Organ-specific distribution of dietary alkaloids in the marine opisthobranch Tylodina perversa

Specimens of the opisthobranch Tylodina perversa that were observed while feeding on the sponge Aplysina aerophoba were transferred to seawater tanks along with their prey and kept under controlled conditions. After one week the opisthobranchs were anaesthetized, dissected and studied for sequestered sponge-derived brominated alkaloids. All parts of T. perversa analyzed including feces, mucus and egg masses that had been produced during captivity contained alkaloids derived from A. aerophoba. The highest total alkaloid concentration (24.6 mg g(-1) dry wt) was found in mantles of T. perversa (compared to 51.2 mg g-l dry wt of total alkaloids in A. aerophoba), Hepatopancreas, egg masses and mucus (respective total alkaloid concentrations ranging from 20.4 to 12.5 mg g(-1) dry wt) were also rich in alkaloids. Whereas in A. aerophoba the isoxazoline alkaloids aerophobin-2 and isofistularin-3 were present in almost equal concentrations, aerophobin-2 constituted by far the major alkaloid (amounting to approximately 70% of all identified alkaloids) in mantles, mucus and egg masses of T. perversa, indicating selective sequestration by the opisthobranchs. Mantles as well as mucus also contained appreciable concentrations (approximately 20% of all identified compounds) of the brominated alkaloid aerothionin; this is not detected in A. aerophoba. It is possible that aerothionin originates from a previous encounter of T. perversa with the sponge A. cavernicola, the latter being closely related to A. aerophoba. The enrichment of aerophobin-2 (and of aerothionin) in mantles, mucus and egg masses that are vulnerable and exposed (mantles and egg masses) to predators and/or pathogens argues for defensive functions of the respective alkaloids even though this hypothesis still needs to be experimentally corroborated. (C) 1999 Elsevier Science Ltd. All rights reserved.