Body size regulation in copepod crustaceans

Body size affects survival probabilities, reproductive output and individua l fitness in many organisms. In freshwater zooplankton. traits ranging from demographic rates to community composition depend on body size, and predat ion is most often identified as the selection pressure determining body siz e. We examined the extent to which stage-specific growth trajectories and b ody sizes of copepod crustaceans are constrained, independent of selection. We used exuviae shed at each molt to quantify the relationship between siz e at moiling and growth during the subsequent instar for two common, herbiv orous calanoid copepods, Boeckella triarticulata and Diaptaomus leptopus. I ndividuals of both species were raised under diets of different food qualit y or quantity, and at different temperatures. Size at molting varied little amen individuals of both species, as a consequence of a persistent negativ e relationship between size at molting and subsequent (absolute) growth inc rement. Individuals that were small when they molted grew more during the s ubsequent instar than individuals that were large. This relationship was st atistically significant for nearly all instars of both species raised in di fferent food or temperature conditions, and not affected by food quality, f ood quantity or temperature. Our results indicate that body size is constra ined or regulated over much of the copepod life cycle, independent of the e ffects of environmental conditions (food. temperature, predation).