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. / Master of Science / Symbiotic relationships are relationships between two or more organisms lasting for long periods of time and are often associated with proximity or touch. In symbiotic relationships there can be a host and a symbiote. The difference between the host and symbiont can be found in their roles such as protection from predators or parasites or by providing nutrients or transportation and the difference in size with the host being larger. Symbiotic relationships are not static and can change over time due to a variety of reasons, such as host size, symbiont abundance, or nutrient availability. The introduction of harmful non-native species, otherwise known as invasive species, can disrupt symbiotic relationships across ecosystems. Invasive species can introduce non-native symbionts, and also can become potential hosts for native symbionts. The relationship between crayfish and Branchiobdellidan worms, an order of small, segmented worms, has been established over decades of research as a useful system for studying symbiosis. Branchiobdellidan worms can provide a beneficial cleaning service by removing harmful symbionts or bacteria from their crayfish host. Alternatively, they can become parasites and feed on crayfish gills if nutrients are not available on the host. Introduced invasive crayfish can decrease the population of brachiobdellidan worms within the Southern Appalachians in Virginia. However, an established relationship between native symbionts and invasive crayfish hosts has not been studied. To investigate the effects of a symbiotic relationship over the time span of several months between invasive hosts and native symbionts, I experimentally reduced the ability of invasive hosts to remove branchiobdellidan symbionts to allow native branchiobdellidan worms time to acclimate on to invasive crayfish and establish a symbiotic relationship. In two experiments over several months, I recorded changes in host growth rates and gill damage. Invasive hosts had an increased growth rate when there was a low abundance of worms. I also compared an invasive host to a native host to see how changes in growth rates, gill chamber damage, and locations of worms on their host may differ. The native host's growth rates increased, but the invasive host had a negative growth rate when worm densities were too high. My findings suggest invasive hosts can have their own unique symbiotic relationship with native symbionts. When invasive hosts are introduced to a region, native symbiont populations may either decrease or native symbionts may find compatible invasive hosts. By examining relationships between native symbionts and invasive hosts, we can understand how invasions may influence symbiotic relationships and how other organisms are affected in the ecosystem.

Crayfish throughout the holarctic are found in association with an order of worms known as branchiobdellidans. This relationship has been confirmed as a cleaning symbiosis in several species. The Conhaway crayfish,​ Cambarus appalachiensis, is a species of crayfish endemic to the New River Basin in Virginia and West Virginia. We studied a population of ​ C. appalachiensis​ in Sinking Creek in Newport, VA from March 2017 until February 2018. We collected morphological data and quantified the branchiobdellidan communities on 986 individuals, and kept note of egg brooding and young of year throughout the study period. The life cycle of C. appalachiensis was found to be similar to other large-bodied species of Cambarus crayfish. Molting occurred throughout the year, peaking in the months of April and September. This molting served as a disturbance effect to the symbiotic branchiobdellidan community and reset community assembly. The worm communities on larger, recently molted crayfish more closely resembled the less diverse communities on smaller crayfish. Most worms on recently molted crayfish were ones that we know are early colonizers. This thesis work provides the first life history information on a newly described species of Cambarus crayfish and provides both seasonal data on its branchiobdellidan associates and one of the first empirical examples of host ontogeny acting as a disturbance on a symbiotic community. / Master of Science / Crayfish throughout North America and Eurasia are the symbiotic partners to a number of small worms. This relationship has been confirmed as a cleaning symbiosis for several crayfish, similar to the cleaning stations at a coral reef. The Conhaway crayfish is a species of crayfish found in the New River Basin in Virginia and West Virginia. We studied a population of Conhaway crayfish in Sinking Creek in Newport, VA from March 2017 until February 2018. We collected data on the physical attributes of the crayfish, quantified the worms present on 986 individuals, and kept note of female crayfish with eggs and baby crayfish throughout the study period. The life cycle of the Conhaway crayfish was found to be similar to other large-bodied species of closely related crayfish. Molting, where the crayfish loses its shell and grows a new one, occurred throughout the year, peaking in the months of April and September. This molting served as a disturbance effect to the symbiotic worms, similar to how a wildfire might displace animals in a grassland ecosystem. The type and number of worms found on larger, recently molted crayfish more closely resemble the type and number of worms found on smaller crayfish. This thesis work provides the first information on the physical and reproductive attributes of a newly described species of crayfish and provides both seasonal data on its symbiotic worms and one of the first empirical examples of host growth and aging acting as a disturbance to symbiotic organisms living on that host.