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

Abstract

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 faster just prior to the onset of reproduction had higher fitness than colonies that grew slower during this time. Growth rates then declined with the onset of reproduction. Our results show that, rather than being a simple consequence of selection on body size, growth rate can directly affect variation in fitness in ways that are not directly attributable to juvenile size. Colony size and growth rate in modular animals are not always reliable surrogates for direct estimates of survival and reproduction, without identifying when and how growth affects fitness.