Persistent locomotor rhythmicity in the prawns Penaeus indicus and P. monodon

The locomotory activity of adult Penaeus indicus Milne Edwards and P. monodon (Fabricius) collected from the Vellar estuary, South India, in 1983 was monitored continuously for a period of 40 d in the laboratory. Both species exhibited persistent circatidal and circadian rhythmicity, the former displayed peak activity during predicated times of high water in their original habitat. The latter exhibited peak activity during periods corresponding to in situ night-time, with almost no activity during the day, except at the time of high tide. The tidal and daily rhythms were evident for only 3 to 4 d and 5 to 6 d, respectively, under laboratory conditions. The amplitude of the rhythms displayed variations in each semilunar period, reflecting the synchrony and dissynchrony of the tidal and daily rhythms. Activity was correlated with phases of the moon, peaking at times of new and full moon, and showing a scattered pattern with smaller peaks during the first and third quarters.