What are the evolutionary constraints on larval growth in a trophically transmitted parasite?

For organisms with a complex life cycle, a large larval size is generally beneficial, but it may come at the expense of prolonged development. Individuals that grow fast may avoid this tradeoff and switch habitats at both a larger size and younger age. A fast growth rate itself can be costly, however, as it requires greater resource intake. For parasites, fast larval growth is assumed to increase the likelihood of host death before transmission to the next host occurs. Using the tapeworm Schistocephalus solidus in its copepod first intermediate host, I investigated potential constraints in the parasite’s larval life history. Fast-growing parasites developed infectivity earlier, indicating there is no functional tradeoff between size and developmental time. There was significant growth variation among full-sib worm families, but fast-growing sibships were not characterized by lower host survival or more predation-risky host behavior. Parental investment also had little effect on larval growth rates. The commonly assumed constraints on larval growth and development were not observed in this system, so it remains unclear what prevents worms from exploiting their intermediate hosts more aggressively

Growth and ontogeny of the tapeworm <it>Schistocephalus solidus</it> in its copepod first host affects performance in its stickleback second intermediate host

<p>Abstract</p> <p>Background</p> <p>For parasites with complex life cycles, size at transmission can impact performance in the next host, thereby coupling parasite phenotypes in the two consecutive hosts. However, a handful of studies with parasites, and numerous studies with free-living, complex-life-cycle animals, have found that larval size correlates poorly with fitness under particular conditions, implying that other traits, such as physiological or ontogenetic variation, may predict fitness more reliably. Using the tapeworm <it>Schistocephalus solidus</it>, we evaluated how parasite size, age, and ontogeny in the copepod first host interact to determine performance in the stickleback second host.</p> <p>Methods</p> <p>We raised infected copepods under two feeding treatments (to manipulate parasite growth), and then exposed fish to worms of two different ages (to manipulate parasite ontogeny). We assessed how growth and ontogeny in copepods affected three measures of fitness in fish: infection probability, growth rate, and energy storage.</p> <p>Results</p> <p>Our main, novel finding is that the increase in fitness (infection probability and growth in fish) with larval size and age observed in previous studies on <it>S. solidus</it> seems to be largely mediated by ontogenetic variation. Worms that developed rapidly (had a cercomer after 9 days in copepods) were able to infect fish at an earlier age, and they grew to larger sizes with larger energy reserves in fish. Infection probability in fish increased with larval size chiefly in young worms, when size and ontogeny are positively correlated, but not in older worms that had essentially completed their larval development in copepods.</p> <p>Conclusions</p> <p>Transmission to sticklebacks as a small, not-yet-fully developed larva has clear costs for <it>S. solidus</it>, but it remains unclear what prevents the evolution of faster growth and development in this species.</p