HOMARINE AS A FEEDING DETERRENT IN COMMON SHALLOW-WATER ANTARCTIC LAMELLARIAN GASTROPOD MARSENIOPSIS-MOLLIS - A RARE EXAMPLE OF CHEMICAL DEFENSE IN A MARINE PROSOBRANCH

The common bright yellow antarctic lamellarian gastropod Marseniopsis mollis was examined for the presence of defensive chemistry. Proton nu clear magnetic resonance (NMR) spectroscopy indicated that a major com ponent of ethanolic extracts purified by reversed-phase column chromat ography was homarine. Further high-performance liquid chromatography ( HPLC) analysis of the mantle, foot, and viscera verified the presence of homarine in all body tissues at concentrations ranging from 6 to 24 mg/g dry tissue. A conspicuous macroinvertebrate predator of the shal low antarctic benthos, the sea star Odontaster validus, always rejecte d live individuals of M. mollis, while readily feeding on pieces of fi sh tail muscle. Filter paper disks treated with shrimp elicited a broa d range of feeding behaviors in the sea star O. validus (movement of d isc to mouth, extrusion of cardiac stomach, humped feeding posture). S hrimp disks treated with homarine (0.4 and 4 mg/disk) were rejected by O. validus significantly more frequently than control disks treated w ith solvent carrier and shrimp or shrimp alone. The highest concentrat ion of homarine tested not only caused feeding deterrence, but in seve ral sea stars a flight response was noted. Homarine was not detected i n the tunic of the antarctic ascidian Cnemidocarpa verrucosa, a presum ed primary prey of M. mollis. Nonetheless, crude extracts of the epizo oites that foul the tunic (primarily the bryozoans and hydroids) conta in homarine, suggesting M. mollis may ingest and derive its chemistry from these organisms. This appears to be only the third example of che mical defense in a member of the Order Mesogastropoda. As the vestigia l internalized shell of M. mollis is considered a primitive condition, the findings of this study lend support to the hypothesis that chemic al defense evolved prior to shell loss in shell-less gastropods.