ABSTRACT.-In northern Wisconsin lakes, the crayfish Orconectes rusticus is replacing 0. propinquus, a previous invader, and 0. virilis, a native crayfish. Because chela size dictates success in aggressive encounters, predatory defense and reproduction, interspecific size dif- ferences could lend insight to this ongoing replacement. To quantify chela size differences among these three species, we measured chela lengths and widths of Form II (FII, nonre- productive) males, Form I (Fl, reproductive) males and females. For Fl and FII males, both 0. rusticus and 0. propinquus had larger chelae than similar-sized 0. virilis. Females differed only slightly in chela size. Females had smaller chelae than their male conspecifics. Chela size differences, coupled with differential body size, likely contributes, via differential predatory susceptibility and reproductive success, to the replacement of 0. propinquus and 0. virilis by 0. rusticus.
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