Abstract
Anguillicola crassus is a nematode parasite of anguillid eel swimbladders. A. crassus is native to Japanese eels, but it has become invasive in both European eels (Anguilla anguilla) and American eels (A. rostrata). American eels are a catadromous species, living most of their lives in freshwater before migrating to the Sargasso Sea to spawn. Research has shown that heavy repeated infection with A. crassus can affect the volume and composition of the gases within the swimbladder. This would cause an increased amount of energy necessary for an eel to control its buoyancy. Extra energy exerted on buoyancy would mean less energy available for both migration and gonad development, possibly putting spawning in jeopardy. Three objectives were studied in this thesis: 1) using digital X-ray to develop a non-lethal technique for the detection of A. crassus, 2) investigating the temperature tolerance of the free-living and adult stages of A. crassus to determine over-wintering survival in the St. Lawrence River region, and 3) using artificial infection techniques to determine the growth rate of A. crassus in American eels at 20°C.While researchers currently use dissection to confirm A. crassus infection, this had led to the dissection of thousands of both American and European eels in the last decade. The digital X-ray technique presented here is a fast and accurate way to non-lethally determine the presence or absence of A. crassus in American eels. There were no false positives, and >70% of infections were detected. The average detectable intensity was 5.86 ± 5.60 parasites per eel. The average detectable parasite length was 17.98 ± 8.13 mm and average detectable coil diameter was 5.28 ± 2.84 mm. Appendix 1 provides additional examples of radiographs showing eel swimbladders infected with A. crassus. It has been hypothesized that A. crassus will not establish a sustainable population in the St. Lawrence River (SLR) due to low winter temperatures. However, small incidences of infections have been found in the SLR in the last decade. Therefore, the survival of both the free-living and adult stages of the parasite at low temperatures was investigated. The free-living larval stage of A. crassus had significantly reduced survival at 1, 2, and 4°C compared to 5, 6, 10, and 15°C. Adult parasites had significantly reduced survival at 2°C compared to 20°C. The reduction of survival in both the L2 and adult stages at extremely low temperatures indicates that the A. crassus life cycle may be disrupted during the winter, slowing the spread of the parasite in the SLR region. Experimental infection techniques have been used for decades to investigate A. crassus infections in European eels but has never been attempted in American eels. The growth rate of A. crassus in American eels is also unknown. Therefore, experimental infection techniques as well as digital X-ray were used track the growth rate of the parasite. Thirteen American eels were artificially infected with A. crassus. One eel contained two adult parasites 18 weeks post-infection. While not enough data was collected to determine a growth rate, this chapter shows that experimental infection of American eels with A. crassus is possible.