The life histories of the crayfish Cambarus robustus and Cambarus bartoni were studied in the Kawartha Lakes region of southern Ontario. There were marked differences in their breeding and molting cycles compared with the familiar pattern of the Orconectes species of this region. Egg extrusion occurred later (July in C. robustus, June in C. bartoni), and juveniles therefore did not become free living until late summer or early fall. With little growing time in their first summer, they measured only 5–10 mm in carapace length (CPL) before growth ceased for the winter. At the end of their second summer the still immature crayfish measured 17–26 mm CPL in C. robustus and 13–20 mm CPL in C. bartoni. Maturity was therefore not attained until the end of the third summer, when most C. robustus matured at 34–45 mm CPL and C. bartoni at 25–30 mm CPL. The majority of individuals apparently reproduced for the first time during their fourth summer; a few apparently survived into another summer, reaching carapace lengths greater than 50 mm in C. robustus and 30 mm in C. bartoni. In males of both species, form 1 and form 2 occur throughout the summer. Although lacking the synchrony of Orconectes species, breeding and molting activities are still confined to the period between April and October. The timing of the life-history events observed in these two Cambarus species may be adaptations to seasonal stresses of the swift water environments that these species inhabit as well as to the relative harshness of the northern temperate climate.

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