Continuous darkness stimulates body growth of the juvenile giant freshwater prawn, Macrobrachium rosenbergii de Man.

The effect of photoperiod on growth of juvenile giant freshwater prawns, Macrobrachium rosenbergii de Man, was tested. The prawns were divided into four groups and each group was reared under one of the following light-dark conditions: continuous darkness (L0:D24), 12 hr light: 12 hr dark (L12:D12), 16 hr light: 8 hr dark (L16:D8), and 20 hr light: 4 hr dark (L20:D4). Body size was determined at the age of 45, 75, and 110 days by measuring total length, orbital length, and carapace length; body weight was determined at the age of 110 days. At 110 days of age, the prawns reared under L0:D24 photoperiod were significantly longer and heavier than those reared under other light-dark conditions. The survival rate of the prawns reared under L0:D24 photoperiod was also higher than that of other groups. This study indicates a positive effect of continuous darkness on growth and survival rate of juvenile giant freshwater prawns, M. rosenbergii.