The relationship between larval size and fitness in the tapeworm Schistocephalus solidus: bigger is better?

In organisms with complex life cycles, fi tness often increases with body size at the transition from larva to adult. Th e translation of larval size into fi tness, however, can depend on the source of size variation, with size, per se, not always increasing adult success. In parasitic worms, many factors infl uence larval growth, but little is known about the consequences of this growth variation. We examined how the size of the tapeworm Schistocephalus solidus in its copepod fi rst intermediate host aff ects infection success and growth in the stickleback second host. Moreover, we assessed whether the conspicuous growth variation caused by copepod size is fi tness-relevant. Using larvae of the same age, we found that larger worms had a substantially higher infection probability and they tended to still be slightly larger after several months of growth in fi sh. However, big larvae from bigger copepods did not have higher fi tness, suggesting that being large relative to the host, but not necessarily large in general, is important. Th ese fi ndings clarify some aspects of the life history strategy of S. solidus (e.g. why there is a fl at ontogenetic reaction norm across copepod stages), but also raise questions (e.g. why growth costs have been hard to document). More generally, our results indicate that larval size can correlate with fi tness in helminths, but that not all size variation is predictive of success in the next host.