Morphological traits distinguish feeding guilds in a Southern Ocean demersal fish community

Morphological traits reflect an organism's ecological niche and role within ecosystems. Identifying how traits are associated with diet will therefore improve our understanding of the drivers of community structure. We combined individual morphological measurements with stomach contents from nine demersal fish species from the subantarctic island of South Georgia, where climate change is impacting the distribution of a key prey species, Antarctic krill. Cluster analysis identified five feeding guilds, with traits such as gape size proving especially useful for determining guild membership. Individuals feeding on fish had larger gapes and higher caudal and pectoral fin aspect ratios, enhancing their ability to capture and consume such large, fast prey. In contrast, benthic feeders had smaller gapes and lower fin aspect ratios, reflecting reliance on suction feeding and higher manoeuvrability. Random Forest analysis reliably predicted feeding guild membership based on these simple traits, highlighting the strong links between ecology and morphology. This study provides an important step forward in the application of trait-based approaches within the Southern Ocean. Continued research into links between morphology and diet will improve our understanding of niche partitioning in marine ecosystems and aid our ability to predict the effects of environmental change on community composition and structure.