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The marble goby (Oxyeleotris marmoratus) is a valuable food fish, but its aquaculture production is often hampered by poor growth performance associated with starvation at an early stage. The objective of this study was to investigate the effects of delayed initial feeding on the growth performance and nutritional condition of the marble goby. Six different first feeding times were examined: 0, 12, 24, 36, 48, and 60 hours after the first feeding (HAFF), and their impacts on growth performances were evaluated based on larval final total length (mm) and survival (%), while nutritional condition was evaluated based on body morphometric changes, gut epithelium height (μm), and gut condition. The experiment was conducted for 15 days. All parameters were measured after larvae were collected at different sampling times, except survival and growth, which were measured at the end of the experiment. The results showed that the onset of the first feeding was 36 h after hatching (hAh) and that a short delay in the first feeding by 12, 24, 36, 48, and 60 HAFF significantly reduced the growth performance of the larvae and severely affected the larval nutrition condition with noticeable shrinkage in body morphometry and gut epithelium height. This study concluded that the onset of first feeding in marble goby occurs at 36 hAH and that first feeding beyond 36 hAH significantly worsens the nutritional condition and growth performance of the larvae.

Highlight Research The reproductive characteristics of L. hoevenii were examined The potentially smallest maturity sizes of female and male L. hoevenii were 350 g and 180 g, respectively The fecundity of a 1,000 g L. hoevenii can be more than 100,000 eggs L. hoevenii was determined as a multiple spawner Abstract Sultan fish ( Leptobarbus hoevenii ) is a high value freshwater fish, cultured in some Southeast Asian countries, including Malaysia and Thailand. However, information on its reproductive characteristics is very scarce. This study examined the gonadosomatic index (GSI), fecundity, egg diameter, and determined whether L. hoevenii is a single- or multiple-spawner. Twenty male and female pond-farmed L. hoevenii broodstock were obtained to measure their total length (TL), body weight (BW) and gonad weight to calculate the GSI. Ten females were randomly sampled from the 20 to determine their fecundity. A total of 1,500 eggs were sampled from each female. The egg diameter was measured then its frequency distribution was analyzed to detect the number of egg class group, and to determine whether L. hoevenii is a single- or multiple spawner. The female L. hoevenii examined were 32.2-47.1 cm and 350-1,200 g, while the males were 30.7-45.8 cm and 180-970 g in TL and BW, respectively. All female specimens contained gonads. The potentially smallest mature samples were recorded at 350 g (female) and 180 g (male). GSI for the female and male L. hoevenii were 1.81-12.28 % and 1.03-5.09 %, respectively. The fecundity was 35,467 -128,067 eggs, while the highest fecundity was observed in a 1,000 g fish. The observed egg diameter ranged from 500 to 1,855 μm. Two to five groups of egg class were detected, indicating that L. hoevenii is a multiple spawner.

The giant mottled eel Anguilla marmorata Quoy & Gaimard is an important aquaculture candidate in eel farming industry. The high economic value of the species leads to intensive cultivation affecting its welfare under farm condition. Since colour environment affects the welfare of confined fishes, basic information on colour vision is necessary. By means of a behavioural experiment of A. marmorata elvers in the laboratory, we determined colour vision under natural daylight. The elvers were subjected to classical conditioning to associate a reward feed with a blue or a green stimulus placed amid seven shades of grey. The elvers learned this visual task after 30 or 36 trials and thus the ability of colour discrimination was confirmed. Since the elvers have the retinal tapetum, it may be suggested that colour vision in dim light enables crepuscular feeding by the A. marmorata elvers, clearly an advantage in aquaculture.

