Variations in infection levels and parasite-induced mortality among sympatric cryptic lineages of native amphipods and a congeneric invasive species: Are native hosts always losing?

Abstract

Shared parasites can strongly influence the outcome of competition between congeneric, sympatric hosts, and thus host population dynamics. Parasite-mediated competition is commonly hypothesized as an important factor in biological invasion success; invasive species often experience lower infection levels and/or parasite-induced mortality than native congeneric hosts. However, variation in infection levels among sympatric hosts can be due to contrasting abilities to avoid infection or different parasite-induced mortality rates following infection. Low parasite infection levels in a specific host can be due to either factor but have drastically different implications in interaction outcomes between sympatric hosts. We assessed acanthocephalan infection levels (prevalence and abundance) among cryptic molecular taxonomic units (MOTU) of the native G. pulex/G. fossarum species complex from multiple populations where they occur in sympatry. We concomitantly estimated the same parameters in the invasive Gammarus roeseli commonly found in sympatry with G. pulex/G. fossarum MOTUs. We then tested for potential differences in parasite-induced mortality among these alternative hosts. As expected, the invasive G. roeseli showed relatively low infection level and was not subject to parasite-induced mortality. We also found that both acanthocephalan infection levels and parasite-induced mortality varied greatly among cryptic MOTUs of the native amphipods. Contrary to expectations, some native MOTUs displayed levels of resistance to their local parasites similar to those observed in the invasive G. roeseli. Overall, cryptic diversity in native amphipods coupled with high levels of variability in infection levels and parasite-induced mortality documented here may strongly influence inter-MOTU interactions and native population dynamics as well as invasion success and population dynamics of the congeneric invasive G. roeseli.

Keywords: Acanthocephalan parasites; Biological invasion; Cryptic diversity; Gammarus fossarum/Gammarus pulex species complex; Gammarus roeseli; Infection levels; Parasite-induced mortality.

Graphical abstract

Fig. 1

Genetic divergence levels (%) among MOTUs of the G. fossarum/G. pulex species complex found in our sampling sites/rivers. Gammarus roeseli was identified morphologically rather than genetically.

Fig. 2

Mean parasite prevalences (proportion of infected individuals in %) among amphipod populations/sampling sites and their bootstrapped 95% confidence intervals in the different MOTUs sampled and for the three acanthocephalan species, separately and overall (all three parasites grouped). Overall prevalences in MOTUs assigned different letters are significantly different at the 0.05 level.

Fig. 3

Mean parasite abundances (mean number of acanthocephalan larvae per individual host) among amphipod populations/sampling sites and their bootstrapped 95% confidence intervals in the different MOTUs sampled and for the three acanthocephalan species, separately and overall (all three parasites grouped). Overall abundances in MOTUs assigned different letters are significantly different at the 0.05 level.

Fig. 4

Parasite abundance as a function of amphipod body size (used as a proxy for age) in each of the 8 amphipod MOTUs. The polynomial effect of body size on parasite abundance is modeled with a general mixed effect linear model with a Poisson distribution and a log link function. The y axis is in log scale for representation purposes. Body size is rescaled to initial values in the graph for representation purposes. Predicted curves are represented in plain black lines with their standard errors in dotted lines.