FIELD OBSERVATIONS OF MOLT CYCLE, FEEDING-BEHAVIOR, AND DIET OF SMALLJUVENILE TIGER PRAWNS PENAEUS-SEMISULCATUS IN THE EMBLEY RIVER, AUSTRALIA

Juvenile tiger prawns Penaeus semisulcatus (2.5 to 5.5 mm carapace len gth) were collected on an intertidal seagrass bed in the Embley River estuary in the Gulf of Carpentaria, Australia. Prawns were sampled eve ry 2 h over one 24 h period in the pre-wet (early October) and over on e 24 h period in the late-wet season (late March) and the moult cycles and foregut contents determined. Premoult prawns were found mostly be tween 12:00 and 22:00 h and ecdysis occurred mostly between 20:00 and 0:00 h. A mean moult period of 3.0 d was estimated from the percentage of the prawns in premoult and ecdysis stages in early evening samples . On a scale of 0 (empty) to 10 (full), the mean foregut fullness of s mall juveniles was greater than 4.5 during both the day and night in e ach season. Copepods, filamentous algae, diatoms and unidentified mate rial were the most common prey in both seasons. Insect larvae were mor e common in the pre-wet and nematodes and ostracods were more common i n the late-wet. Little diel variation in diet was noted. Rapid foregut passage rates under field starvation conditions showed that between 5 5 and 60 % of foregut contents was cleared in 1 h. This result, and th e values for mean foregut fullness over 24 h, suggested that small pra wns fed continuously through the day and night. Further field experime nts in the presence of food showed that both hard and soft prey items were cleared from the foregut within 2 h and also that moult stage aff ected the feeding behaviour of prawns. Intermoult prawns maintained me ans of foregut fullness greater than 7, while premoult prawns ate less .