Distribution and taxonomy of three species of Orconectes (Decapoda: Cambaridae) in Illinois, U.S.A.

Abstract

We examined Ohio and Mississippi River specimens of Orconectes that had been identified previously as O. placidus and collected new specimens or examined types of Orconectes luteus, O. placidus, O. forceps, O. medius, and O. rusticus from Illinois and other states. Our analyses revealed the presence of three species of Orconectes in the Ohio and Mississippi rivers, Orconectes luteus, O. placidus, and O. sp. (undescribed), based on chela morphometrics, gonopods, carpal spines, chela denticles, and life colors. All the Mississippi River records of O. placidus were O. luteus, and many of the Ohio River records of O. placidus were O. n. sp. Distinguishing characteristics for all three species, along with an addition to the dichotomous key to Illinois decapods, were included. The ranges of O. luteus, O. placidus, and O. n. sp. in Illinois were amended. Several problems were noted with the syntypes of O. placidus such as erroneous locality, misidentification, and mixing of specimens between localities. The ratio dactylus length/palm mesial margin length, which has been considered a distinguishing characteristic between O. forceps and O. placidus, was invalidated as a useful discriminator for these species, whereas presence or absence of a distomedian carpal spine was reliable. We consider Orconectes luteus and O. n. sp. native inhabitants of Illinois, and the latter species likely occurs in Kentucky portions of the Ohio River as well.