Lack of seasonal variation in the life-history strategies of the trematodeCoitocaecum parvum: no apparent environmental effect

Abstract

SUMMARYParasites with complex life cycles have developed numerous and very diverse adaptations to increase the likelihood of completing this cycle. For example, some parasites can abbreviate their life cycles by skipping the definitive host and reproducing inside their intermediate host. The resulting shorter life cycle is clearly advantageous when definitive hosts are absent or rare. In species where life-cycle abbreviation is facultative, this strategy should be adopted in response to seasonally variable environmental conditions. The hermaphroditic trematodeCoitocaecum parvumis able to mature precociously (progenesis), and produce eggs by selfing while still inside its amphipod second intermediate host. Several environmental factors such as fish definitive host density and water temperature are known to influence the life-history strategy adopted by laboratory raisedC. parvum. Here we document the seasonal variation of environmental parameters and its association with the proportion of progenetic individuals in a parasite population in its natural environment. We found obvious seasonal patterns in both water temperature andC. parvumhost densities. However, despite being temporally variable, the proportion of progeneticC. parvumindividuals was not correlated with any single parameter. The results show thatC. parvumlife-history strategy is not as flexible as previously thought. It is possible that the parasite's natural environment contains so many layers of heterogeneity thatC. parvumdoes not possess the ability to adjust its life-history strategy to accurately match the current conditions.