When does growth rate influence fitness in a colonial marine invertebrate?

Growth rate affects body size, and larger body sizes are often associated with the capacity to produce more surviving offspring. However, the assumption that growth rate should positively relate to fitness is rarely tested, especially in colonial marine invertebrates where size and age can be decoupled. We measured growth, survival, and reproduction through repeated census of 97 colonies from two populations of a marine bryozoan in the field from settlement to the end of their reproductive season in the northern Gulf of Mexico. Despite large population differences in fitness when grown in a common garden setting, selection within populations on variation in relative growth rate prior to reproduction was similar. In both populations, colonies that grew faster early after settlement, hence were larger, did not consistently have higher fitness than colonies that grew slower after settlement. Instead, early juvenile growth was uncorrelated to later juvenile growth, and colonies that grew fa...