The taxonomy of the ostracod genus Mytilocypris is based in part on characters of shell morphology. Specifically, M. minuta, M. mytiloides and M. tasmanica chapmani lack distinctive internal characters and are differentiated largely on size and shape of the shell. The three taxa also differ in the salinity of the habitats they occupy. A field study showed a steady decrease in adult size, spanning the size range of all three taxa, over the course of a season within single lakes, in association with changing salinity and other environmental variables. The present study was conducted to investigate whether each phenotype could be produced under laboratory conditions of variable salinity. Offspring of the three parental phenotypes were raised under high and low salinity. Regardless of parental type, offspring grew more quickly, but reached a smaller size at high salinity, and grew more slowly, but reached a larger size at low salinity. The shape of the shell was found to have a significant size component. Plasticity in size and shape suggests these are unreliable characters for taxonomy. Accelerated development may be an adaptation to living in an ephemeral habitat, with salinity being one of the cues Mytilocypris uses to gauge habitat duration.