The Ecology and Evolution of Inducible Defenses in a Marine Bryozoan: Cues, Costs, and Consequences

Membranipora membranacea rapidly deploys defensive spines in response to waterborne cues from a trophically specialized nudibranch, Doridella steinbergae. The defensive response by unattacked colonies is enhanced by a high concentration of predators or an attack on a nearby colony. Small colonies require a higher concentration of inducer than larger colonies, perhaps because defensive costs increase as size decreases. Spines reduce the feeding rate of nudibranchs to approximately 40% of the normal level, but they are produced at a cost in colony growth: colonies producing spines initially grow at 85% of the rate of unspined colonies. In colonial animals, a growth decrement is directly translated into a reduced output of sexual propagules because fecundity is directly proportional to colony size. In addition, intraspecific competition is so intense in M. membranacea that an initial decrement in growth can severely reduce final size. This study supports predictions that the evolution of inducible defenses is favored when (1) prey are not killed in initial encounters with predators, (2) attacks cannot be predicted from cues other than the presence of a predator, and (3) the cost of defense is substantial. Thus, predator-induced defenses are common among organisms not killed in initial encounters with consumers, such as clonal and modular organisms and hosts of parasites.