Arctic cod (Boreogadus saida) represents the most widespread and abundant fish in the Arctic and is a critical trophic link in its ecosystems. Like many species endemic to the region, it has lost essential habitat as the extent and thickness of sea ice have declined substantially in recent decades. Extreme warming induced by climate change continues to deteriorate polar marine environments. Thus, understanding how Arctic cod uses and partitions its habitat/environment is central to its conservation. We assessed Arctic cod functional morphology using traditional (including gill rakers) and geometric morphometrics and explored whether these differed among habitats and age classes using multivariate techniques. While distinct ecotypes have been proposed, these were not detected in our analyses. Rather, results show similar patterns in the external morphology of Arctic cod across habitats and age classes in the Beaufort Sea. However, analysis of gill rakers revealed concurrent habitat- and age-specific changes likely associated with dietary preferences. Findings indicate that although Arctic cod does not specialise in external morphological features in any habitat, important aspects of its internal feeding morphology shift as it grows, likely underpinning important distributional changes and the species' critical role in transferring energy in Arctic marine ecosystems.

Arctic cod were collected along predefined transects in the Beaufort Sea as part of the Canadian Beaufort Sea Marine Ecosystem Assessment program run by Fisheries and Oceans Canada. Collected fish were frozen and sent to the Fish Population and Conservation Genetics laboratory at McGill University for processing. Here, fish were thawed and photographed in standardised manner. Because some fish were in poor conditions (after collection, freezing, and thawing) not all fish were usable in all analyses (as described in the text). Fish pictures were analysed for geometric morphometrics to assess shape and shape features. Traditional morphology was also assessed from standardised pictures. The gill arch on the right side of each usable fish was extracted under a dissecting microscope and the length of the gill arch and the number of gill rakers was recorded.