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 (see Related Datasets). Seawater temperature was recorded at high tide at study sites prior to and during the outplant experiment. All temperature data were recorded by Tidbit temperature loggers (Onset Computer Corporation) placed in the intertidal zone at +1.5 meters Mean Low Water. Temperature was sampled at 5-minute intervals and temperature values corresponding to daily high tides were extracted for statistical analyses.