Migratory behavior of postlarval white, Penaeus schmitti, and river shrimps, Macrobrachium olfersi and Macrobrachium acanthurus, in their zone of overlap in a tropical lagoon

The daily activity of postlarval Penaeus schmitti and two river shrimps, Macrobrachium olfersi and M. acanthurus. was studied during 1 year by means of fortnightly samples taken hourly during 24-h periods in Tacarigua, a coastal lagoon in Venezuela. P. schmitti and M. olfersi occurred throughout the year, whereas M. acanthurus was present only in January, November and December. The three species showed an alternation of benthic and planktonic distribution. Planktonic activity is nocturnal in the three species, but occurs during incoming tide in P. schmitti and during outgoing tide in Macrobrachium. Activity observations in experimental chambers showed the presence of a bimodal endogenous rhythm of a 12-h period with outbursts of activity at dawn and dusk in M. olfersi, whereas in P. schmitti a similar endogenous rhythm is phased with the times of high water. The postlarvae of P. schmitti possibly represented older stages of larvae that use currents to move onto inshore nursery grounds, and which still retained their complex pattern of vertical movements, responding to circadian illumination cycles and non-circadian tidal cycles, related to the need of the species to arrive and stay in the lagoon, whereas the activity observed in Macrobrachium during the seaward flow, when salinity decreases, causes the postlarval to remain within the limits of salinity required for their normal development.