Two species of moss growing in Moss Lake on Signy Island, South Orkney Islands, had unusual morphologies with large leaves and long internodes. Both Calliergon sarmentosum and Drepanocladus cf. aduncus differed from their terrestrial counterparts and from each other in the character of their leaves. The two genera differed in the ability of the terrestrial forms to develop a large-leaved growth form. Calliergon, which was represented by the same species in both environments, changed to the aquatic morphology when separated shoots were grown either submerged or under damp conditions. Ught intensity was not an important factor influencing change in morphology. In contrast, terrestrial Drepanocladus uncinatus, the closest taxonomic counterpart of D. cf. aduncus on Signy Island, did not show any adaptation under similar conditions. The aquatic forms also showed a corresponding degree of plasticity in their natural habitat. Calliergon varied from robust shoots to microphyllous or even leafless stems whereas Drepanocladus cf. aduncus only grew in the robust form. These differences were related to the success of the two species at different depths and it is suggested that the very low compensation points of the two mosses (reported in a separate study) resulted from the morphology of the plants.