. Bulletin. Natural history; Natural history. August 1985 Crayfishes and Shrimps of Illinois 405. Fig. 139.—Gravel riffles in the lower portion of Big Creel<, Hardin County, Illinois (10 March 1976) provide the best habitat in Illinois for Orconectes placidus. Among the crayfishes of Illinois, populations herein assigned to O. placidus arc the most troublesome tax- onomically. The only large population is that in Big Creek in Hardin County; other populations are small, highly localized ones inhabiting rocky areas along the banks of the Ohio and Missis- sippi rivers. Although all are assigne
. Bulletin. Natural history; Natural history. August 1985 Crayfishes and Shrimps of Illinois 405. Fig. 139.—Gravel riffles in the lower portion of Big Creel<, Hardin County, Illinois (10 March 1976) provide the best habitat in Illinois for Orconectes placidus. Among the crayfishes of Illinois, populations herein assigned to O. placidus arc the most troublesome tax- onomically. The only large population is that in Big Creek in Hardin County; other populations are small, highly localized ones inhabiting rocky areas along the banks of the Ohio and Missis- sippi rivers. Although all are assigned to O. placidus, these populations show variations usually indicative of specific distinctiveness in Orconectes. The gonopod of the form I male lacks any hint of a shoulder on its dorsal surface in Big Creek specimens but has a defi- nite shoulder (although not as pro- nounced and angular as in O. rusticus) in Mississipi^i Ri\er specimens (no form I males from the Ohio River are avail- able). Big Creek specimens always lack a carina on the rostrum; those from the Ohio and Mississippi rivers may or may not have one. In general, western j)op- ulations differ the most from typical O. placidus occupying the Cumberland and Tennessee river systems in Ken- tucky and Tennessee (Hobbs 1971b). Variation in O. placidus and its rela- tives is badly in need of study. Distribution and Habitat.—O. placi- dus occupies streams of the Cumber- land, Tennessee, and lower Ohio rivers in Kentucky, Tennessee, and Illinois (Fig. 137). It lives in rocky riffles and pools, using cavities and interstices as hiding places. Recent Illinois records exist for Big Creek, Hardin County, the Ohio River, Massac and Pulaski counties, and the Mississippi River, Randolph and Jack- son counties (Fig. 138). Most of the large-river collections were made along rocky banks or in rocky backwater areas. In the Big Creek system, O. placidus is found mainly in downstream gravel and rubble riffles (Fig. 139), and O. ken
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