Bristle patches on the carapace behind the eyes were found for the first time in both males and females of the four species of mud crab Scylla spp. Electrocardiography revealed that the bristle patches of Scylla paramamosain were sensitive to touch (with a glass capillary) but not to chemical stimulation (with sugarcane juice). To elucidate the function of these touch signals, the courtship behaviour of S. paramamosain was video-recorded in a broodstock tank. A male approached a female, extended a walking leg over her carapace, touched her bristle patches with a dactyl, mounted her, and probed the bristle patches further with the third maxillipeds. Touch stimulus may be a signal for courtship in Scylla and one of the chain stimuli for the completion of mating behaviour.

The giant freshwater prawn Macrobrachium rosenbergii larvae have apposition eyes and are positively phototactic, whereas the postlarvae (PL) have superposition eyes and are negatively phototactic. M. rosenbergii has colour vision as early as larval stage. We discovered that M. rosenbergii alters background colour preference after metamorphosis from larvae to PL in association with nature of phototaxis. The test circular glass aquaria covered with a pair of two-colour papers contained with a group of 100 larvae or 20 Pl, and the number of individuals in each colour background was recorded five times for each colour pair. The background colours tested were light blue, green, yellow, red, white and black. The numbers of larvae or PL in each colour background of different pairs were analysed by the Thurstone's law of comparative judgment. In the larvae, significant bias towards yellow was evident. In the PL, of the four pairings of black with other colours, all biased to black. The mean z-scores were highest for yellow in the larvae, and for black in the PL. To determine the possible background brightness preference of the larvae and PL, six different colour backgrounds were presented in pairs. The larvae significantly preferred light blue over dark blue, white over yellow and white over black. The PL exhibited reversed preference. The relationship between z-scores and light reflectance levels of five colour papers was significantly positive in the larvae and negative in the PL. The observed background colour preference was probably due to relative brightness rather than chromaticity difference.

Crabs and shrimps (order Decapoda) use colours in various tasks such as foraging and mate choice. Colour vision requires at least two types of photoreceptors with different spectral sensitivities. Previous physiological studies revealed that most crabs including Scylla mud crabs have a single visual receptor system, i.e. they are colour blind. We determined colour vision by means of a behavioural experiment on hatchery-produced and wild-captured purple mud crab Scylla tranquebarica in a roofed hatchery. Adult crabs (8-10 cm carapace width) were subjected to classical conditioning to associate a food reward with a blue or a green stimulus placed among seven shades of grey. The hatchery-produced crabs learnt this task after 14 days of reward training, and thereafter distinguished blue in 27 non-reward trials, and green in 39 non-reward trials. The wild-captured crabs did so after 25 days of reward training, and distinguished green in 49 non-reward trials. These results indicated colour vision in S. tranquebarica. However, the crabs were unable to distinguish blue or green in dim light of 4.4 cd/m2 (which is slightly brighter than full moon light). The high colour vision threshold was attributed to the small optic parameters of the apposition compound eyes of S. tranquebarica.

While the olfactory cue hypothesis has been proposed for spawning migration of silver eels, it has been shown that olfactory cells and associated mucus cells degenerate in male and female eels after hormonally induced sexual maturation. However, the degeneration of the olfactory organ could be a real event in the sequence of maturation, or may be an unnatural side effect of the hormone treatment itself. We morphologically and histologically examined the olfactory rosettes of hormone-untreated and hormone-treated (mixture of hCG and PG) giant mottled eel (Anguilla marmorata) and Japanese eel (A. japonica). The olfactory rosette from all the hormone-treated specimens significantly degenerated at various degeneration levels even in sexually immature specimens, indicating the side effect of the hormone-treatment. However, a sexually immature non-hormone treated female A. marmorata (87.4 cm TL, 199.4 g BW, at less advanced maturity) had slightly degenerated olfactory rosette. Further studies should focus on conducting natural degeneration of the olfactory rosette during the sexual maturation in tropical eels.

