Fishes can be found in almost any body of water, be it an underground cave, beneath the arctic ice, in the deepest marine trench, in a fast moving mountain stream, or on a tropical reef. In each of these environments, fish have evolved combinations of physiological, morphological and biomechanical traits which adapt them to their particular habitat. In particular, almost all fish swim. Many functional traits are integrated to perform swimming activities, at many different levels of organisation ranging from cell energy production, to myocyte and muscle contraction, to cardiac and respiratory function, and to the systems for neural and endocrine regulation, integration and control. These traits have presumably all co-evolved, and contribute not only to locomotion but also to environmental adaptation in its broadest sense. It is generally held that the diversity of fish swimming modes and performance is a direct reflection of their ecological diversity. For instance, illuminated open 297oceans may select for high locomotory capacity, with a streamlined drag-minimising, torpedo-shaped morphology. In the abyss, on the other hand, the absence of light and the reduction in the distance over which predators and prey interact may have relaxed selection pressures on the body shapes and swimming abilities of the resident fish (Verity et al., 2002; Seibel and Drazen, 2007). It seems intuitive that swimming ability should contribute directly to ecological performance and evolutionary fitness in fishes, although there is currently very little direct experimental evidence for this (Billerbeck et al., 2001; Lankford et al., 2001; Ghalambor et al., 2003). Nonetheless, the assumption that swimming performance is of ecological significance has been the impetus for many studies to understand the anatomical and physiological mechanisms that contribute to swimming performance, and how performance is influenced by factors in the environment (Domenici et al., 2007).