Convergence in Diet and Morphology in Marine and Freshwater Cottoid Fishes
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Habitat transitions provide opportunities for drastic changes in ecology, morphology, and behavior of organisms. The goal of this study is to determine whether the numerous evolutionary transitions from marine to freshwaters have altered the pattern and pace of morphological and lineage diversification within the sculpins (Cottoidea). The broad global distribution and wide-ranging ecology of sculpins make them an ideal study system in which to analyze marine invasions in northern latitudes. The sheer diversity of sculpins in isolated systems like Lake Baikal has led some to suggest these fishes (particularly Cottus) underwent an adaptive radiation upon their invasion of freshwaters in north Asia and Europe. Marine sculpins appear to be more diverse than freshwater sculpins, and while cottoids show signs of explosive radiation early in their evolutionary history, our study shows that unequal patterns of clade disparity among these lineages has led to constant rates of morphological and lineage diversification. Feeding morphology traits are highly conserved in cottoids, with both marine and freshwater species displaying similar morphologies despite widely-varying diets. While convergence in feeding morphology and dietary ecology is widespread in freshwater and marine cottoids, some specialist taxa, including planktivores and piscivores, show notable departures from the ancestral sculpin body plan. These include planktivores like Comephorus dybowskii, as well as piscivores Hemitripterus bolini and Myoxocephalus polyacanthocephalus. C. dybowskii’s unique feeding morphology, including a high posterior mechanical advantage and long dentary bones, may be further explained by its habitation of the pelagic environment.