Downstream drifting of Macrobrachium (Decapoda: Palaemonidae) larvae in the Shimanto River, Japan

This study was undertaken to clarify the downstream drifting of Macrobrachium larvae in the Shimanto River by 24-hour sampling. Stage I zoeae accounted for most larvae collected in fresh and brackish waters. M. formosense and M. japonicum dominated in brackish and fresh waters, respectively, while M. nipponense occurred only in brackish waters. The amount of yolk droplets of M. formosense and M. japonicum reduced over time in freshwater. However, no larvae that had completely used up their yolks were observed. Even though M. japonicum larvae drifted from farther upstream than those of M. formosense, they had a higher residual amount of yolk droplets. Therefore, long-distance and long-time drifting in M. japonicum may be enabled by delaying the absorption rate of droplets.