The role of desiccation pressures and surface area/volume relationships on seasonal zonation and size distribution of four intertidal decapod crustacea from New Zealand: implications for adaptation to land

Abstract

Seasonal changes in size distribution of four decapod crustacean species from New Zealand were assessed as a function of increasing elevation above the low tide level. These results were compared with desiccation data for representatives of the same species with known surface areas and volumes. Surface areas were measured by applying a thin, even coat (20 Å deep) of aluminium over crabs and polythene sheets of known surface area mounted on an S.E.M. planetary workholder. The amount of aluminium coating the crabs and polythene sheets was then measured using atomic absorption spectrometry. Data suggest that the absence of individuals smaller than 6.5 mm (carapace width) among the two grapsid species studied, the seasonal shifts of modal size, and a general pattern of retreat from upper levels of the shore during the summer months can be explained primarily on the basis of desiccation pressures acting upon crabs within given ratios of surface area/volume. These findings conform to the hypothesis of exponential relationships between surface area and volume as a chief limiting factor in the landward movement of crabs.