INDUCIBLE DEFENSES AND ALLOCATION SHIFTS IN A MARINE BRYOZOAN

Most theoretical models of the evolution of conditional responses require some form of cost to balance the benefits of an intermittently deployed character. The inducible spines, keels, and helmets of marine and freshwater invertebrates have proven to be tractable subjects for testing this hypothesis. Membranipora membranacea, a marine bryozoan, produces large spines within 2 d of detecting chemical cues from a predatory mollusc. The spine response, which increases with the level of predator attack and varies with colony size, is associated with reduced growth of colonies in short-term laboratory experiments. Because of the plastic nature of allocation shifts in colonial invertebrates, lifetime fitness consequences for inducibly defended colonies in the field were measured in this study. In two successive years, spined colonies grew somewhat more slowly, but also reproduced sooner than less-spined colonies. Spined colonies also senesced sooner than unspined colonies in the one year measured. The differences in growth trajectories between spined and less-spined colonies detected in the field were small relative to those predicted from short-term laboratory experiments, emphasizing the importance of field studies to evaluate the costs of defense in nature.