Within‐Host “Infraecology”: Asymmetric Interactions Between Two Co‐infecting Eye Parasites

Parasites within the host body can occupy similar ecological niches and, therefore, compete for resources. Similarly to macroecological environments, within-host habitats often provide unequal access to resources for different parasitic species. Parasites that live closer to the source of nutrients likely "intercept" resources, thus inhibiting the growth of "downstream" competitors. The eyes of vertebrates are an example of such asymmetric habitats, as lenses have priority access to nutrients over the vitreous humor. However, parasitic interactions in asymmetric environments have been rarely purposefully examined. We experimentally infected fish with lens- and vitreous-dwelling parasites (Diplostomum pseudospathaceum and Tylodelphys clavata, respectively) to compare their infection success and growth in the presence/absence of each other. We found evidence of antagonistic interactions between parasites during the infection process and their growth within hosts. Each parasite species decreased the infection success of the other to a similar extent. Importantly, in accordance with our initial hypothesis, interactions between the parasites during the growth phase were asymmetric. In the presence of D. pseudospathaceum living closer to the nutrient source (in the lens), vitreous-dwelling T. clavata grew slower than in monoinfections. In contrast, the presence of T. clavata, the "downstream" parasite, did not influence the growth of the D. pseudospathaceum. Our results highlight how within-host interactions and spatial heterogeneity affect the infection success of parasites and the size structure of infracommunities. We emphasize that such "infraecological" considerations should be considered when studying parasitic communities since within-host ecological interactions can influence infection dynamics and virulence.