In general, African catfish shows higher survival rates in the dark conditions than in the light conditions. In this study, larval behavior of African catfish was observed under 0, 0.01, 0.1, 1, 10, and 100 lx using a CCD camera to investigate the reason why African catfish larvae show higher survival rates in dark conditions. The larvae showed significantly higher swimming activity under 0, 0.01, and 0.1 lx than that under 10 and 100 lx. The larvae also showed significantly increased aggressive behavior under 10 and 100 lx; the swimming larvae attacked resting individuals more frequently under 10 and 100 lx than under 0, 0.01, and 0.1 lx. The aggressive behavior and sharp teeth of the attacking larvae appeared to induce skin surface lesions on injured larvae. Chemical substances were then generated from the injured skin surface, and these chemical stimuli triggered cannibalistic behavior in other fish near the injured fish. The results of this study demonstrate that the higher survival rates of African catfish larvae under dark conditions are a result of inactivity and subsequent increase in chemical releasing stimuli concentrations around inactive individuals that triggers feeding behavior in nearby active catfish. Therefore, we recommend larval rearing of African catfish in dark or dim conditions, as it improves catfish survival rates.

The present study was conducted to elucidate the effects of light and dark condition on the feeding activity of shrimp, Litopenaeus vannamei. Examination on the ingestion rate of shrimps at different sizes (0.5, 1.0 and 1.5 cm total length, TL) under light and dark condition was conducted using newly hatched frozen Artemia nauplii. For each condition, shrimp were let to ingest known number of Artemia nauplii for 30 minutes, thereafter the remaining Artemia nauplii was counted. For the observation of eye structures under light and dark conditions, 1.0 cm TL shrimps were preserved in Bouin's solution for histological observation. Another feeding trial was conducted to examine the growth performance and survival of shrimps (initial size 1.0± cm, TL) under different photoperiod regimes (24 hours dark: 24D, 24 hours light: 24L and 12 hours light and dark: 12LD) for three weeks. Results showed that, the 0.5cm TL shrimp significantly ingested more Artemia nauplii under light condition compared to dark condition (P = 0.000). The 1.0 and 1.5 cm TL shrimps consumed Artemia naulplii equally under both conditions. The shrimp attained a complete eye structure which can be differentiated into crystalline cone, clear zone, rhabdom and fasciculated zone at 1.0cm TL. This study also showed that growth (P = 0.557) and survival (P = 0.686) of shrimps did not vary significantly among different photoperiod. This study suggests that the feeding activity of the smallest shrimp (0.5 cm TL) is affected by light condition. However, feeding activity, growth and survival of bigger sized shrimp (> 1.0 cm TL) were not affected by light and photoperiod regimes.

This study was conducted to investigate the effect of light and dark conditions on feeding activity and eye adaptations of post larvae (PL5, PL10, PL20 and PL30) Penaeus vannamei fed with frozen Artemia. Shrimp PL were placed individually in beakers and after acclimatization under the light or dark condition, PL were left to ingest known number of Artemia for 30 minutes. Thereafter, each PL was subsequently anesthetized by putting an ice cube into the beakers followed by adding few drops of paraformaldehyde. The results showed that the PL5 ingested significantly more frozen Artemia under light condition compared to dark condition (P<0.05). The eye structures of PL5 comprises of crystalline cone, rhabdom and fasciculated zone. However, it was incomplete due to the lack of clear zone and no migration of the screening pigment granules was observed under light and dark conditions. On the contrary, the number of frozen Artemia ingested by the latter stages PL10, PL20 and PL30 showed no significant differences (P>0.05) under both light conditions and these PL have complete eye structures with define clear zone. The width of clear zone was found to increase proportionally with the growth of the PL. Besides that, the screening pigment granules were able to migrate under light and dark conditions. The ability of the PL10, PL20 and PL30 to ingest almost similar numbers of Artemia under light and dark conditions suggests that different lighting conditions did not affect the feeding activity of the PL and other sensory organs may play roles in detecting food, while PL5 need light to improve their feeding activity. Based on these results, we suggest that in aquaculture practice, during the rearing of early stage of PL (<PL5), a brighter environment or light should be provided to enhance larval feeding activities. Whereas, later stages of PL (>PL10) can be cultured under any light condition.

The effects of feeding rotifers and Artemia enriched with different types of enrichment in the first feeding and later stages of Scylla tranquebarica larvae were investigated. Two different experiments were conducted each with five enrichment treatments including live and digestible Nannochloropsis sp. and Tetraselmis sp. or commercial product. In Experiment I, the zoea 1 (Z1) larvae were fed with enriched or unenriched rotifers as their first feed until they moulted into the zoea 2 (Z2) stage, while in Experiment II, the zoea 3 (Z3) larvae were fed with enriched or unenriched Artemia until they metamorphosed into megalopa stage. In Experiment I, Z1 fed enriched rotifers showed higher survival, larval stage index (LSI) and carapace length than those in unenriched treatment. However, the highest survival was recorded in larvae fed Nannochloropsis sp. treatment. In Experiment II, Z3 fed Artemia enriched with digestible Tetraselmis sp. exhibited the highest survival and LSI, lowest chela‐carapace length ratio and abnormal moulting frequencies. In comparison to live microalgae, digestible treatment resulted in a smaller chela‐carapace length ratio with lower abnormal moulting. In conclusion, enrichments tested in this study can be used to enrich rotifers for the first feeding of Z1, while later stage (Z3 onwards) fed Artemia enriched with digestible Tetraselmis sp. improved survival and decreased the abnormal moulting frequencies of Z5 to megalopa stage.