Developmental inflexibility of larval tapeworms in response to resource variation

The timing of habitat switching in organisms with complex life cycles is an important life history characteristic that is often influenced by the larval growth environment. Under starvation, longer developmental times are frequently observed, probably as a consequence of developmental thresholds, but prolonged ontogeny sometimes also occurs under good conditions, as organisms may take advantage of the large potential gains in body size. I investigated whether variation in growth conditions affects the larval development time of a complex life cycle tapeworm (Schistocephalus solidus) in its copepod first host. Moreover, I reviewed patterns of developmental plasticity in larval tapeworms to assess the generality of my findings. Copepod starvation weakly retarded parasite growth but did not affect development. Worms grew bigger in larger copepods, but they developed at a similar rate in large and small hosts. Thus, S. solidus does not delay ontogeny under good conditions nor does it fail to reach a developmental threshold under poor conditions. Although unusual in comparison to free-living organisms, such inflexibility is common in tapeworms. Plasticity, namely prolonged ontogeny, has been mainly observed at high infection intensities. For S. solidus, there were large cross-environment genetic correlations for development, suggesting there may be genetic constraints on the evolution of developmental plasticity.