Head shape disparity of the cod icefishes Trematominae (Notothenioidei, Teleostei)

The suborder Notothenioidei (Teleostei) has undergone a remarkable adaptive radiation in the Southern Ocean. Within this suborder, the subfamily Trematominae is endemic to Antarctic waters and represents a dominant component of the shelf fish fauna. After recent advances in molecular phylogenetics, 14 species of Trematomus are currently recognized (including Pagothenia and Cryothenia spp.) comprising both considerable morphological and ecological diversity. Here, we aim to illustrate the main axes of shape variation in Trematomus and explore the evolution of their morphology. A dataset of 96 specimens representing 10 species of Trematomus from the collection of the Natural History Museum of Paris was assembled, and landmark-based geometric morphometrics was applied to quantify head shape disparity. Regular regression analysis revealed significant interspecific allometry while a low percentage of shape variation was explained by size (R2 = 0.11; P < 0.001). Main shape variation across species was explored using a principal component (PC) analysis on shape variables. Two groups diverged along PC1: (1) T. bernacchii, T. hansoni, T. pennellii and T. tokarevi have short cephalic profiles with larger cheeks (lowest values along PC1); and (2) T. lepidorhinus, T. eulepidotus and T. newnesi show lengthened cephalic profiles with larger eyes (highest values along PC1). Trematomus scotti differed from all other species mainly along PC3 indicating more elongated cheeks. Phenogram based on Procrustes shape distances will be compared to molecular phylogenetic trees and morphometric data will be mapped onto phylogenetic trees in order to illustrate the mode of phenotypic diversification of Trematomus during evolution.