Study of the eco-morphological diversification of the family Iphimediidae (Crustacea, Amphipoda) on the Antarctic continental shelf

Abstract

The Antarctic shelf and the fauna living on it have been moulded by millions of years of extreme seasonality, extreme isolation, extreme cold and recurrent glacial periods. Iphimediidae is a family of amphipods well-represented on the Antarctic shelf with at least 13 genera and 46 species. Recent molecular studies have shown that these are monophyletic and are in fact for the most part species complexes. Iphimediidae present a large diversity in mouthpart morphologies. Their mandibles are marked by the positioning of the incisor along the median line enabling it to cut in a frontal plane like scissors. This might have been one of the major reasons for their ecological success. Conversely, the basis of the maxilliped seems to conserve its general morphology across taxa. Using a time calibrated phylogeny of the family from a recent study, isotopic data as a proxy for the trophic niche and geomorphometric data of the mandible and the maxilliped’s basis from 50 putative species of Iphimediidae, this study set out to (i) explore the mouthparts’ morphological diversity, (ii) study the relation between morphology and trophic ecology and (iii) analyse the evolution of the morphological traits along the phylogeny. For (i), measures of phylogenetic signal are significant but low for both mouthparts indicating variability among related species. The phylomorphospaces indicates that different clades present different modes of diversification. It also indicates that many species cluster in a limited region of morphospace. For (ii), using a MANOVA in a penalized likelihood framework to test if isotopic data could explain morphology no significant results were obtained suggesting a one-to-many situation in mandibles and a many-to-one for the maxilliped’s basis. For (iii), fitting models of trait evolution to our data, we find that the Ornstein-Uhlenbeck is the best fitting model where morphology is driven by selection towards an optimum. A study of disparity through time finds that disparity is higher than expected under a stochastic model. The high diversification within subclades is likely due to recurrent extinction and allopatric speciation events due to icesheet advance and isolation of species in refugia.