COMPARATIVE STUDIES ON OSMOREGULATORY CAPACITIES OF TWO COMMERCIALLY IMPORTANT PORTUNID CRABS: IMPLICATOINS FOR AQUACULTURE  

The mud crab, Scylla serrata and the blue swimmer crab, Portunus pelagicus, are two commercially important portunid crab species that have been targeted for aquaculture in the Indo-Pacific. Comparative studies on their osmoregulatory capacities showed that despite they are taxonomically closely related, their capacities on osmoregulation vary substantially. The juvenile mud crab S. serrata was shown as a strong osmoregulator, who was particularly adaptive to low salinities. The survival of juvenile S. serrata was found not significantly affected by a broad salinity range from 4 ‰ - 44 ‰ while the optimal salinity for their growth was identified as between 4 ‰ - 28 ‰. On the other hand, the juvenile blue swimmer crab P. pelagicus was identified as a weak osmoregulator, who was particularly sensitive to low salinities. It was shown that salinity ≤15 ‰ led to significant higher mortality of juvenile P. pelagicus while significantly lower growth was detected at salinity ≤ 10 ‰ and ≥ 45 ‰. The optimal salinity range for juvenile P. pelagicus was hence identified as with a comparatively narrow range of 20 ‰ - 35 ‰.

The above knowledge on osmoregulation ability difference of the two portunid crabs have significant implications on their aquaculture, including suitable farm site selection and adopting pre-emptive measures during monsoon season in the tropics for P. pelagicus culture. Since weak osmoregulation ability, particularly high sensitivity to low salinity, could represent an issue during monsoon season for P. pelagicus culture, experiments were conducted to evaluate whether the osmoregulatory capacity of the crab could be enhanced through dietary approach. The results showed that the resistance of juvenile P. pelagicus to osmotic shock substantially improved when they were fed diets with high phospholipid contents. Similarly, higher dietary highly unsaturated fatty acid (HUFA) contents significantly improved survival of juvenile P. pelagicus at low salinities while growth was significantly improved at both low and high salinity conditions, which was likely through improved haemolymph ion maintenance. These results demonstrated that dietary approach potentially can be adopted as a practical method to improve osmoregulatory capacity of cultured P. pelagicus under osmotically stressful conditions to improve their culture productivity.