Some Current‐Directed Movements of Macrobrachium Acanthurus (Wiegmann 1836) (Decapoda, Palaemonidae) Under Laboratory Conditions

The larvae of some species of freshwater shrimp of the genus Macrobrachium will develop only in brackish water. The increased incidence of these shrimp within estuaries and in the lower reaches of rivers at certain times of the year suggests that regular down—stream migrations are carried out to facilitate the transport of larvae to a brackish estuarine environment. Current chamber experiments conducted in the laboratory indicated that gravid individuals tend to swim consistently downstream whereas other adult females tend to swim consistently upstream. Other current chamber experiments showed that larval stages, held in brackish water, will drop to a position low in the water column when salinity is reduced (simulating an ebb tide) and revert to swimming higher in the water column when the salinity is increased (simulating a flood tide). These responses are interpreted as a mechanism whereby larvae are prevented from being carried out to sea, thus facilitating their eventual migration upstream to a freshwater environment.