Since the Antarctic Polar Front developed more then 20 million years ago,temperatures in the southern-most continent have fallen, forcing many of the species trapped there to adapt as the climate hardened. Notothenioid fish comprise more then 50% of the species in Antarctica's waters, and one of their responses to the harsher conditions has been to increase their muscle fibre size. Could the fishes' larger muscle fibres be a derived feature that emerged as Notothenioid species diverged? Ian Johnston was intrigued by the enormous muscle fibres, but to get to the bottom of the evolutionary question, he says`you can't just look at the fish in the context of today's environment'because not all of the fish have stayed firmly trapped in Antarctica's icy conditions; some have returned to the warmer waters near Tierra del Fuego. So the species that passed time in Antarctica experienced very different climatic conditions to the Notothenioids that stayed put in warmer waters. If he was going to be able to compare the fishes' muscle structures, without interference from the fishes' confusing climatic past, Johnston would need to build the Notothenioid's family tree(p. 2595.

But it took Johnston four years of travel and help from colleagues at the British Antarctic Survey before he had collected 16 species of Notothenioid fish from Antarctica, as well as the warmer waters around Tierra del Fuego and in the Beagle Channel. Back in the lab, Johnston and his coworkers analysed the fishes' muscle structure, measuring the fibres' dimensions and counting the fibre numbers. Then they constructed a phylogentic tree for the fish,based on mitochondrial 12S RNA, and compared how the muscle fibre size had changed since the species began diverging. From the tree, Johnston could see that the most recently derived species had the largest muscle fibres but, more surprisingly, they had far fewer muscle fibres than their distant relatives. The loss in fibres also followed a phylogentic trend.

Johnston suggests that the increase in fibre diameter could reduce the fishes' energy costs. As the fibre's diameter increases, its surface-to-volume ratio decreases, and so the fibre requires fewer energetically costly ion-pumping proteins to maintain its ionic balance. By reducing the number of pumps in the fibre's membrane the fish will potentially conserve energy, which they can then divert to other metabolic demands such as antifreeze production to protect themselves from Antarctica's perishing conditions.

Johnston, I. A., Fernández, D. A., Calvo, J., Vieira, V. L. A., North, A. W., Abercromby, M. and Garland, T., Jr(
). Reduction in muscle fibre number during the adaptive radiation of notothenioid fishes: a phylogenetic perspective.
J. Exp. Biol.