Relationships between shell shape, water reserves, survival and growth of highshore littorinids under experimental conditions in New South Wales, Australia

The shell morphologies of the highshore littorinids, Littorina unifasciata Gray and Nodilittorina pyramidalis (Quoy & Gaimard) have previously been shown to vary at a variety of spatial scales, including among replicate sites at the same height, from height to height and from shore to shore. In this study, the relationships between morphology of the shell, the reserves of water held within the shell, the size of the foot and survival on different shores and rates of growth in different habitats were examined for L. unifasciata and, to a lesser extent, N. pyramidalis. Reserves of water were not consistently related to size or shape of the shell, but did increase as relative weight of shell increased. This may be due to the relatively smaller body providing more internal volume for extra-corporeal water. Water reserves and the amount of free water held in the shell were also not related to loss of water or survival during extended periods of emersion. Although the shape of the shell on sheltered and exposed shores was correlated with size of the foot, with the snails on an exposed shore having larger apertures and feet than those on a sheltered shore, transplant experiments did not show differential mortality between morphs from the different shores. All translocated and transplanted snails disappeared from the exposed shore at a greater rate than from the sheltered shore, but this was probably due to the snails dispersing out of the experimental areas rather than due to mortality. Therefore, many of the large-scale models that have previously been used to describe patterns of shell shape in intertidal gastropods do not appear to be important in these highshore littorinids. Finally, field experiments on growth of juvenile L. unifasciata indicated that rate of growth, largely governed by opportunity to feed rather than type and quantity of food, is the most likely explanation for the small-and large-scale patterns of shell shape that have been previously described in this species.