LAUSR

The fluvial prawn Macrobrachium yui lives in caves during the free-swimming zoea larval stages, and the hatched larvae require cave stream water for normal development. To develop larval culture techniques for M. yui in the absence of cave stream water, we determined the relationship between salinity tolerance of hatched larvae and egg size in Macrobrachium species and tested the effect of rearing-water salinity (0, 1.5, 3.5, and 10.5 ppt) on larval survival during free-swimming zoea larval stages. When reared at 3.5-ppt salinity, the hatched larvae successfully settled on the bottom and had the highest survival, with more than 90% of the hatched larvae developing into postlarvae. This indicates that we can successfully rear the free-swimming zoea larvae using 3.5-ppt seawater in place of cave stream water (0.18- to 0.27-ppt salinities). To test the acclimation of the postlarvae to freshwater, we reared the postlarvae for 2 weeks after settling, under three treatment conditions (I: rearing at 0 ppt for 2 weeks; II: rearing at 1.7 ppt for the first week and 0 ppt for the second week; III: rearing at 3.5 ppt for 2 weeks), and compared their survival under the different conditions. The postlarvae had the highest survival (78%) in treatment II. This suggests that M. yui has the ability to gradually adapt to freshwater, after the free-swimming zoea larvae have settled on the bottom and developed into postlarvae. These results would be critically useful for larval culture of M. yui, helping to restock the wild with the larval prawns from captive stocks to maintain the endangered local population.