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Small crustaceans, such as the mysid Neomysis americana (S.I. Smith 1873), are a central component of coastal food webs and, while generally tolerant of a wide-range of environmental conditions, can be negatively affected by poor water quality. In this study, daily growth rates (GRD) and clutch size metrics of N. americana collected during the early and late summer of 2018–2019 were evaluated for the Choptank and Patuxent rivers, major tributaries of Chesapeake Bay known to exhibit different oxygenation regimes. Genetic variation in the mitochondrial CO1 locus was assessed to evaluate the potential intraspecific genetic structure within Chesapeake Bay. CO1 haplotype network analysis, phylogenetic analysis, and analysis of molecular variance revealed no genetic differences between Choptank and Patuxent river populations, with all Chesapeake Bay individuals belonging to a single genetic lineage (lineage C), of the N. americana cryptic species complex. Total and size-specific clutch size were approximately 18% and 53% higher, respectively, in the normoxic Choptank River during the early summer. Embryos within the marsupium, corrected for clutch size and female length, were consistently larger in the Choptank River during later larval development stages. Size-specific clutch size showed correlations with bottom water dissolved oxygen concentration (positive) and water temperature (negative). GRD did not differ between rivers or seasonally but juveniles grew twice as fast as adults. Given that all individuals genotyped from both rivers belonged to lineage C of the N. americana cryptic species complex, it is hypothesized that bottom water hypoxia (rather than genetic differentiation) is responsible for reduced clutch size in the Patuxent River. Our findings build on other recent work by providing evidence of a direct, negative relationship between hypoxia and local population dynamics of N. americana, a key ecological component of Chesapeake Bay’s food web. 

The movement of viruses in aquatic systems is rarely studied over large geographic scales. Oceanic currents, host migration, latitude-based variation in climate, and resulting changes in host life history are all potential drivers of virus connectivity, adaptation, and genetic structure. To expand our understanding of the genetic diversity of Callinectes sapidus reovirus 1 (CsRV1) across a broad spatial and host life history range of its blue crab host (Callinectes sapidus), we obtained 22 complete and 96 partial genomic sequences for CsRV1 strains from the US Atlantic coast, Gulf of Mexico, Caribbean Sea, and the Atlantic coast of South America. Phylogenetic analyses of CsRV1 genomes revealed that virus genotypes were divided into four major genogroups consistent with their host geographic origins. However, some CsRV1 sequences from the US mid-Atlantic shared high genetic similarity with the Gulf of Mexico genotypes, suggesting potential human-mediated movement of CsRV1 between the US mid-Atlantic and Gulf coasts. This study advances our understanding of how climate, coastal geography, host life history, and human activity drive patterns of genetic structure and diversity of viruses in marine animals and contributes to the capacity to infer broadscale host population connectivity in marine ecosystems from virus population genetic data.