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ABSTRACT ObjectiveWe experimentally tested whether adult Black Sea Bass Centropristis striata belonging to the northern stock could theoretically overwinter in Long Island Sound (LIS) and whether doing so would affect their survival, growth, and gonadal investment and the lipid and lean content of their gonad, liver, and white muscle tissues. MethodsFish were caught via hook and line in LIS before and after their offshore winter migration (October 2022 and May 2023, respectively). Fifty individuals from October were reared for 200 d under flow-through conditions and fed diets of crushed mussels or herring. At the end of the experiment, laboratory and wild fish were assessed for their growth, gonadosomatic index, hepatosomatic index, and tissue-specific lipid and lean contents. ResultsLaboratory fish experienced unfavorable winter temperatures (∼5–12°C) for more than 5 months, exhibiting negligible growth and high mortalities. Mortalities began accruing after temperatures had reached their seasonal minimum of about 5°C in early February (day 120). Mortalities were lower for fish on the mussel diet (40%) than for those on the herring diet (68%), but survivors from the latter group had higher tissue lipid contents. Wild Black Sea Bass returning to LIS in spring had higher tissue lipid contents and greater gonadosomatic indices than surviving laboratory fish on either diet. ConclusionsAt present, overwintering in LIS appears possible but likely disadvantageous for Black Sea Bass because offshore winter migration results in greater energy reserves and subsequent reproductive investment. In the future, however, warming coastal waters will continue to shorten the duration of unsuitable winter temperatures, which could become conducive to year-round inshore residency or partial migration patterns in the northern stock of Black Sea Bass. 

Synopsis This series of papers highlights research into how biological exchanges between salty and freshwater habitats have transformed the biosphere. Life in the ocean and in freshwaters have long been intertwined; multiple major branches of the tree of life originated in the oceans and then adapted to and diversified in freshwaters. Similar exchanges continue to this day, including some species that continually migrate between marine and fresh waters. The series addresses key themes of transitions, transformations, and current threats with a series of questions: When did major colonizations of fresh waters happen? What physiographic changes facilitated transitions? What organismal characteristics facilitate colonization? Once a lineage has colonized freshwater, how frequently is there a return to the sea? Have transitions impelled diversification? How do organisms adapt physiologically to changes in halohabitat, and are such adaptive changes predictable? How do marine and freshwater taxa differ in morphology? How are present-day global changes in the environment influencing halohabitat and how are organisms contending with them? The purpose of the symposium and the papers in this volume is to integrate findings at multiple levels of biological organization and from disparate fields, across biological and geoscience disciplines. 

Synopsis Ecological transitions across salinity boundaries have led to some of the most important diversification events in the animal kingdom, especially among fishes. Adaptations accompanying such transitions include changes in morphology, diet, whole-organism performance, and osmoregulatory function, which may be particularly prominent since divergent salinity regimes make opposing demands on systems that maintain ion and water balance. Research in the last decade has focused on the genetic targets underlying such adaptations, most notably by comparing populations of species that are distributed across salinity boundaries. Here, we synthesize research on the targets of natural selection using whole-genome approaches, with a particular emphasis on the osmoregulatory system. Given the complex, integrated and polygenic nature of this system, we expected that signatures of natural selection would span numerous genes across functional levels of osmoregulation, especially salinity sensing, hormonal control, and cellular ion exchange mechanisms. We find support for this prediction: genes coding for V-type, Ca2+, and Na+/K+-ATPases, which are key cellular ion exchange enzymes, are especially common targets of selection in species from six orders of fishes. This indicates that while polygenic selection contributes to adaptation across salinity boundaries, changes in ATPase enzymes may be of particular importance in supporting such transitions.