LIFE HISTORY EVOLUTION OF TROPICAL AMERICAN CUPULADRIID BRYOZOANS

Cupuladriids are abundant free-living bryozoans that commonly dominate benthic communities to depths of 100 m. A major component of their ecological success is the ability to produce new colonies asexually via fragmentation of adult colonies as well as by larvae derived from sexual reproduction. The existence of these alternative strategies raises basic questions about the environmental factors that should favor sexual versus asexual reproduction. Theories about the maintenance of sex and dispersal predict that asexual propagation should persist in stable environments while sex should be more beneficial in areas of high environmental instability. In contrast, life history theory of growth and recruitment predicts that asexual propagation should be favored in areas of high productivity versus sexual reproduction in low productivity environments. We tested these alternative predictions for eight common species of Cupuladria in the southwestern Caribbean over the past 10 Ma. Oceanographic conditions in the region changed from high variability and productivity to low variability and productivity between 5-3 Ma as the rising Isthmus closed off the Caribbean from the Pacific. Remarkably, most of the life history variability was intraspecific with seven of the eight species persisting for 10 to 4.5 Ma. Among the eight species, the proportion of sexually derived colonies increased by 10 to 55% and the calcification index increased from 33 to 90%. Four of the eight species also became more dome-shaped. Thus, colonies of each species became progressively stronger, squatter, and smaller, all of which are traits that reduce rates of asexual reproduction by fragmentation. These trends are consistent with life history theories about growth and recruitment and contradict models regarding maintenance of sex or dispersal. However, the trends persisted long after the period of major environmental change, suggesting that ongoing changes in overall community composition were also important selective factors for increased sexual reproduction. The results are strong evidence for the importance of environmental change as a driving factor in life history evolution.