Variation in the preoperculomandibular canal of the johnny darter, Etheostoma nigrum, with associated zoogeographical considerations

A total of 1267 specimens (from 87 stations) of the johnny darter, Etheostoma nigrum, were studied to examine the geographic variation in the numbers of pores on the preoperculomandibular canal. The pore count is bimodal for the total sample. These modes correspond to distinct geographic regions. Fishes from northern Ontario (west and north of Lake Nipigon), Manitoba, Saskatchewan, Minnesota, Wisconsin, Iowa, and the Upper Peninsula of Michigan usually have 7 or fewer pores (mode = 6). Populations from northern Ontario (east and south of Lake Nipigon), southern Ontario, Quebec, Illinois, Indiana, Kentucky, Tennessee, Maryland, Ohio, New York, Pennsylvania, West Virginia, Virginia, North Carolina, South Carolina, and the Lower Peninsula of Michigan have 8 or more pores (mode = 9). The differentiation between morphs predates their present distribution and the morphs probably occupied distinct geographic areas within the Mississippi refugium during the last glaciation. Etheostoma nigrum dispersed north, following two postglacial routes: (i) via the Mississippi River to Lake Agassiz (12 800 years BP) then eastward to the Hudson Bay and James Bay drainages via Lake Barlow–Ojibway (9500 years BP), and (ii) via a northeastern spread from the Great Lakes and Ohio River drainages to the St. Lawrence River and Ottawa River drainages (12 000 years BP).