VARIATION IN THERMAL TOLERANCE AND ROUTINE METABOLISM AMONG SPRING-WATER AND STREAM-DWELLING FRESH-WATER SCULPINS (TELEOSTEI, COTTIDAE) OF THE SOUTHEASTERN UNITED-STATES

Evolutionary theory predicts that some aquatic organisms may adapt by directional selection to limiting physical environmental conditions, y et empirical data are conflicting. We sought to test the assumption th at sculpins (family Cottidae) inhabiting thermally stable springs of t he southeastern United States differ in temperature tolerance and meta bolism from populations inhabiting more thermally labile stream habita ts. Spring populations of pygmy sculpins (Cottus pygmaeus) and Ozark s culpins (C. hypselurus) differed interspecifically in thermal toleranc e from populations of stream-dwelling mottled (C. bairdi) and Tallapoo sa sculpins (C. tallapoosae), and both stream and spring populations o f banded sculpins (C. carolinae). No intra- or interspecific differenc es in thermal tolerance were found among populations of C. bairdi, C. tallapoosae, or C. carolinae. Cottus pygmaeus acclimated to 15 degrees C differed intraspecifically in routine metabolism from fish acclimat ed to 20 degrees and 25 degrees C. Cottus pygmaeus and stream-dwelling C. bairdi and C. carolinae acclimated to temperatures of 20 degrees a nd 25 degrees C showed no interspecific differences in routine metabol ism. Our results suggest that some spring-adapted populations or speci es may be more stenothermal than stream-dwelling congeners, but a grea ter understanding of the interactions of other physical and biological factors is required to better explain micro- and macrohabitat distrib utions of eastern North American sculpins.