Penaeid shrimp live in saline water ranging from 15 to 50 ppt, but salinity tolerance is usually more restricted in the adults than the juveniles or larvae. Spawning adults are generally restricted to habitats where the salinity is no less than 28 ppt. Species specific salinity optima differences have been noted. P. japonicus are adapted to a narrow, more oceanic salinity range, and larval development is best at salinities of 27 - 32 ppt. Juvenile pink shrimp are found in water of 5 ppt during summers. They generally avoid salinities in excess of 65 ppt. Adults are nearly always confined to 33 - 36 ppt sea water (Tabb et al., 1972). P. monodon can, as an adult, tolerate the low and variable salinity of fish ponds (Shigueno, 1975). Reduced salinity is rarely a problem as long as the shrimp are able to migrate to deeper, more saline water (Korringa, 1975) or into outer coastal regions away from river mouths (Munro, 1975). The inshore shallow water habitats of many juvenile penaeids are generally either continuously or intermittently low in salinity. Penaeids cope with this by being euryhaline which greatly increases the inshore area that can be used as nursery grounds. Large postlarvae had a wider salinity tolerance than smaller individuals of the same age. Larger and older postlarval stages tend to be better adapted to low salinity conditions than are either the earlier stages or the adults. Tolerance is not the only way of coping with low salinity: another method is to avoid unsuitable salinities (Dall et al., 1990).

The low salinity that characterizes many nursery areas appears to be used by some species of penaeids as a means of identifying these areas the behavioural responses of penaeid postlarvae to low salinity have been proposed as a mechanism for locating and entering estuaries. Salinity responses are also involved in the emigration of some species of penaeids from estuaries (Dall et al., 1990).