A combined morphometric and phylogenetic analysis of an ecomorphological trend: pelagization in Antarctic fishes (Perciformes: Nototheniidae)

The Antarctic fish family Nototheniidae (Perciformes) presumably originated from a benthic ancestor, and several lineages have evolved to live or at least feed in the water column, a trend called pelagization. Here, we use information on phylogeny, allometric growth, and diet composition for an integrated analysis of morphological and ecological diversification in this group, mainly focusing on the subfamilies Trematominae and Pleuragramminae. A phylogenetic analysis of data published in earlier systematic studies produced eight equally parsimonious trees, all indicating that several previously recognized taxa are paraphyletic. These phylogenetic trees all suggest multiple origins of pelagic life styles. Multivariate morphometric analyses including nine species showed that juveniles and adults grow according to a common pattern of ontogenetic allometry. The morphometric differences among species are mosdy the result of lateral transpositions of the growth trajectories, indicating that embryonic and larval development is more important as a determinant of morphological variation than allometric growth as juveniles and adults. We studied patterns of interspecific variation with principal components and the covariation between morphometric variables and food composition with a partial least-squares analysis. Both analyses revealed a gradient from benthic to pelagic foragers. Measurements of structures involved in swimming have a prominent role in these analyses, suggesting adaptive evolution of these traits. Tracing morphometric traits on the phylogenetic trees revealed a considerable amount of evolutionary plasticity, showing that species related phylogenetically need not be morphologically similar, but can diverge considerably, perhaps as a response to natural selection and adaptation to different habitats and foraging modes. In accordance, a test of phylogenetically independent contrasts showed that bursts of increased morphological change accompanied habitat shifts.