Exploitation of the same trophic link favors convergence of larval life-history strategies in complex life cycle helminths

Switching from one host to the next is a critical life history transition in parasites with complex life cycles. Growth and mortality rates are thought to influence the optimal time and size at transmission, but these rates are difficult to measure in parasites. The parasite life cycle, in particular the trophic link along which transmission occurs, may be a reasonable proxy for these rates, leading to the hypothesis that life cycle should shape life history strategy. We compiled data on the size and age at infectivity for trophically-transmitted helminths (i.e. acanthocephalans, cestodes, and nematodes), and then categorized species into trophic links (e.g. planktonic crustaceans to fish, insects to terrestrial vertebrates, etc.). Comparative analyses that explicitly included stabilizing selection within trophic links fit the data significantly better than random walk models, indicating that parasites with different life cycles have different optimal times/sizes for host switching. The...