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Oceanographic changes are occurring more rapidly in recent decades, with new implications for ocean ecosystems and adjacent human communities. It is important to bring attention to these changes while they are unfolding rather than after they have occurred. Here we report on a rapid shift toward colder, fresher water in the deep Gulf of Maine that, as of mid-June 2024, has persisted for at least six months. The shift likely represents an influx of Labrador Slope Water and resembles conditions that predated a major warming shift that occurred in 2011–2012. Deep-water oceanographic conditions in the Gulf of Maine have a strong influence on ecosystem dynamics, including the prey of critically endangered North Atlantic right whales, the seasonal and disease dynamics of American lobster, and the distribution and abundance of kelp forest communities, among others. Oceanographic surprises have an important role in this system, and monitoring how this shift unfolds, oceanographically and ecologically, will give new insights into how oceanographic signals can inform our understanding of ecosystem responses. 

Abstract The American lobster (Homarus americanusMilne Edwards, 1837) is an ecologically and economically valuable invertebrate in the Northwest Atlantic. Its geographic range is shifting northward due to ocean warming. While extensive research on the thermal tolerance of this species has been performed on adults and postlarvae, there have been few studies focused on its multiple early developmental stages. We applied transcriptomics to investigate transcriptional changes in laboratory-reared American lobster developmental stages I through V. Changes in gene expression were contextualized in the ontogenetic shifts in distribution that these different life history stages experience, with highly active stage IV exhibiting increased cellular metabolism and shell-building processes. We identified differential expression of transcripts related to thermal and UV stress in planktonic stages I-IV compared to benthic stage V, which suggests innate molecular defenses against these stressors. Together, these findings further our understanding of crustacean development in the context of climate change and can be used to inform population distribution modeling efforts. They also provide evidence for the need to investigate the potential trade-offs associated with responding to a changing environment on a stage-by-stage basis.