Abstract
Identifying environmental predictors of phenotype is fundamentally important to many ecological questions, from revealing broadscale ecological processes to predicting extinction risk. However, establishing robust environment-phenotype relationships is challenging, as powerful case studies require diverse clades which repeatedly undergo environmental transitions at multiple taxonomic scales. Actinopterygian fishes, with 32,000+ species, fulfil these criteria for the fundamental habitat divisions in water. With four datasets of body size (ranging 10,905-27,226 species), I reveal highly consistent size-by-habitat-use patterns across nine scales of observation. Taxa in marine, marine-brackish, euryhaline and freshwater-brackish habitats possess larger mean sizes than freshwater relatives, and the largest mean sizes consistently emerge within marine-brackish and euryhaline taxa. These findings align with the predictions of seven mechanisms thought to drive larger size by promoting additional trophic levels. However, mismatches between size and trophic-level patterns highlight a role for additional mechanisms, and support for viable candidates is examined in 3439 comparisons.