The present study was aimed at evaluating the growth performance and feed utilization of marble goby (Oxyeleotris marmorata) juveniles fed with the acidified diets (AD). In feeding trial I, five fish meal-based diets were prepared [control (pH 6.0), AD 5.3, 4.3, 3.2 and 2.5]. Each diet was fed to triplicate batches of wild-caught O. marmorata (19 fish/ tank; total length, TL = 4.72±0.46 cm) for 8 weeks. The control, AD 5.3, and AD 4.3 treatments were terminated at week 4, due to drastic decline in total feed intake (TFI=0–0.05 g) and weight loss (weight gain, WG = -15.3 to -16.9%) in the fish. The AD 3.2 and AD 2.5 treatments were continued until week 8. Fish fed with the AD 3.2 showed significantly higher (P < 0.05) TFI (0.98 g) compared to those fed with the AD 2.5 (0.73 g) at the end of the trial. Feeding trial II was done to assess the long-term effects of AD for another 7 weeks. Thirty fish specimens were randomly selected from each of AD 3.2 and AD 2.5 treatments and stocked individually in 7 L aquaria to eliminate the territorial behaviour that was observed in the first trial. At the end of the experiment, fish fed AD 3.2 attained significantly higher (P < 0.05) WG (34%) than those fed AD 2.5 (13%). However, the growth performance and feed utilization results of the present study were very poor compared to those fed the normal fishmeal-based diet as is evident from synthesis of data from the literature review. Feeding of acidified diets was, therefore, not recommended for an extended period (> 3 weeks). Nevertheless, based on the strong preference of the fish for AD, it is worth trying to develop a weaning protocol using these diets as the starter feed and then slowly replacing them with the normal diet.

Feeding attractiveness of 21 fresh herbs was examined for the giant freshwater prawn (Macrobrachium rosenbergii) and the whiteleg shrimp (Litopenaeus vannamei) at different growth stages in tanks. The feeding attractant and enhancement were assessed by behavioural observations of the test animals in response to pelleted feeds incorporated with the herbal extracts in different concentrations. For M. rosenbergii, peppermint (Mentha piperita) and dokudami (Houttuynia cordata) were the most attractive herbs at all stages, and garlic (Allium sativum) strongly attracting to the juveniles but only weekly to the adults, postlarvae and larvae. Peppermint significantly enhanced feeding of M. rosenbergii at all stages except for the larvae. Garlic was a significant feeding enhancer for M. rosenbergii juveniles and postlarvae. The inclusion of the herbal extract at high concentrations had a negative effect on the feeding. As far as L. vannamei is concerned, the red chilli (Capsicum annuum) was the best feeding attractant and ginger (Zingiber officinale) and peppermint were moderate attractants for adults and juveniles. While galangal (Alpinia galangal), ginger and yellow onion (Allium cepa sp.) enhanced feeding in L. vannamei adults, but red chilli, garlic and peppermint did not enhance feeding. Evidently, the strong feeding attractants were not necessarily effective feeding enhancers. The incorporation of several herb extracts at higher concentrations caused a negative effect on the feeding of the test animals. This could be due to the presence of feeding deterrents in the herb such as saponins which are known to lower food palatability in insect, decapod crustaceans and fishes. More long-term work is warranted to determine if inclusion of feeding enhancing herbs in artificial feed promotes growth performance of the shrimp.

The previous behavioural studies on vision of decapod crustaceans were often based on the assumption that the visual field of the test animals was all around and there was little or no blind area above or to the rear of the animals. In the present study, we determined the visual field of the wild captured purple mud crab (Scylla tranquebarica) and the farmed whiteleg shrimp (Litopenaeus vannamei) by eliminating the directions in which vision is anatomically blocked in all directions around the eyes. The mud crab had the visual field covering the entire visual world except for the ventral-most blind area. The whiteleg shrimp has the visual filed with a 66Ëš binocular field and can see all around but is morphologically blocked by the scaphocerite extended forward between the eyes. While the transparent scaphocerites transmit 80 % of light from 400 to 700 nm wavelengths, an object seen through the scaphocerites is faded due to the light refraction, indicating that the morphological blocking is not always negligible. The trait of these visual fields should be taken into consideration in the design of visual behaviour experiments.

