Salinity-driven size variability in Cyprideis torosa (Ostracoda, Crustacea)

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 ppm up to about 8 ppm 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 ppm in this study). This switch in size-salinity relationship coincides with a physiologically important switch between hypo- and hyper-osmotic regulation at about 8 ppm known for C. torosa.