Water permeability [based on the half-time of exchange of body water with the environment (T1/2)] of the upper estuarine prawn Palaemon longirostris was measured in a range of salinities at 4, 12 and 20°C. Prawns acclimated for 7 days were compared with prawns exposed to acute salinity changes. Acclimation to low salinity and low temperature caused a significant reduction in permeability. Transfer of prawns from 34 to 0.5‰ had no immediate effect on permeability; however, transfer from 0.5 to 34‰ caused an immediate significant rise in permeability. Heart rate of P. longirostris acclimated to 0.5 and 34‰ at 4, 12 and 20°C was unaffected by salinity, but was significantly reduced at low temperature.
The permeabilities of three other prawn species (Palaemonetes varians, Crangon crangon and Palaemon elegans), which have different horizontal distributions in estuaries and different salinity tolerance ranges, were also studied and compared with the values obtained for P. longirostris. P. varians and C. crangon showed significantly reduced permeabilities at low compared with high salinities; however, the permeability of P. elegans was unchanged at the two salinities used for this species (22 and 34‰). At each experimental salinity, permeabilities followed the sequence: P. longirostris<P. varians<C. crangon<P. elegans. Results show that reduction of permeability is an important physiological adaptation to life in dilute saline regions and may be involved in separating species with overlapping salinity tolerance ranges.