The present study aimed to evaluate the feeding performance of juvenile marble goby (Oxyeleotris marmorata) on acidified diets. Five fish meal-based diets at various pH levels (2.5, 3.2, 4.3, 5.3, and 6.0 (control)) were prepared, and each of them was fed to 10 wild-caught O. marmorata juveniles (body length 7.7–9.3 cm; each fish was placed in an aquarium, hence each dietary treatment was in 10 replicates) for 20 days. Throughout the feeding trial, O. marmorata juveniles showed clear increments in the daily ingestion ratio (IR) values of AD 2.5, AD 3.2, and AD 4.3. The control diet and AD 5.3 were almost totally rejected by the fish. On day 20, the IR of AD 2.5 (0.46) and AD 3.2 (0.36) were significantly higher (P < 0.05) than those of AD 5.3 (0.1) and the control diet (0.02). In addition, 70% of fish fed AD 2.5 achieved a high IR value (0.6 – 1.0), and this was the best result among all the treatments (AD 3.2, 60%; AD 4.3 50%; AD 5.3 and control, 10%). These results suggested that acidified diets can be used to wean O. marmorata juveniles, and the recommended pH level was 2.5.

Swimming crabs have a characteristic fifth pair of legs that are flattened into paddles for swimming purposes. The dactyl of these legs bears a thick seta along its edge. The chemoreceptive and feeding properties of the seta are supported with scientific evidence; however, there is no available data on the sensitivity of the setae in portunid crabs. The underlying mechanisms of the chemo- and mechano-sensitivity of appendages and their involvement in feeding activities of the mud crab (Scylla paramamosain) were investigated using electrocardiography and behavioural assay, which focused on the responses of the mud crab to chemical and touch stimulus. Electrocardiography revealed the sensory properties of the appendages. The dactyls of swimming legs and the antennules were chemosensitive, but not mechanosensitive and vice versa for the antennae. However, the mouthparts, claws, and walking legs were chemo- and mechanosensitive. Only the chemosensitive appendages, including the swimming legs, were directly involved in feeding. The flattened dactyls of the swimming legs were more efficient than the pointed dactyls of the walking legs in detecting the food organism crawling on the substrate. The structural features enhanced the capacity of the crab in coming into contact with scattered food items. This study revealed that the swimming legs are important appendages for feeding in the mud crab.

Oxyeleotris marmorata is an ambush predator. It is known for slow growth rate and high market demand. Farming of O. marmorata still remains a challenge. In order to establish a proper feeding practice to stimulate growth, knowledge of its metabolic processes and cost should be examined. Therefore, this study was designed to investigate the diel osmorespiration rhythms of O. marmorata in response to feeding challenge by using an osmorespirometry assay. The results have shown that oxygen consumption rate of the fed fish was approximately 3 times higher than that of the unfed fish in early evening to support specific dynamic action. Digestion and ingestion processes were likely to be completed within 18–20 h in parallel with the ammonia excretion noticeable in early morning. Under resting metabolism, metabolic oxygen consumption was influenced by diel phase, but no effect was noted in ammonia excretion. As a nocturnal species, O. marmorata exhibited standard aerobic metabolic mode under dark phase followed by light phase, with high oxygen consumption rate found in either fed or unfed fish. It can be confirmed that both the diel phase and feeding have a significant interactive impact on oxygen consumption rate, whereas ammonia metabolism is impacted by feeding state. High metabolic rate of O. marmorata supports the nocturnal foraging activity in this fish. This finding suggested that feeding of O. marmorata should be performed during nighttime and water renewal should be conducted during daytime.

