  Shape variability represents an important direct response of organisms to
selective environments. Here, we use a combination of geometric morphometrics
and generalised additive mixed models (GAMMs) to identify spatial patterns of
natural shell shape variation in the North Atlantic and Arctic blue mussels,
Mytilus edulis and M. trossulus, with environmental gradients of temperature,
salinity and food availability across 3980 km of coastlines. New statistical
methods and multiple study systems at various geographical scales allowed the
uncoupling of the developmental and genetic contributions to shell shape and
made it possible to identify general relationships between blue mussel shape
variation and environment that are independent of age and species influences.
We find salinity had the strongest effect on the latitudinal patterns of
Mytilus shape, producing shells that were more elongated, narrower and with
more parallel dorsoventral margins at lower salinities. Temperature and food
supply, however, were the main drivers of mussel shape heterogeneity. Our
findings revealed similar shell shape responses in Mytilus to less favourable
environmental conditions across the different geographical scales analysed. Our
results show how shell shape plasticity represents a powerful indicator to
understand the alterations of blue mussel communities in rapidly changing
environments.
