SUMMARY 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 μm diameter was used as the criterion for the cessation of fibre recruitment. FDmaxincreased linearly with standard length (SL), reaching 500-650 μm 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<0.001). Estimates of trait values at nodes of the maximum likelihood phylogenetic tree were consistent with a progressive reduction in fibre number during part of the notothenioid radiation, perhaps serving to reduce basal energy requirements to compensate for the additional energetic costs of antifreeze production. For example, FNmax in Chaenocephalus aceratus (12 700±300, mean ± s.e.m., N=18) was only 7.7% of the value found in Eleginops maclovinus (164 000±4100, N=17), which reaches a similar maximum length (85 cm). Postembryonic muscle fibre recruitment in teleost fish normally involves stratified followed by mosaic hyperplasia. No evidence for this final phase of growth was found in two of the most derived families(Channichthyidae and Harpagiferidae). The divergence of the notothenioids in Antarctica after the formation of the Antarctic Polar Front and more recent dispersal north would explain the high maximum diameter and low fibre number in the derived sub-Antarctic notothenioids. These characteristics of notothenioids may well restrict their upper thermal tolerance, particularly for Champsocephalus esox and similar Channichthyids that lack respiratory pigments.

Reproductive aspects of a peripheral population of Galaxias maculatus are described and the relationship with the physiology and handling of energetic reserves under marginal environmental conditions is investigated. The G. maculatus population of Tierra del Fuego has an extended reproductive season, with differences in timing and duration compared to other populations of continental Patagonia, New Zealand and Australia. Elevated gonadosomatic indexes (I G ) were observed during this period in both sexes (maximum I G ¼ 33.49% males; 35.94% females). The high abundance of mature males (with high I G values) on the spawning grounds during the reproductive season suggests that they were waiting for the return of the mature females. Larger females reached total maturation at the beginning of the reproductive season, whereas the size of maturing females diminished toward the end (mean TL ¼ 96 mm, October; 70 mm, February). Both sexes showed an extremely high investment in reproduction, reaching a maximum I G of about 35% for both sexes. At the beginning of the reproductive season females reached the maximum median I H (3.37%) and males the minimum (0.96%), suggesting differences in the role of the liver in the management of energetic reserves during sexual maturation. The variation in the fat index (I F ) suggests that fat reserves were used to survive winter (maximum median I F > 1%, autumn; minimum about 0.2%, spring).

No abstract