Dataset: Summary data for whole genome sequencing on Lottia samples collected from Northern California to Baja from Nov 2020 to Jan 2022

We sampled 19 sites on the Pacific coast of North America, spanning most of the contemporary range of Lottia gigantea (urn:lsid:marinespecies.org:taxname:456593). We non-lethally excised a small piece of foot muscle tissue. For all locations except the four northern sites, we collected tissue from roughly 30 individuals per site, comprised of 10 individuals within each of the following size ranges: 10–25 mm (small), 30–40 mm (medium), and >40 mm (large). Sampling of the four northern sites (i.e., expanded range) also consisted of roughly 10 small, medium, and large individuals per site. However, as individual limpets at these four sites have been closely monitored over the past 8 years, these sites had known cohorts based on the estimated year of settlement. Size classes at sites for which we do not have monitoring data were chosen based on our data and previous growth curves (Kido & Murray, 2003) to approximately group individuals into those that settled during the marine heatwaves (MHWs), soon after the MHWs (late 2016 or late 2017), or well after the MHWs (late 2018 or late 2019). Tissue was stored in 90% ethanol at −20°C. We used the Qiagen DNeasy extraction kit to perform DNA extractions following the manufacturer's protocols. DNA quantity and quality were assessed with NanoDrop, Qubit, and gel electrophoresis. Library preparation followed the Nextera Lite protocol, with adaptations following Rowan et al. (2019). Briefly, this consisted of normalizing samples, tagmentation, PCR, pooling samples, and bead size selection. The purified samples were run on a High Sensitivity DNA Bioanalyzer chip and then sent to BGI Genomics for whole-genome sequencing on the DNBSEQ-T7 PE150 platform on four lanes of sequencing.