Effects of photoperiod on egg production and hatching success, naupliar and copepodite development, adult sex ratio and life expectancy of the tropical calanoid copepod Acartia sinjiensis

摘要

The tropical calanoid copepod Acartia sinjiensis has good potential for mass culture as live feed for reef fish larvae. The present study was conducted to evaluate the effects of photoperiod on various parameters related to A. sinjiensis productivity in culture. Five photoperiods of Light:Dark=0:24; 6:18; 12:12; 18:6 and 24:0h were setup. Daily egg production of individual females under each photoperiod was monitored for 8 consecutive days. The females were randomly selected daily from stock cultures kept under respective photoperiods and discarded after experiment. The results showed a clear trend of increasing egg production with longer illumination period. Under constant darkness, acclimatization was evident as egg output increased steadily over the 8 day period. Statistics showed that photoperiod significantly (p<0.005) affected mean daily egg production, with the highest egg output recorded at 18L:6D and 24L:0D (17.6+/-1.7 and 17.6+/-1.8 eggs/female/day respectively), which were significantly higher than all other treatments. Photoperiod also significantly affected 48 h egg hatching success (p<0.005), a trend of increased hatching success with longer light phase was demonstrated. The highest hatching rate (87.2+/-1.4%) was recorded at 24L:0D, which was significantly higher than the 0L:24D and 6L:18D treatments but not significantly different from the second highest (85.3+/-2.6%) hatching rate of 18L:6D treatment. Photoperiod was further confirmed to significantly (p<0.005) affected naupliar and copepodite development with accelerated development observed with increased illumination period of photoperiods. Mean development time from egg to adult was the shortest at 6.00+/-0.33 days under constant light (24L:0D), followed by 6.24+/-0.24 days at 18L:6D, both were significantly shorter than that of 0L:24D and 6L:18D treatments although no significantly difference was detected between themselves. Adult life expectancy was also found significantly (p<0.005) affected by photoperiod with the shortest adult life span recorded under constant light (24L:0D) (9.4+/-0.4 days), which was significantly shorter than all other photoperiods tested. Adult sex ratio was the only parameter tested that was not significantly affected by photoperiod, a skewed sex ratio in favor of female was found across all photoperiod treatments. Based on results of current study, it is recommended that a photoperiod of 18L:6D being adopted for A. sinjiensis culture to maximize its productivity for aquaculture hatcheries.