Effect of Salinity on Survival and Metamorphosis from Zoea to Megalopa of the Mud Crab Scylla serrata Forskal (Crustacea: Portunidae)

Abstract

Survival and metamorphosis of each of the five zoeal stages (Z1, Z2, Z3, Z4, Z5) of Scylla serrata larvae hatched and reared at 32 ppt and abruptly transferred to salinities of 12, 16, 20, 24, 28 and 32 ppt were compared. These were monitored at 1, 3, 6, 12, 24, 48 and 72 h of exposure. Larvae in all zoeal stages were able to tolerate abrupt transfer to 20, 24, 28 and 32 ppt salinities up to the 72 h of exposure, with consistent low survival at Z3 stage in all salini-ties. Total mortality at 12 ppt occurred on 12 h at Z1, 48 h at Z2, and 24 h at Z3 and Z4, while very few Z5 larvae survived up to 72 h. There was very low survival at 16 ppt in all zoeal stages. Metamorphosis to the next larval stage occurred at salinities of 20 to 32 ppt. In another experiment, zoeae were continuously reared from Z1 up to megalopa in 5 different water salinity regimens. In treatment 1, a 32 ppt salinity was maintained throughout culture period; in treatment 2, from 32 ppt, there was 1 ppt reduction in salinity every day; in treatment 3, 24 ppt salinity was maintained throughout the culture period, in treatment 4, 32 ppt at Z1, 24 ppt at Z2 and 20 ppt from Z3 to megalopa; and in treatment 5, there was a gradual decrease from 32 ppt in Z1 to 25 ppt in Z5. Highest metamorphosis to megalopa occurred where salinity was constant at 32 ppt. The computed population development indexes of larvae in all salinity regimens showed no significant differences, suggesting that salinity did not affect the duration of development of the surviving larvae.