Correlations with ocean salinity levels
To test for associations between genotypic variation and local ocean salinity within P. vaginatum, we used the local ocean salinity measurements from the World Ocean Atlas dataset (Zweng et al., 2019) to perform two statistical comparisons. First, we used a two-tailed t-test to test for differences in local ocean salinity between diploids and triploids. Then we used a fixed-effects linear model to test the effects of copy number of both the P. vaginatum subgenome (NPv) and the other unidentified subgenome (Nother). We compared the models with an interaction (salinity ~ NPv + Nother+ NPv:Nother) and without it (salinity ~ NPv + Nother). We chose the model without an interaction because it had a lower AIC (200.27 vs. 201.09) and because the interaction term was not significant when it was included.