This study was conducted to determine the potential of a prototype feeding stimulant (FS) comprised of amino acids and nucleotides to improve the weaning of Oxyeleotris marmoratus. Two diets (control and FS) were formulated and fed to 2 groups of fish for 14 days. Six fish were in each group. Each fish was in one aquarium and each fish was a replicate. The body lengths were 7.7-9.3 cm. Each fish was given 5 pelleted feeds at 17:00 daily and the ingested feeds were recorded the next day at 12:00. Fish fed the FS diet started ingestion on the 1 st day and attained a higher ingestion ratio of 0.5 and 50% were successfully weaned. However, on the 8th day, fish fed the control diet had an ingestion ratio of 0.17 and 16.7% were successfully weaned. In conclusion, the FS efficiently promoted feed intake and it can be used to improve the weaning of O. marmoratus.

The effect of low pH on the tactile sense of Macrobrachium rosenbergii postlarvae was determined in the laboratory by means of two behavioural assays: shelter (netting) occupancy and jumping response to touch stimuli (taps) by a glass micropipette. The postlarvae were acclimated to pH 4, pH 5, pH 6 and pH 7.5 (control) in 45 L aquaria 5–7 d before the experiments. Shelter occupancy decreased with pH and was significantly lower at pH 4 and pH 5 than at pH 6 and in the control. The jumping response instantly followed a tap 93–98% of the time in the control, pH 6 and pH 5 treatments. However, the postlarvae showed significantly lower jumping response (65%) at pH 4, indicating an impaired tactile sense. Low pH 4–5 probably degrades the chitin of the sensory setae and inhibits the surface mechanoreceptors of the prawn postlarvae.

Behavioral test was conducted to determine the level of food pH which was preferable by the juvenile marble goby, Oxyeleotris marmorata (50 individuals or replicates; total length 6.6–7.0 cm) using agar gel pellets. Eight pH levels of agar gel pellets were prepared (pH 2.4, 3.0, 3.2, 3.4, 4.1, 4.9, 5.1, and the pure agar gel pellet without pH modification—pH 5.9). The ingestion ratio for each pH treatment of agar gel pellet was calculated, and the binary data (ingested or rejected) was analyzed using binomial test. Negative relationship was found between the ingestion ratio and pH level of the agar gel pellets. The highest ingestion ratio was found in the agar gel pellet with pH 2.4 (ingestion ratio, 94%), following by pH 3 (85%), pH 3.2 (3% agar gel powder; 65%), pH 3.2 (2% agar gel powder; 58%), pH 3.4 (24%), pH 4.1 (6%), pH 4.9 (6%), pH 5.1 (6%), and pH 5.9 (2%). The ingestion ratio of pH 2.4 agar gel pellet was significantly higher (P

Food colour preference is a fundamental aspect of food recognition and has practical application in the formulation of artificial diets and the design of visual baits. Food colour affects growth performance and feed efficiency in captive fishes. This study determined the food colour preference of the juveniles of the African catfish Clarias gariepinus under natural light in a roofed hatchery. A group of 20 juveniles were held in a transparent aquarium whose background colour was made white, blue, green, red, or black. The test food was diced shrimp flesh dyed blue, green, red, and yellow, with non-dyed natural flesh as the white control. In the food colour preference test, the catfish were presented the shrimp flesh in a pair of different colours at a time. The colour of the food that was first approached and ingested, i.e., the first response, was recorded. All the 10 possible food colour pairs were tested against five background colours. Quantitative analysis of the frequency of first response was done by χ 2 test and Thurstone’s law of comparative judgment. The juveniles exhibited clear food colour preference that varied with the background colours. Against the black background, the first response was significantly biased to red shrimp flesh. Against the red and blue backgrounds, red and blue shrimp flesh were significantly preferred. Against the white and green backgrounds, no significant color preference was detected. Overall, juvenile African catfish preferred red and blue coloured shrimp flesh. This distinct preference for red and blue food could not be explained by associative learning (brown feed in the hatchery) and seemed innate.

