Invasive Hosts and their Context Dependent Relationships with Native Symbionts

Symbiotic relationships display plasticity through time, depending on a variety of factors that include host properties, symbiont densities, and environmental conditions. Invasive species can affect symbiotic relationships by introducing invasive symbionts, reducing the population of native symbionts, or competing for native symbionts as a resource. There is an established symbiotic relationship between crayfish and annelid worms in the order Branchiobdellida. Branchiobdellidan worms can have a mutualistic cleaning symbiosis with crayfish, or at times become parasitic and feed on crayfish gill tissue if nutrients on the host are low. With the introduction of invasive crayfish in the Southern Appalachians in Virginia, branchiobdellidan worm populations have sharply declined due to invasive crayfish being less competent hosts for the symbionts. However, degree of competency as a host may differ among invasive species to, as invasive hosts have their own unique context-dependent symbiotic relationships. To investigate how symbiotic relationships differ between invasive hosts, I encouraged symbiotic relationships between invasive hosts Faxonius virilis and Faxonius cristavarius and native symbionts Cambarincola ingens. In two experiments spanning several months, I observed changes in growth rates of hosts and damage to gill tissues over varying levels of symbiont exposure. One species of invasive host, F. cristavarius, had increased growth rates when exposed to native symbionts at low symbiont densities, while for the other invasive host, F. virilis, growth rates and gill chamber damage was not impacted by the presence of symbionts. I also compared an invasive host F. cristavarius to a native host Cambarus appalachiensis to measure the response of growth rate, symbiont damage to gills, and behavior of worms across a gradient of symbiont exposure. The native host's growth rates increased over time, but not due to an effect of symbionts. However, the invasive host exhibited effects from parasitism when symbiont densities were high. My findings suggest that invasive hosts can have their own unique context-dependent relationship with native symbionts. Because there is no one-size-fits-all rule for invasive hosts, when invasive hosts enter a region, new symbiotic relationships can be formed that are beneficial for invasive hosts and native symbionts. Invasive hosts or native symbionts could also be rejected by the other which may lead to decreases in either of their populations.