The endemic Western Australian littorinids Nodilittorina nodosa (Gray, 1839) and N. australis (Gray, 1826) were described on the basis of distinctive shell morphologies: N. nodosa is characterized by distinct orange nodules, whereas N. australis typically has spiral grooves. In some places, intermediate phenotypes occur, and these have been interpreted as possible hybrids. In his 1989 review of the Littorinidae, Reid considered N. nodosa and N. australis to be conspecific, on the basis of their indistinguishable genital morphologies. However, a firm decision on their reproductive relationships requires reexamination of the morphological differences that were the basis of the original descriptions.In some places, intermediate phenotypes are common. However, these intermediates generally produce alternative phenotypes at different stages of their growth, suggesting that the differences are not genetically determined. A preliminary translocation experiment confirmed the plasticity of these morphotypes, supporting the interpretation that the two forms are conspecific. Comparisons of allozymes also revealed no differences between N. nodosa and N. australis, even in areas where the morphotypes are distinct. Furthermore, extremely low levels of geographic variation of allelic frequencies indicate extensive gene flow. Thus, the contrasting shell forms are responses to environmental conditions faced by recruits from a common pool. An unusual feature of this plasticity is that the sculpture of new growth can change abruptly in response to a change in habitat.