Correlations between estrogen and testosterone concentrations, pairing status and acanthocephalan infection in an amphipod

Mechanisms by which parasites induce phenotypic alterations are poorly understood in many host–parasite systems. Amphipods are an intermediate host for a range of acanthocephalan parasites, and there is substantial evidence that parasites manipulate the physiology and behavior of amphipods in a variety of ways, including significant reproductive impacts. Evidence suggests such manipulations may result from disruption of serotonergic and dopaminergic pathways. However, correlations with other hormones have not been widely investigated, in part because we lack understanding of the production and action of hormones such as estrogen and testosterone in amphipods. We investigated how estrogen (17β‐estradiol) and testosterone concentrations vary between unparasitized male and female amphipods Gammarus pseudolimnaeus that were and were not in precopulatory pairs. We also investigated whether infection with the acanthocephalan Corynosoma sp. is correlated with differences in estrogen or testosterone concentrations when compared to unparasitized conspecifics. Our results indicate that estrogen and testosterone differ significantly between male and female amphipods, whether paired or unpaired, suggesting they may have sex‐specific functions. The biological relevance of these hormones merits increased attention in light of the prevalence of endocrine‐disrupting compounds in aquatic systems. Further, we found that encysted acanthocephalan parasites are correlated with elevated concentrations of estrogen and depressed concentrations of testosterone in male, but not female amphipods. This correlation suggests the need for a broader investigation of the mechanisms underlying phenotypic alterations of amphipod behavior and physiology by acanthocephalan parasites.