Nycthemeral variations of ammonia excretion in Penaeus japonicus (Crustacea, Decapoda, Penaeidae)

Day and night variations (nycthemeral) of ammonia excretion were studied in post-larvae and adults (male and female) of Penaeus japonicus. When the photoperiod was 16:8 ( L D ), ammonia excretion by adults varied only at night, with two maxima: 21.00–23.00 h and 01.00–03.00 h. Ammonia excretion by the post-larvae exhibited variations during the whole day, with four maxima: 21.00–23.00 h, 01.00–03.00 h, 07.00–09.00 h and 13.00–15.00 h. In adults, hourly specific excretion was enhanced during the dark period (× 2). There was no such difference for post-larvae between day and night. The daily specific excretion did not differ between males and females except for some periods of the daily cycle. The daily specific excretion by post-larvae (47.8 μatg NNH 3NH 4·g −1 live weight·24 h −1) was about five-fold higher than excretion by adults (10.3 μatg NNH 3NH 4·g −1 live weight·24 h −1 at 23°C). The relationships between behaviour and ammonia excretion variations are discussed.