ASSOCIATION BETWEEN COLONY LIFE-HISTORY AND POLYMORPHISM IN CHEILOSTOME BRYOZOANS

Polymorphism in cheilostomes is well developed, with colonies of some species expressing as many as five or more types of heterozooids. Polymorphic zooids within colonies have a division of labor, some zooids are defensive or structural specialists and others focus on reproduction. Polymorphism is not primarily ecophenotypic; even though colonies with a single species can vary in the number of polymorphs types expressed, species do have a characteristic repertoire of polymorphs and these differences are not primarily caused by environmental heterogeneity. In a comprehensive survey of polymorphism in colonial animals, it was found that reproductive division of labor always evolves prior to function polymorphism. This result suggests that the evolution of polymorphic members of a colony is contingent on the evolution of K-selected life history strategies associated with increased competitive ability, longevity, ecological persistence, and investment in offspring with a high probability of survival. Cheilostomes provide an ideal group to identify the association between polymorphism and life history strategies because their life histories and their degree of polymorphism are easily quantifiable. I count the numbers of polymorph types in a colony and compare this number to the proportion of zooids that are sterile. I find greater polymorphism in species where reproductive investment is high, but limited to relatively few zooids in the colony. This occurs in colonies where the proportion of sterile heterozooids is high, and larvae are brooded in few ovicells. But this highly skewed reproductive division of labor is not always associated with high numbers of polymorphs. It is still unknown if the species with highly skewed reproductive division of labor but limited polymorphism have not yet had sufficient time evolve high polymorphism or if their specific life histories do not permit its evolution.