RESOURCE QUANTITY AND OFFSPRING QUALITY IN DAPHNIA

Many species of zooplankton display temporal changes in the size of the offspring they produce. We address the question, is offspring size variation by Daphnia a predictable response to changes in resource abundance? For each of two species of Daphnia (D. pulicaria and D. pulex), we raised several distinct genotypes at each of three different food levels. Differences in neonate body length, dry mass, and C:N ratio were measured at each food level. The two species responded differently to food level; neonate mass decreased in D. pulex with decreasing food, but the largest (mass) D. pulicaria neonates were produced at the intermediate food level. For both species, the lightest (mass) neonates were produced at the lowest food, but these neonates were often equal (D. pulex), or longer (D. pulicaria) in body length than those produced at high food. The production of small-mass, but relatively large-length neonates in the lowest food level was associated with a lower C:N ratio of both adults and neonates compared with the higher food levels. We conclude that qualitative changes in the production of offspring can result from simple quantitative changes in resource abundance. We suggest that these responses may represent adaptive plasticity in reproduction.