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Insights into the generation of diversity in both plants and animals have relied heavily on studying speciation in adaptive radiations. Russia's Lake Baikal has facilitated a putative adaptive radiation of cottid fishes (sculpins), some of which are highly specialized to inhabit novel niches created by the lake's unique geology and ecology. Here, we test evolutionary relationships and novel morphological adaptation in a piece of this radiation: the Baikal cottid genus,Cottocomephorus, a morphologically derived benthopelagic genus of three described species. We used a combination of mitochondrial DNA and restriction site associated DNA sequencing from allCottocomephorusspecies. Analysis of mitochondrial cytochrome b haplotypes was only able to two resolve two lineages:C. grewingkiiandC. comephoroides/inermis. Phylogenetic inference, principal component analysis, andfaststructureof genome‐wide SNPs uncovered three lineages withinCottocomephorus:C. comephoroides,C. inermisandC. grewingkii. We found recent divergence and admixture betweenC. comephoroidesandC. inermisand deep divergence between these two species andC. grewingkii. Contrasting other fish radiations, we found no evidence of ancient hybridization amongCottocomephorusspecies. Digital morphology revealed highly derived pelagic phenotypes that reflect divergence by specialization to the benthopelagic niche inCottocomephorus. AmongCottocomephorusspecies, we found evidence of ongoing adaptation to the pelagic zone. This pattern highlights the importance of speciation along a benthic‐pelagic gradient seen inCottocomephorusand across other adaptive fish radiations.

Atlantic Cutlassfish,Trichiurus lepturus, have large, barbed, premaxillary and dentary fangs, and sharp dagger‐shaped teeth in their oral jaws. Functional teeth firmly ankylose to the dentigerous bones. We used dry skeletons, histology, SEM, and micro‐CT scanning to study 92 specimens ofT. lepturusfrom the western North Atlantic to describe its dentition and tooth replacement. We identified three modes of intraosseous tooth replacement inT. lepturusdepending on the location of the tooth in the jaw. Mode 1 relates to replacement of premaxillary fangs, in which new tooth germs enter the lingual surface of the premaxilla, develop horizontally, and rotate into position. We suggest that growth of large fangs in the premaxilla is accommodated by this horizontal development. Mode 2 occurs for dentary fangs: new tooth germs enter the labial surface of the dentary, develop vertically, and erupt into position. Mode 3 describes replacement of lateral teeth, in which new tooth germs enter a trench along the crest of the dentigerous bone, develop vertically, and erupt into position. Such distinct modes of tooth replacement in a teleostean species are unknown. We compared modes of replacement inT. lepturusto 20 species of scombroids to explore the phylogenetic distribution of these three replacement modes. Alternate tooth replacement (in which new teeth erupt between two functional teeth), ankylosis, and intraosseous tooth development are plesiomorphic to Bluefish + other Scombroidei. Our study highlights the complexity and variability of intraosseous tooth replacement. Within tooth replacement systems, key variables include sites of formation of tooth germs, points of entry of tooth germs into dentigerous bones, coupling of tooth germ migration and bone erosion, whether teeth develop horizontally or immediately beneath the tooth to be replaced, and how tooth eruption and ankylosis occur. Developmentally different tooth replacement processes can yield remarkably similar dentitions.