Repeated morphological diversification in endemic Antarctic fishes of the genus Trematomus

The iterative nature of ecomorphological diversification is observed in various groups of animals. However, studies explicitly testing the consistency of morphological variation across and within species are scarce. Antarctic notothenioids represent a textbook example of adaptive radiation in marine fishes. Within Nototheniidae, the endemic Antarctic genus Trematomus consists of 15 extant species, some with documented large intraspecific variability. Here, we quantify head shape disparity in 11 species of Trematomus by landmark-based geometric morphometrics, and we illustrate repeated events of divergence and convergence of their head morphology. Taking advantage of the polymorphism observed in some species of Trematomus, we also show that two closely related species or clades (e.g., Trematomus bernacchii and T. hansoni) are characterised by the same level of morphological disparity as observed at the level of the entire genus. Interestingly, the same main axes of shape variation are shared between and within species, indicating repeated morphological diversification. Overall, we illustrate a similarity of intra- and interspecific patterns of phenotypic diversity providing new insights into the mechanisms that underlie the diversification of Antarctic fishes.