The living ostracod Cyprideis torosa (Jones, 1850) is geographically widespread, often abundant, occurring in modern and late Quaternary marginal-marine and athalassic environments world-wide. The species is capable of withstanding varying salinity over short (diurnal) timescales as well as adjusting to longer-term changes. Much attention has been paid in the past to the development of eco-phenotypic nodes and the shape of sieve-type pores on the external, lateral surfaces as indicators of particular salinity levels. In this paper we demonstrate a bimodal distribution between shell size (which can be determined directly from optical microscopy) and the salinity of the water in which the carapace formed. Between almost ‘freshwater’ salinity of about 1‰ up to about 8‰ the length of C. torosa increases linearly by about 10%, after this point there is a sharp break in the size–salinity relationship with carapace length reverting to values at or below those of freshwater and gradually declining in size by about 5% through the observed range (a maximum salinity of almost 40‰ in this study). This switch in size–salinity relationship coincides with a physiologically important switch between hypo- and hyper-osmotic regulation at about 8‰ known for C. torosa.