Samples of Carcinus maenas (urn:lsid:marinespecies.org:taxname:107381) were collected by large network of collaborators, usually by trapping but approaches vary. Crabs were first frozen at -20C, then muscle or gill tissue was dissected out, preserved in 95% ethanol, and archived at -80C. Genomic DNA was extracted with Chelex 100 resin. Targeted genotyping of ~200 SNPs was conducted following the approach of Campbell et al. 2015; target SNPs were identified in prior population genomics analysis in the region based on RNA-Seq data. Samples were prepared in two groups, with overlapping but not identical targets; some samples were normalized using SequalPrep plates and others were not, following results of replicate sequencing with ("norm" in sequence names) and without normalization showing that this step did not improve genotyping success. Libraries were individually dual-indexed with plate and well barcodes, and multiplexed 800-1500 / lane on two lanes of 150 bp paired-end Illumina NovaSeq X 10B sequencing at the University of California at Berkeley's QB3 Genomics Core lab. Some samples were re-run across multiple plates / lanes as technical replicates ("repA" and "repB" in sequence names).