RESUMEN
Giant freshwater prawns, Macrobrachium rosenbergii de Man, were reared under three different lighting conditions: continuous darkness (DD), 12 hr of light and 12 hr of darkness (LD 12:12) and continuous light (LL). After one month, the prawns were sacrificed and optic lobes isolated from the eyestalks were determined for N-acetyltransferase (NAT) activities and melatonin concentrations. Gonads were weighed and examined under light microscopy. The optic lobes from LL prawns contained significantly higher activities of NAT than those from LD 12:12 prawns. The melatonin concentrations and size and histological features of the gonads from the three groups of prawns did not differ. The results indicate that continuous light increases NAT activities in the optic lobe of M. rosenbergii but has no drastic effect on gonadal growth.