Although many impacts of invasion have been well studied, less well understood is how invasive species impact native symbiotic systems. In Virginia, the non-native Spiny Stream Crayfish (Faxonius cristavarius) has spread throughout three river basins in the Mountain Lake Region. Each of these basins hosts a unique native crayfish community with a diverse assemblage of ectosymbiotic annelid worms (Order: Branchiobdellida). These worms engage in a cleaning symbiosis with their crayfish hosts that shifts between mutualism and parasitism based on symbiont density. While studying these symbiont communities, we observed that the presence of non-native crayfish resulted in significant reductions in native symbiont abundance and richness as compared to survey sites occupied by only native crayfish. We have found these negative impacts to symbiont communities to be due to tolerance differences between invasive and native crayfish. While native crayfish often host diverse worm assemblages consisting of tens of worms, invasive crayfish actively host symbiont communities of significantly reduced abundance and diversity. To better understand the process by which invasive hosts alter native symbiont communities, we carried out a two-part study consisting of dispersal experiments under controlled settings and a widespread survey of the three basins in the Mountain Lake Region.
Results/Conclusions
Dispersal experiments that simulated the progressive displacement of native crayfish by invasive crayfish found that as invasive crayfish displace native crayfish, symbiont dispersal rates initially increase due to behavioral differences between native and invasive crayfish before dropping to near zero with full displacement. Symbiont survival was shown to decrease across the displacement gradient. Our findings support the hypothesis that these symbiotic worms do not distinguish between host quality when dispersing. Although dispersal may increase as invasive crayfish become present in crayfish communities, symbiont survival decreases due to the lower symbiont tolerance of invasive crayfish. Survey results show that the displacement gradient we simulated in displacement studies exists in all three basins. Each basin hosts sites with complete native crayfish assemblages, sites exhibiting partial displacement by invasive crayfish, and sites exhibiting complete displacement. Analysis of sampled symbiotic worm assemblages shows that as expected, the progressive displacement of native crayfish by invasive crayfish results in reduced diversity and abundance in native symbiont communities. Our findings suggest a compounding negative effect of invasion. As native hosts are displaced by invasive hosts, native symbiont communities may be negatively impacted, further exacerbating the negative effects of invasion.