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ABSTRACT Many coastal marine species experienced Pleistocene gene flow between the North Pacific and Atlantic. Understanding historical connectivity between ocean basins should aid in predicting how regional faunas will respond to recent warming that has intensified trans‐Arctic dispersal. Wetland fauna of the Northwestern Atlantic may have survived in estuarine refugia throughout glacial cycles, or recolonised from the southern coast, North Pacific or Northeastern Atlantic. Here, we used multilocus genetic markers and historical climate data to investigate lineage distribution and connectivity among populations of the nominally cosmopolitan sea slugAlderia modesta, sampled from mudflats on both coasts of the North Pacific and North Atlantic. Mitochondrial DNA clades from European and North American populations were deeply divergent and reciprocally monophyletic; differences at seven polymorphic nuclear loci indicated prolonged absence of trans‐Atlantic gene flow. A Pacific ancestor likely first colonised the Atlantic during the marine biotic interchange of the middle Pliocene ~3.5 Ma. Both mtDNA phylogenetics and nuclear genotype assignments support repeated trans‐Arctic colonisation of the Northwestern Atlantic from the Pacific during inter‐glacial cycles; no gene flow was evident since the last glacial maximum, however. Time‐calibrated coalescent phylogenies, Bayesian skyline plots and haplotype networks all indicated recent population expansions in the Pacific and Europe, but not Northwestern Atlantic. In both the Pacific and Northwestern Atlantic, older lineages persisted in patchy refugia north of glacial margins, while a derived clade of Pacific haplotypes indicates northward post‐LGM expansion. The biogeographical history ofAlderiacontrasts with rocky‐shore taxa that were largely extirpated by glacial advance and recolonised from refugia following the last glacial maximum. Based on molecular differences and distinctions in radular and penial stylet morphology, we resurrect the nameAlderia harvardiensisGould 1870 forAlderiafrom the Northwestern Atlantic and North Pacific;A. modestarefers exclusively to European slugs. 

Among biocollections, mollusks are a particularly powerful resource for a wide range of studies, including biogeography, conservation, ecology, environmental monitoring, evolutionary biology, and systematics. U.S. mollusk collections are housed in stand-alone natural history museums, at universities, and in a variety of governmental and non-governmental institutions. Differing in their histories, specializations, and uses, they share common needs for long-term development, and collectively contribute to biodiversity knowledge at regional, national, and global scales. Commitment by dedicated staff, collectors, and volunteers, institutional investments, philanthropy, and governmental funding have built and maintained these collections and their support infrastructure. Efforts by the North American malacological collection community since the early 1970s led to coordination in database design but left the data isolated in individual institutions. Collection digitization developed through a combination of individual/institutional initiatives and federally supported projects funded by the National Science Foundation (NSF) and the Institute of Museum and Library Services (IMLS). Advances in digital technology enabled the shift toward nationally and globally unified collections. Networking and collaboration were greatly accelerated by NSF’s Advancing Digitization of Biodiversity Collections (ADBC) program, which created a central coordinating organization (iDigBio) and funded Thematic Collections Network (TCN) projects. One such TCN was developed to mobilize nearly 90% of the known U.S. museum-collections-based data of the U.S. Atlantic and Gulf coasts (Mobilizing Millions of Marine Mollusks of the Eastern Seaboard—ESB). The project, involving 16 museum collections (plus the Smithsonian Institution as federal partner), combines data from approximately 4.5 million specimens collected from the ESB region and makes them available to the TCN portal InvertEBase and other aggregators such as iDigBio and GBIF. In addition to fostering community and expanding the corpus of available digitized mollusk records through new data entry and georeferencing (GEOLocate, CoGe) and standardizing taxonomy, the project drove key innovations for the invertebrate collections community. For instance, it worked with the Biodiversity Information Standards (TDWG) group to create a new Darwin Core standard term, “Vitality”, expanded GEOLocate to support complex geospatial types, integrated global elevation and bathymetric datasets directly into georeferencing workflow, and developed various education and outreach public outreach products. Synthesizing from the 15 following articles with individual histories of ESB-participating mollusk collections, several topics are discussed—such as what defines a “good” mollusk collection in the digital age and the importance of federal support for this national resource.