Natural Hybrids of the Freshwater Sculpins Cottus bairdi and Cottus cognatus (Pisces: Cottidae): Electrophoretic and Morphometric Evidence

Abstract

Natural hybrids of the sculpins Cottus bairdi and C. cognatus in the Sus- quehanna River drainage are electrophoretically identifiable on the basis of at least seven enzyme loci. Identification of Cottus hybrids on morphological grounds is, how- ever, somewhat speculative. Meristic characters overlap widely between the parental species, and hybrids generally do not possess intermediate character states. Hybrids can be tentatively identified on the basis of their intermediacy for two diagnostic characters which unambiguously distinguish the two species elsewhere in the Susque- hanna drainage: presence/absence of the palatine tooth patch, and number of pelvic fin rays. Principal component and discriminant analyses of suspected hybrids and samples of the parental species show the presumed hybrids to be somewhat intermedi- ate in morphology but to largely resemble C. cognatus in body form and meristic counts. Diagnostically intermediate specimens are more variable in their meristic and mensural characters than are nonintermediate individuals, and also display more bi- lateral asymmetry. F2 and backcross progeny were not detected in electrophoresis samples, but morphological evidence suggests that some introgression may be occur- ring.