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The Pribilof Islands blue king crab ( Paralithodes platypus) fishery collapsed over two decades ago, is considered overfished, and has not recovered in the absence of fishing. Red king crab ( Paralithodes camtschaticus) abundance estimates have exceeded blue king crab for over three decades. We investigated the roles of larval recruitment and habitat availability as potential mechanisms limiting blue king crab recovery. We conducted young-of-year abundance and habitat assessments near Saint Paul Island from 2017 to 2019 and compared these results to 1983–1984 historical data to assess changes in larval supply and benthic substrates. Historically abundant blue king crab settlers were rarely encountered in our surveys. Red king crab settlers, once rare in historical surveys, are now more common throughout the region in low abundance. Benthic habitats did not change over time, as 90% of resampled sites had similar substrates. We conclude that larval supply and not benthic habitat is limiting juvenile recruitment. Our results could inform future fishery rebuilding efforts for blue king crab, which could include approaches to increase larval supply and juvenile recruitment through aquaculture. 

ABSTRACT Ocean acidification can affect the ability of calcifying organisms to build and maintain mineralized tissue. In decapod crustaceans, the exoskeleton is a multilayered structure composed of chitin, protein and mineral, predominately magnesian calcite or amorphous calcium carbonate (ACC). We investigated the effects of acidification on the exoskeleton of mature (post-terminal-molt) female southern Tanner crabs, Chionoecetes bairdi. Crabs were exposed to one of three pH levels – 8.1, 7.8 or 7.5 – for 2 years. Reduced pH led to a suite of body region-specific effects on the exoskeleton. Microhardness of the claw was 38% lower in crabs at pH 7.5 compared with those at pH 8.1, but carapace microhardness was unaffected by pH. In contrast, reduced pH altered elemental content in the carapace (reduced calcium, increased magnesium), but not the claw. Diminished structural integrity and thinning of the exoskeleton were observed at reduced pH in both body regions; internal erosion of the carapace was present in most crabs at pH 7.5, and the claws of these crabs showed substantial external erosion, with tooth-like denticles nearly or completely worn away. Using infrared spectroscopy, we observed a shift in the phase of calcium carbonate present in the carapace of pH 7.5 crabs: a mix of ACC and calcite was found in the carapace of crabs at pH 8.1, whereas the bulk of calcium carbonate had transformed to calcite in pH 7.5 crabs. With limited capacity for repair, the exoskeleton of long-lived crabs that undergo a terminal molt, such as C. bairdi, may be especially susceptible to ocean acidification.