The complexity of phenotypic plasticity in the intertidal snail Nodilittorina australis

The intertidal snail Nodilittorina australis has distinct nodulose and striate morphs, which are plastic, developmental responses to sloped and vertical rocky shores, respectively. The nodulose morph cools more quickly, which should be advantageous on the sunnier, sloped shores. In contrast, the striate morph responds more quickly to submersion, which should be advantageous on the more wave‐intense vertical shores. Transplant experiments revealed that even large snails can alter development of new shell. The developmental transitions indicate partial dissociation of colour and sculpture. However, the incomplete association of component traits is seen in very few natural populations, indicating that canalization of nodulose and striate pathways has evolved to the extent that is adaptively relevant. There is a distinct asymmetry of the lability of development, whereby nodulose snails are much more likely to switch to development of striate shell than the reverse; this directionality occurs naturally as well as in transplant experiments. Rather than representing an underlying constraint, the asymmetry probably reflects a greater advantage of nodules in smaller snails. The only clear developmental constraint affecting this dimorphism is the association of the production of nodules with slower growth, an expected consequence of the rate‐limiting process of shell deposition.