Distribution of Gambusia (Poeciliidae) in a southeastern river system and the use of fin ray counts for species determination

Recent collections from the Conasauga River system revealed the presence of Gambusia holbrooki, previously unreported in the Coosa River portion of th e Mobile River drainage. We analyzed Gambusia specimens in 36 collections f rom the Conasauga River system to determine the relative distribution of G. affinis, G. holbrooki, or their hybrid progeny. We used gonopodial charact ers and ray counts from the anal and dorsal fins, which required reconcilin g differences among published accounts, to make species determinations. We counted all fin rays separately for dorsal and anal fins and recommend this method to clarify discrepancies among previous reports. This method yielde d counts of seven dorsal and 10 anal rays for G. affinis and eight dorsal a nd 11 anal rays for G. holbrooki. Both species and their hybrids were colle cted in the Conasauga River system. Gambusia affinis is widespread in the C onasauga River system and is presumably native. Gambusia holbrooki currentl y has a limited distribution but appears to be expanding its range. We attr ibute the occurrence of G. holbrooki to human introduction.