Diversity in mouthpart morphology and trophic niche in Antarctic Iphimediidae (Amphipoda)

The fauna living on the Antarctic shelf has been moulded by millions of years of extreme seasonality, extreme isolation, extreme cold and recurrent glacial periods. With at least 13 genera and 46 species, the family Iphimediidae is one of the predominant amphipod taxon of the Antarctic shelf. Recent molecular studies show the monophyly of these species and the presence of species complexes. The high diversity in mandible morphologies observed in some species as well as their gut content have led to the hypothesis that Iphimediidae are micropredatory browsers with specialist feeding behaviours. Their mandibles are marked by the positioning of the incisor medially enabling it to cut in a frontal plane like scissors, probably one of the major reasons for their local ecological success. Conversely, the basis of their maxillipeds seems to vary less between species with different ecologies.
This study sets out to (i) explore the diversity of mouthpart morphologies and (ii) study the relation between morphology and trophic ecology in Antarctic Iphimediidae. To achieve this, we used a recent time-calibrated phylogeny of the family to control for the effect of relatedness in form-function relationships, isotopic data (δ13C, δ15N) as a proxy for the trophic niche, and 3D-geometric- morphometric data from μCT-scans of the mandible and the maxilliped’s basis from 50 putative species of Iphimediidae.
Measures of phylogenetic signal and phylomorphospaces show that Iphimediidae present high diversity within subclades while simultaneously high morphological convergence in distant species. Links between isotopic data and morphology are not straightforward. Indeed, different adaptations can be found in one functional group and similar adaptations can be found in different functional groups. Morphologies that allow for adaptability and opportunistic behaviour are of interest in such a seasonally variable environment as the Antarctic shelf.