea) lives in and feeds on the filamentous cyanobacterium (blue-green alga) Microcoleus lyngbyaceus (Kutz.) Crouan, from which it sequesters the secondary metabolites malyngamide A and B, converting some B into its acetate. To determine if an unwillingness to consume secondary metabolites from potential foods limited the dietary range of Stylocheilus, we fed it a series of artificial diets containing secondary metabolites (malyngamide A and malyngamide B from Microcoleus; caulerpenyne, caulerpin, halimedatetraacetate, pachydictyol A, and ochtodene from other algal species; chondrillin, luffariellolide, and a brominated diphenyl ether from sponges). We performed one set of no-choice feeding trials and two sets of choice feeding trials. The details of the results varied between experiments; however, every secondary metabolite significantly reduced feeding by Stylocheilus in at least one set of trials. Overall, malyngamide A and B were as or more effective at deterring feeding by Stylocheilus than were most other secondary metabolites, suggesting that Stylocheilus is not particularly adapted to the secondary chemistry of Microcoleus, and that the secondary chemistry of other algae does not limit Stylocheilus to a specialized diet. Widespread dispersal of Stylocheilus during its planktonic larval phase probably precludes local adaptation to the geographically variable secondary metabolites found in Microcoleus. Several alternate hypotheses might explain why Stylocheilus specializes despite disliking the secondary metabolites of its host; these hypotheses remain to be tested.