Dataset: Shoot measurements (sheath length and width) for the eelgrass (Zostera marina) shoots sampled for whole genome sequencing collected from Bodega and Tomales Bay, CA, USA from July to September 2019

We collected 2 to 3 shoots attached by a rhizome from fifteen putative genets (separated by approximately 5 to 10 meters) at 14 sites across Tomales Bay and Bodega Harbor from a height below 0.0 mean lower low water (MLLW) (i.e., not sampling the uppermost or lowermost vertical distribution of Zostera marina). For two of the sites sampled in Bodega Harbor (Mason's Marina and Westside Park), we also collected a deeper set of specimens (at least -0.6 meters below MLLW) to test for differences between shallower versus deeper plants. We transported plants back to the University of California, Davis in a cooler with ice packs, and stored them for no more than 1 day in a recirculating seawater table before measuring the terminal shoot sheath width using calipers and sheath length using a ruler. The sheath width was measured at the abscission point and the length from the abscission point to the base of the shoot. Once measurements were made, we dissected out the tissue from within the leaf sheath and flash-froze them in liquid nitrogen and stored at -80 degrees Celsius (°C).