Distinct evolutionary signatures underlie body shape diversity across deep sea habitats

The deep sea is known for challenging abiotic and biotic conditions; yet, deep-sea fishes have been shown to have higher phenotypic diversity than shallow relatives. An open question is whether different habitats within the deep sea differentially contribute to this surprising phenotypic diversity. Here, we explore the joint effects of two major environmental dimensions, the benthic-pelagic axis and ocean depth, on body shape diversification in marine teleost fishes. We found that increasing ocean depth shifted axes of phenotypic evolution and promoted diversification for benthic, demersal, and pelagic fishes alike. However, body shape diversity and rates of body shape evolution did not scale consistently across habitats. For benthic fishes, rate increased more strongly than diversity with increasing ocean depth, while the reverse was true for pelagic fishes. Analyses of habitat transitions suggested that independent invasions may help explain the diversity of deep-pelagic fishes without invoking high evolutionary rates. Relaxed selection may also explain this diversity, as suggested by the wide range of deep-pelagic forms observed along an evolutionary axis of body elongation. Overall, our results reveal a mosaic of pathways through which body plan diversity accumulated across a vertebrate radiation, underscoring the importance of considering finer-scale habitat variation in broad-scale studies.