Physiological variability of sodium regulation in geographically separated brackish-water and marine populations of Sphaeroma (Crustacea: Isopoda: Flabellifera)

Abstract

Sodium influx and efflux in individuals from marine and brackish-water populations of the isopod genus Sphaeroma were measured. These were taken from Mediterranean littoral lagoons experiencing different salinity conditions, and from the Atlantic coast. Differences in haemolymph Na + concentrations, transepithelial potential differences (TEP), and sodium permeability ( P Na ) were observed between populations of S. serratum Fabricius, and between this species and S. hookeri Leach. In the latter low salinity acclimation reduced P Na from 2.36 to 1.13 × 10 −8 m · s −. Their active Na + -uptake systems also showed intra- and interspecific population differences, namely in maximum (Na + -depleted) rate ( V max ) and affinity ( K m ). Exchange diffusion and passive influx components were identified. In some populations Na + balance in low salinities is achieved by increased V max which offsets high net (passive) losses. These losses may be reduced in S. hookeri by a lowered haemolymph Na + concentration and P Na . In the latter, a relatively rapid acclimatory response may be important in reducing net loss rate. Acclimation (1 month) did not, however, significantly alter either V max or K m for Na + uptake in this species. The significance of physiological variability in the adaptation to low salinities of the Sphaeroma species-complex is discussed.