Temperature is one of the most important extrinsic factors determining muscle performance in ectotherms. In considering the effects of temperature on muscle contraction in fish it is important to distinguish between sustainable and maximum effort, since they are supported by different muscle fibre types. A great variety of muscle phenotypes are observed, brought about by the differential expression of different protein isoforms, and by varying the amounts of ion channels, membrane pumps and cellular organelles (see Johnston & Altringham, 1991, for a recent review). Slow swimming activity largely involves the recruitment of slow twitch muscle fibres, which are dependent on aerobic metabolism, and complex respiratory and circulatory support systems to supply oxygen and substrates (Bone, 1966; Johnston, Davison & Goldspink, 1977). Slow twitch fibres, with their high concentrations of myoglobin and mitochondria and well developed capillary supply, are the main fibre type in red muscle (Johnston, 1981; Altringham & Johnston, 1988a).
As swimming speed increases, faster contracting muscle fibres are recruited (Johnston et al., 1977; Rome, Loughna & Goldspink, 1984). Maximum performance is achieved during fast-starts associated with escape responses and predation and involves the recruitment of the entire white muscle mass (Johnston, Franklin & Johnson, 1993). The white muscle is typically composed of a single fibre type expressing fast isotypes of the myofibrillar proteins and containing high concentrations of the cytoplasmic Ca2+-binding protein parvalbumin (Gerday et al., 1979; Rowlerson et al., 1985; Crockford & Johnston, 1993).