Collaborative Research: Anti-Predator Defenses of Marine Hydroids: Alternative Strategies, Biogeographic Patterns and Ecological Implications

University Of North Carolina At Chapel Hill, Chapel Hill NC

Investigators

Abstract

Optimality theory suggests that preferential allocation of resources to one form of defense should occur among a series of alternative defenses resulting in a single prevailing defense strategy. Although this idea has been widely accepted, few studies provide rigorous evidence to support the notion of trade-offs among defense systems, particularly in marine systems. The PI's recent data indicate that, with stinging organelles (nematocysts) and secondary metabolites as likely alternative defenses (Stachowicz & Lindquist 1997, in press), marine hydroids may provide an excellent system with which to test optimality theory and its implications for the evolution of defensive strategies among marine organisms. Using proven experimental methodologies to distinguish between nematocyst-defended and chemically-defended hydroids, The investigators will rigorously test the hypothesis that defensive nematocysts and secondary metabolites do not co-occur in marine hydroids and thus represent alternative defensive strategies. They will also test a second hypothesis that hydroids are not also defended by low nutritional value by examining correlations among whole polyp palatability, nematocyst types, chemical palatability, and nutritional parameters. Where chemical defenses are apparent, they will isolate and structurally characterize deterrent metabolites so that they can examine secondary biosynthetic diversity among hydroids relative to their phylogeny, patterns and scales of intraspecific chemical variability, compound localization, and elemental composition as they relate to hydroid anti-predator strategies and potential costs of the chemical defense. In addition the PIs will conduct a separate set of field and laboratory studies examining the ecological implications of altemative anti-predator defenses for other trophic levels. Specifically, they will test the hypothesis that epifauna abundance and diversity will be lower among nematocyst defended hydroids than among chemically defended hydroids because stinging nematocysts can harm associated epifauna. Because hydroids form the "foundation" on which an entire epifaunal community depends, negative effects of some hydroid defenses on epifauna could have important consequences for community composition and the function of these ecosystems. They will test these hypotheses on 3 different hydroid faunas: (1) the temperate Atlantic (NC); (2) the Caribbean (FL); and (3) the temperate (but less seasonal) eastern Pacific (northern CA), allowing biogeographic comparisons of hydroid defenses against predators. The results of this study will provide a conceptual framework for comparing ecological and evolutionary consequences of chemical vs. nematocyst defenses among broader groups of cnidarians that presently seem to rely largely on chemical defenses (e.g., octocorals like gorgonians and soft corals) vs. those such as sea anemones and hard corals that seem to rely on nematocysts for defense.