Sustained swimming performance and muscle structure are altered by thermal acclimation in male mosquitofish

Abstract

Males of the eastern mosquitofish ( Gambusia holbrooki) possess one of the widest reproductively active temperature ranges for any ectotherm, ranging across seasons from at least 18°C to 34°C. In this study, we tested the ability of male G. holbrooki to acclimate their sustained swimming performance following long-term exposure to 18°C or 30°C. We also investigated some of the possible physiological mechanisms associated with thermal acclimation responses in swimming performance, including changes in slow muscle fibre size and abundance and the expression of myosin heavy chains (MyHC). We found that U crit of 18°C-acclimated G. holbrooki was 20% greater at 18°C than 30°C-acclimated fish, and the U crit of the 30°C-acclimated group was more than 15% greater at 30°C. Slow, fast and intermediate muscle fibres were identified on the basis of their myosin ATPase staining reaction. Although the number of slow and intermediate muscle fibres was similar between groups, the total cross-sectional area of aerobic fibre types was 40% greater in 18°—than 30°C—acclimated fish, reflecting an increase in the average fibre diameter. An S58 antibody raised against chicken slow skeletal muscle myosin stained a sub-set of the slow fibres identified by myosin ATPase staining. The number of S58-positive muscle fibres was 50% greater in 30°C—than 18°C—acclimated fish, implying that different MyHCs are being expressed in cold and warm acclimated individuals. Given the sexually coercive mating system of this species, increases in the sustained swimming performance via thermal acclimation may benefit the ability of males to maintain a high rate of sneaky copulations.