The complexity of phenotypic plasticity in the intertidal snail Nodilittorina australis

The intertidal snail Nodilittorina australis has distinct nodulose and stri ate morphs, which are plastic, developmental responses to sloped and vertic al rocky shores, respectively. The nodulose morph cools more quickly, which should be advantageous on the sunnier, sloped shores. In contrast, the str iate morph responds more quickly to submersion, which should be advantageou s on the more wave-intense vertical shores. Transplant experiments revealed that even large snails can alter development of new shell. The development al transitions indicate partial dissociation of colour and sculpture. Howev er, the incomplete association of component traits is seen in very few natu ral populations, indicating that canalization of nodulose and striate pathw ays has evolved to the extent that is adaptively relevant. There is a disti nct asymmetry of the lability of development, whereby nodulose snails are m uch 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 d evelopmental constraint affecting this dimorphism is the association of the production of nodules with slower growth, an expected consequence of the r ate-limiting process of shell deposition. (C) 2001 The Linnean Society of L ondon.