The present study aimed to determine if betaine can function as a feeding stimulant or feed enhancer for the juvenile grouper, Epinephelus fuscoguttatus through a behavioural experiment with video recording. Agar gel pellet was used as the medium to deliver the chemical test substances-betaine (BET), amino acids mixture (AAM) and

African catfish Clarias gariepinus hatched with morphologically immature features; however, sensory organs developed rapidly with fish growth. Although the eyes of newly hatched larvae were immature without pigment, in 2 day-old larvae, the retina of the eyes had already developed except for the rod cells. No free neuromasts were observed in newly hatched larvae. In 1 day-old larvae, however, free neuromasts were observed on the head and trunk. Free neuromasts increased with larval growth. Newly hatched larvae had simple round-shaped otic vesicles; however, all sensory epithelia of the inner ear were observed until the larvae were 3 days old. Two day-old larvae swam horizontally, had sharp teeth, commenced ingesting rotifers and also artificial feed (small-size pellets) under both light and dark conditions; by then the larvae already had many taste buds. Three day-old larvae showed negative phototaxis and cannibalism by eating their conspecifics. Most of the free neuromasts observed in this study had the peculiar feature of many microvilli around the sensory cells on the apical surface. Detected free neuromasts as ordinary type lateral-line organs were not observed in previous reports in teleosts. In 10 day-old larvae, there were two lines of free neuromasts on the flank and lower edge of the trunk; presumptive canal neuromasts were oval shaped and had begun to sink under the skin. The direction of maximum sensitivity of the neuromasts was parallel with the longitudinal axis of their elliptical apical surface.

Sutchi catfish Pangasianodon hypophthalmus hatch with morphologically immature sensory organs; however, sensory organs develop rapidly with larval growth. Two-day-old larvae commenced ingesting Artemia nauplii. The larvae displayed many taste buds on the barbels, the head surface, and in the buccal cavity. Other sense organs were also well developed at this stage. Feeding experiments revealed that 2-day-old larvae ingested Artemia under both light and dark conditions, moreover, the larvae could ingest frozen dead Artemia. The ingestion rates for 4-and 7-day-old larvae were significantly higher under dark conditions than under light conditions. The rates using frozen dead Artemia were mostly higher than the rates using live Artemia. Therefore, feeding behavior under dark conditions is most likely not mediated by visual or mechanical senses, but rather by chemosensory senses, such as taste buds. Larval fish are vulnerable to predators; thus, if they can search for and eat food at night, they can avoid diurnal predators. The behavior observed here appears to represent their survival strategy. Moreover, these results suggest a new possibility that sutchi catfish larvae can be reared under dark or dim light conditions in order to improve survival and growth rates as in the case of African catfish Clarias gariepinus.

Larvae of the sutchi catfish Pangasianodon hypophthalmus hatch with morphologically immature features, but sensory organs develop rapidly as the fish grow. By 1 day old, yolk-sac larvae showed notochord flexion, and by 2 days old larvae were observed to have consumed a large part of the yolk sac. At this stage, larvae had welldeveloped eyes, olfactory organs with ciliated receptor cells, inner ears with semicircular canals, and numerous taste buds, and they commenced ingestion of rotifers, Artemia nauplii, and artificial compound feed. Two-dayold larvae had many free neuromasts on the surface of the head and flanks and clearly showed rheotaxis. By 20 days old, free neuromasts in postflexion larvae had sunk under the skin. At this later stage, larvae swam against a water current and schooled along the side of the fish tank. Rapid development of sensory organs and notochord flexion would be an adaptation for survival in conditions of flowing water, as in the Mekong River. In this study, we show that development of the lateral line in the postflexion stage seems to be closely related to larval behavior, suggesting that these developments could be essential for sutchi catfish larvae survival.

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.