This dataset consists of the results of the genetic comparison of different species of the genus Jaera, a marine isopod, in the UK and USA. Jaera albifrons, J. forsmani, J. ischiosetosa, J. nordmanni and J. praehirsuta were collected from the underside of rocks at intertidal sites in the the UK. In the USA, J. posthirsuta, J. albifrons, J. ischiosetosa were similarly collected from sites in Maine and Massachusetts. Sample collection took place in 2015 and 2016. Sbf1 gene Restriction site Associated DNA (RAD) libraries were generated from total DNA using Illumina paired end DNA sequencing. In addition, individual microbiomes were characterised for Jaera albifrons, J. ischiosetosa, J. nordmanni, J. posthirsuta from UK populations using 16S rDNA metabarcode amplicon sequencing. The project aimed to assess parallel signatures of evolution across isolated populations and pinpoint regions of elevated genetic differentiation. All sampling and sequencing pipelines were undertaken by Marius Wenzel and Stuart Piertney (University of Aberdeen), with funding from NERC solo grant NE/M015661/1. Data are archived at the National Center for Biotechnology Information (NCBI).