Origin, Distribution, and Postglacial Dispersal of a Swimbladder Nematode, Cystidicola stigmatura

New distributional records indicate that Cystidicola stigmatura occurs in lake trout (Salvelinus namaycush) and Arctic char (S. alpinus) in the Great Lakes and St. Lawrence River drainage basins, in lakes along an arc extending from northwestern Ontario to Great Bear Lake and into the Canadian Arctic Archipelago but not elsewhere. This distribution can be explained by the parasite's probable postglacial history. I hypothesize that the nematode survived in only one glacial refugium. During deglaciation the parasite probably dispersed with lake trout from its refugium in the upper Mississippi River region into the Lake Michigan basin. Confluence of glacial waters would have given the nematode access to the drainage basins of Lakes Huron, Erie, and Ontario. Invasion of the St. Lawrence River basin occurred during a postglacial connection with the Lake Ontario basin. Dispersal from the Lake Huron basin was via glacial Lake Algonquin and later via glacial Lake Barlow–Ojibway and gave rise to present populations in eastern Ontario and western Quebec. Northward dispersal was probably via the glacial waters of the Lake Michigan–Lake Superior basin through glacial Lakes Agassiz, McConnell, and Coppermine to the Arctic Ocean. Arctic char would have first acquired the parasite at this time. Extant parasite populations along this dispersal route occur in Minnesota, northwestern Ontario, Manitoba, Saskatchewan, and the Northwest Territories. Invasion of the Arctic Archipelago has been with anadromous fishes. The apparently restricted preglacial distribution of the nematode and recent history of its intermediate host (Mysis relicta) in freshwater suggests that C. stigmatura may have arisen as a species during the last glaciation.