Salinity tolerance and sodium balance in the prawn Palaemonetes vulgaris (say) compared with P. pugio

1. 1. Larvae (first zoeae) of Palaemonetes vulgaris are relatively stenohaline (optimum salinity = 20%.), adults euryhaline (96-hr LD₅₀ values: 0.8 and 51%.). 2. 2. The concentration of blood sodium remains nearly constant over the salinity range 5-45%. 3. 3. Adult P. vulgaris are less tolerant of dilute (1-20%.) media than sympatric P. pugio but equally tolerant of higher salinities (35-45%.). Palaemonetes vulgaris maintains a slightly more constant and higher (average) sodium concentration in the blood than P. pugio. 4. 4. It is suggested that these differences contribute to habitat partitioning of these species and that they reflect the greater affinity of P. vulgaris for a euhaline milieu.