Reduction in muscle fibre number during the adaptive radiation of notothenioid fishes: a phylogenetic perspective

The fish fauna of the continental shelf of the Southern Ocean is dominated by a single sub-order of Perciformes, the Notothenioidei, which have unusually large diameter skeletal muscle fibres. We tested the hypothesis that in fast myotomal muscle a high maximum fibre diameter (FDmax) was related to a reduction in the number of muscle fibres present at the end of the recruitment phase of growth. We also hypothesized that the maximum fibre number (FNmax) would be negatively related to body size, and that both body size and size-corrected FNmax would show phylogenetic signal (tendency for related species to resemble each other). Finally, we estimated ancestral values for body size and FNmax. A molecular phylogeny was constructed using 12S mitochondrial rRNA sequences. A total of 16 species were studied from the Beagle Channel, Tierra del Fuego (5–11°C), Shag Rocks, South Georgia (0.5–4°C), and Adelaide Island, Antarctic Peninsula (–1.5 to 0.5°C). The absence of muscle fibres of less than 10·mm diameter was used as the criterion for the cessation of fibre recruitment. FDmax increased linearly with standard length (SL), reaching 500–650·mm in most species. Maximum body size was a highly significant predictor of species variation in FNmax, and both body size and size-corrected FNmax showed highly significant phylogenetic signal (P