Goldfish (Family Cyprinidae, Carassius auratus) and killifish (Family Cyprinodontidae, Fundulus heteroclitus) were acclimated to 10, 20 and 35 °C for 4 weeks. The thermal acclimation of C-start (escape swimming) performance and the physiological properties of fast twitch muscle fibres that underlie it were investigated in these species at the molecular (myosin isoform expression), biochemical (myofibrillar ATPase activity), cellular (contractile kinetics) and organismal levels of organisation. Peptide maps were obtained for fast muscle myosin heavy chains, isolated from 10 °C- and 35 °C-acclimated fish. Different myosin heavy chain isoforms were expressed in response to a change in acclimation temperature in goldfish, but myosin heavy chain isoform expression was unaffected by acclimation temperature in killifish. Compared with fish acclimated to 35 °C, acclimation to 10 °C increased the activity of fast muscle myofibrillar ATPase assayed at 10 °C fivefold in goldfish and only 50 % in killifish. Muscle twitch contraction time at 10 °C decreased significantly in response to acclimation to 10 °C in both species; however, the magnitude of this response was much greater in goldfish (100 %) than in killifish (30 % or less). In goldfish, these changes in the physiological properties of fast twitch fibres during 10 °C acclimation resulted in a six- to eightfold increase in the speed and turning velocity of fish performing C-starts at 10 °C. By comparison, the somewhat smaller acclimatory response of killifish fast muscle properties was accompanied by only a minor (50 % or less) adjustment in locomotor performance. Thermal acclimatory responses of fast muscle at the molecular, biochemical and cellular levels of organisation are clearly reflected in alterations in organismal escape performance.

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