Colonization of novel habitats can result in marked phenotypic responses to the new environment that include changes in body shape and opportunities for further morphological diversification. Fishes have repeatedly transitioned along the benthic–pelagic axis, with varying degrees of association with the substrate. Previous work focusing on individual lineages shows that these transitions are accompanied by highly predictable changes in body form. Here, we generalize expectations drawn from this literature to study the effects of habitat on body shape diversification across 3344 marine teleost fishes. We compare rates and patterns of evolution in eight linear measurements of body shape among fishes that live in pelagic, demersal and benthic habitats. While average body shape differs between habitats, these differences are subtle compared with the high diversity of shapes found within each habitat. Benthic living increases the rate of body shape evolution and has led to numerous lineages evolving extreme body shapes, including both exceptionally wide bodies and highly elongate, eel-like forms. By contrast, we find that benthic living is associated with the slowest diversification of structures associated with feeding. Though we find that habitat can serve as an impetus for predictable trait changes, we also highlight the diversity of responses in marine teleosts to opportunities presented by major habitats.

We present a dataset that quantifies body shape in three dimensions across the teleost phylogeny. Built by a team of researchers measuring easy-to-identify, functionally relevant traits on specimens at the Smithsonian National Museum of Natural History it contains data on 16,609 specimens from 6144 species across 394 families. Using phylogenetic comparative methods to analyze the dataset we describe the teleostean body shape morphospace and identify families with extraordinary rates of morphological evolution. Using log shape ratios, our preferred method of body-size correction, revealed that fish width is the primary axis of morphological evolution across teleosts, describing a continuum from narrow-bodied laterally compressed flatfishes to wide-bodied dorsoventrally flattened anglerfishes. Elongation is the secondary axis of morphological variation and occurs within the more narrow-bodied forms. This result highlights the importance of collecting shape on three dimensions when working across teleosts. Our analyses also uncovered the fastest rates of shape evolution within a clade formed by notothenioids and scorpaeniforms, which primarily thrive in cold waters and/or have benthic habits, along with freshwater elephantfishes, which as their name suggests, have a novel head and body shape. This unprecedented dataset of teleostean body shapes will enable the investigation of the factors that regulate shape diversification. Biomechanical principles, which relate body shape to performance and ecology, are one promising avenue for future research.

Deep‐sea fishes have long captured our imagination with striking adaptations to life in the mysterious abyss, raising the possibility that this cold, dark ocean region may be a key hub for physiological and functional diversification. We explore this idea through an analysis of body shape evolution across ocean depth zones in over 3000 species of marine teleost fishes. We find that the deep ocean contains twice the body shape disparity of shallow waters, driven by elevated rates of evolution in traits associated with locomotion. Deep‐sea fishes display more frequent adoption of forms suited to slow and periodic swimming, whereas shallow living species are concentrated around shapes conferring strong, sustained swimming capacity and manoeuvrability. Our results support long‐standing impressions of the deep sea as an evolutionary hotspot for fish body shape evolution and highlight that factors like habitat complexity and ecological interactions are potential drivers of this adaptive diversification.

The shape of shark teeth varies among species, but traditional testing protocols have revealed no predictive relationship between shark tooth morphology and performance. We developed a dynamic testing device to quantify cutting performance of teeth. We mimicked head-shaking behaviour in feeding large sharks by attaching teeth to the blade of a reciprocating power saw fixed in a custom-built frame. We tested three tooth types at biologically relevant speeds and found differences in tooth cutting ability and wear. Teeth from the bluntnose sixgill (Hexanchus griseus) showed poor cutting ability compared with tiger (Galeocerdo cuvier), sandbar (Carcharhinus plumbeus) and silky (C. falciformis) sharks, but they also showed no wear with repeated use. Some shark teeth are very sharp at the expense of quickly dulling, while others are less sharp but dull more slowly. This demonstrates that dynamic testing is vital to understanding the performance of shark teeth.

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