A new antifouling assay method: Results from field experiments using extracts of four marine organisms

Antifouling is one possible defensive function of marine natural products isolated from sessile benthic organisms, but there is little experimental evidence to support this claim. We developed a technique in which crude organic extracts of marine organisms are incorporated into hard, stable gels that serve as substrata for larval settlement in the held. These gels contain extracts at concentrations that are volumetrically equivalent to those in living tissues, and compounds diffuse from assay gels in a manner that may mimic their natural release from some organisms. After 21 days in flowing seawater, a mean of 56% of the mass of crude extract from the sponge Hymeniacidon heliophila (Parker) remained in gels. Extracts from two sponges, Aplysilla longispina (George & Wilson) and Hymeniacidon heliophila, an ascidian, Eudistoma hepaticum (VanName), and an alga, Codium decorticatum (Woodward & Howe), were incorporated into gels and deployed in the held over a period of 28 days; extracts from A. longispina deterred settlement of invertebrates and algae relative to control gels, while extracts from C. decorticatum enhanced settlement. Mean settlement on control gels was similar to that on plexiglas plates of the same size. This technique represents a more ecologically relevant method for assaying the antifouling properties of extracts of marine organisms because (1) assay gels are exposed to a natural population of settling propagules, (2) extracts are incorporated into gels at natural volumetric concentrations, and (3) extracts within the gel matrix do not alter the physical characteristics of the settlement surface.