P. platycheleswas found to be euryhaline while the relatedP. longicornisproved to be stenohaline. Prior acclimation to 60 % s.w. improved survival in 40 % s.w. inP. platycheles. Salinity preference was demonstrated inP. platycheles, which could avoid low salinities and move along salinity gradients to do so; at high salinities the Weber‐Fechner “law” may operate for this crab as it could avoid high salinities but could not differentiate between two high salinities, separated by 3.4‰.P. platychelesdifferentiated between solutions on the basis of their total osmolarities, and the ability to do so was significantly impaired by removal of the antennules.P. longicornisexhibited no ability to distinguish between salinity levels.