Hydroid defenses against predators: the importance of secondary metabolites versus nematocysts

Marine hydroids are commonly thought to be defended by stinging organelles called nematocysts that penetrate predator tissues and inject proteinaceous venoms, but not all hydroids possess these nematocysts. Although an increa sing number of bioactive secondary metabolites have been isolated from mari ne hydroids, ecological roles of these compounds are poorly known. To test the hypothesis that nematocysts and noxious secondary metabolites represent alternative defenses against predation, we examined hydroids from North Ca rolina, United States for: (1) the palatability of whole polyps before and after nematocysts had been deactivated; (2) the palatability of their chemi cal extracts; and (3) their nutritional value in terms of organic content, protein content, and levels of refractory structural material (chitin). All hydroids were avoided by a generalist predator, the pinfish Lagodon rhombo ides, compared with palatable control foods. Two of these (Halocordyle dist icha and Tubularia crocea) became palatable after being treated with potass ium chloride to discharge their nematocysts, suggesting that these species rely on nematocysts for defenses against predators. Chemical extracts from nematocyst-defended species had no effect on fish feeding. The four species that remained unpalatable after nematocysts had been discharged (Corydendr ium parasiticum, Eudendrium carneum, Hydractinia symbiolongicarpus, Trident ata marginata) possessed chemical extracts that deterred feeding by pinfish . We have isolated and characterized the structures of the deterrent metabo lites in two of these species. We found no differences in nutritional conte nt or levels of chitin between nematocyst-defended and chemically defended species, and no evidence that either of these played a role in the rejectio n of hydroids as prey. Our results suggest that, among hydroids, chemical d efenses may be at least as common as nematocyst-based defenses and that the two may represent largely alternative defensive strategies. The four hydro id species with deterrent extracts represent four families and both sub-ord ers of hydroids, suggesting that chemical defenses in this group may be wid espread and have multiple origins.