Dataset: Littorina obtusata shell length, shell thickness, and tissue mass measured during a field experiment conducted at 12 sites in the Gulf of Maine from April to August 2021

To assess local and regional variation in shell thickness plasticity, we conducted a field experiment that utilized 12 Littorina obtusata populations in the Gulf of Main (GOM), with 6 in the northern Gulf and 6 in the southern Gulf. Hence, in addition to the 4 sites surveyed for crab abundance within each region (see Related Datasets), our experiment included two additional sites to increase site replication and power within each region. Juvenile L. obtusata (5-6 millimeters (mm) in length) were also collected from each population in late April, returned to the laboratory, and individually tagged with color-coded paint that was sealed with cyanoacrylate glue. Initial shell length and shell thickness were measured with digital calipers (Trussell 1996) and initial tissue mass was determined using a non-destructive weighing technique (see Palmer 1982). The initial measurements of shell length, shell thickness, and tissue mass were made on April 28th and April 29th, 2021.

In mid-May, 2021 snails were placed in replicate chambers and returned to their native sites in the mid-interidal zone (+1.5 meters Mean Low Water). In each chamber, we placed four snails (hereafter "response snails") in one of two paired chambers (10 x 10 x 7 centimeters (cm), length x width x height) that had mesh windows (mesh size = 3mm) to allow water flow and the transmission of risk cues. Each chamber also contained approximately 60 grams (g) of brown algae (Ascophyllum nodosum) to serve as food for response snails. The other chamber (the "stimulus chamber") was perforated with small holes on all sides which allowed response snails to be exposed to water-borne cues signaling either the presence (Crab, + C) or absence (No Crab, – NC) of predation risk. The 'Crab' treatment was created by placing a mature male green crab, Carcinus maenas, and 30 adult L. obtusata to serve as food for the crab in the stimulus chamber, whereas stimulus chambers for the 'No Crab' treatment received just 30 adult L. obtusata. These paired chambers were housed within a larger plastic chamber (14 x 14 x 16 cm) that also had mesh windows (mesh size = 3mm) to permit water flow. At each site, replicate chambers (n = 4 for each treatment) were anchored beneath the fucoid canopy and Tidbit loggers (Onset Computer Corp.) placed within a subset of chambers recorded seawater temperature at 5-minute intervals. The food supply for crabs was replaced approximately every 21 days while fucoid algae that served as food for the response snails was replaced at day 42. Replicate chambers remained in the field for 90 days before their return to the Northeastern University Marine Science Center (Nahant, Massachusetts) for final measurements of shell length, shell thickness, and tissue mass in response snails. The final measurements were made on August 10th and August 11th, 2021.