The sessile suspension-feeding wormsnail Dendropoma maxima, Sowerby 1825 (Vermetidae) secretes a mucus net to capture planktonic prey. The nets are spread out over the corals and often have remarkable deleterious effects on them like changes in growth form and pigmentation shifts not uncommonly resulting in tissue necrosis. Until now, there is no explanation for this phenomenon although the indication as well as theories about its genesis is mentioned in several publications. Vermetids are well studied concerning the intraspecific competition with neighboring individuals but not in their interaction with other taxa like corals or fish. We did extensive in situ video recording and observed that fish avoided the plankton-load nets although several specialized taxa are known to be molluscivores, mucivores, and/or feed on plankton. As many molluscs use chemical weapons to combat feeding pressure and to defend themselves against predators, we screened empty and plankton-load mucus nets for potential bioactive metabolites. Bioactivity testing was performed with a recently developed system based on a chromatographic separation (high-performance thin-layer chromatography (HPTLC)) and a bioassay with luminescent bacteria Vibrio fischeri. Thus, we found at least two active compounds exclusively accumulated by the wormsnails themselves. This is the first record of bioactive properties in the whole family of Vermetidae. 1. Introduction The vermetid gastropod Dendropoma maxima, Sowerby 1825 (Vermetidae, Littorinimorpha, Gastropoda) is a dominant, very abundant encrusting species of outer tropical reefs and widespread throughout the IndoPacific [1–3]. In the Red Sea, D. maxima populations can attain substantial densities, a phenomenon largely restricted to that area [4, 5]. Wormsnails mainly live in the infralittoral and upper-circalittoral between the breaker zone and the outer reef edges facing the current [1, 2, 6]. To combat the competition for space and nutrients, wormsnails developed particularly strong substrate preferences. In the Red Sea, substrates are living corals like hydrozoan Millepora spp. (Figure 1(a)) and Porites spp., dead coral rock next to the reef edge, or they can be found on the nonliving reef flat substratum [2, 7]. Figure 1: Deleterious effects of the vermetid gastropod Dendropoma maxima on associated corals. Several individuals growing inside the hydrozoan fire coral Millepora platyphylla that minimizes the contact area between coral tissue and nets because of its erected and branching growth form (a). Morphology and pigmentation shift of
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