Transient marine species disproportionately expand the morphospace of North American continental freshwater fishes.

Abstract

Marine taxa have repeatedly introduced diversity to continental freshwater fish faunas, yet their contribution to total morphological disparity remains poorly quantified. We analysed body shape disparity across 642 North American fish species using three-dimensional geometric morphometrics and kernel density hypervolumes to assess the extent to which transient marine species expand morphospace beyond that occupied by exclusively freshwater species. Marine-affiliated taxa account for 7.7× the hypervolume of freshwater species and expand the total morphospace by 144%, significantly exceeding chance expectations (p < 0.0001). Marine and freshwater morphospaces exhibited minimal overlap, with marine taxa extending the morphospace along multiple axes of body shape disparity. Despite comprising only 27% of the species, marine-affiliated taxa contribute disproportionately to morphological variance and exhibit greater within-group dispersion. These patterns support a 'museum effect', in which marine environments preserve the morphological diversity lost from more unstable freshwater systems. Our results demonstrate that regional freshwater morphological diversity depends partly on connectivity to marine source pools, with implications for understanding evolutionary constraints and conserving functional diversity in freshwater ecosystems.