Weaning from live to formulated feeds is a major challenge in the culture of marble goby Oxyeleotris marmorata . It can be solved through dietary supplementation of suitable taste substance as feeding stimulant. However, there is still no information on the suitable feeding stimulant for this species. The present study was conducted to evaluate the feeding response of juvenile O. marmorata to 13 organic acids, 19 amino acids, 5 sugars and 2 classical taste substances in a concentration of 0.1 M. The response was judged through behavioural observations to determine the potential feeding stimulant. Fifty O. marmorata were conditioned to accept agar gel pellet, and then tested with the agar gel pellets that contained each of the taste substances. Pure agar gel pellet was used as the negative control. All agar gel pellets with organic acids were fully ingested by the fish (100%). Among the 19 amino acids tested, aspartic acid attained the highest ingestion ratio (94%) which was significantly different (P O. marmorata for organic acids seemed to be influenced by the feed pH. Strong negative linear correlation ( r = −0.828; P O. marmorata prefers acidic food. Evidently, all the organic acids and aspartic acid tested were potential feeding stimulants for O. marmorata . In developing weaning diets by using these taste substances, the dietary pH should be monitored as it may be one of the factors that influences the fish taste preference.

The culture of African catfish (Clarias gariepinus) faces a number of challenges which have to be addressed in view of the growing economic importance of this fish. The captive larvae and early juveniles are aggressive and cannibalistic, resulting in considerable economic losses and shortage of seed to meet demands of the farmers. Improving larval survival and health while the fish is held in captivity is a subject of great interest. C. gariepinus generates a weak electric discharge during the aggressive interactions with the conspecifics and their electroreceptors are highly sensitive. While very little is known on the role of electroreception in the aggressive behaviour, we attempted a sensory control of the cannibalism in the larvae by applying an excess direct current (DC) stimulus (0.67 V/cm or 2.65 V/cm) and ablated the function of the electroreceptors. The number of surviving individuals and those which were lost were compared between the control and the treated groups. Six trials were conducted involving larvae at the different initial ages, from 4 to 9 days after hatching, and with different observation periods ranging from 5 to 11 days. The electric treatment resulted in higher survival in five trials and the number of fish lost to cannibalism was significantly lower in the treated group than in the control in one trial. While the electrically-ablated electroreceptors might regenerate, the ablation seemed to be effective during whole larval phase.

The Bokashi leachate (BL) is a by-product from the anaerobic digestion of food waste, following the Bokashi composting method. Bokashi leachate is acidic and it contains effective microorganisms hence it has potential to be a functional feed additive to the plant proteins based diets for fish farming. This study evaluated the growth performance and feed utilization of the red tilapia (Oreochromis sp.) fingerlings fed with the BL supplemented soybean meal (SBM) based diets. After an 8-week feeding trial, fish fed with the 5% BL supplemented SBM diet attained the highest weight gain. This result was significantly higher (p 0.05) than those fed with the 0% BL supplemented SBM diet, and comparable (p 0.05) to those fed with the control full fish meal (FM) diet. Generally, dietary inclusion of BL enhanced the fish feed intake on the SBM diet but it did not show clear sign of improvement in their feed utilization. In addition, no significant difference was found across the hepatosomatic index and viscerosomatic index from all dietary treatments. These outcomes concluded that dietary inclusion of BL can enhance the feed intake and growth performance of the red tilapia fingerlings fed with the SBM based diet without compromising their health, and the optimum BL inclusion level was 5%. Nevertheless, further study on the properties and substances content of the BL produced from different types and ratios of food waste is strongly recommended. In this study, BL was also discovered to be capable of reducing the crude fiber content in the SBM diets. Such observation deserves a further exploitation on the application of BL to manipulate the crude fiber content in the plant proteins based diets in fish farming.

Japanese eel is already magnetosensitive at the glass eel phase. However, currently, there is no evidence that they can use geomagnetic cues for orientation. We examined orientation behaviour in a radially symmetrical test arena in which the horizontal component of the geomagnetic field could be manipulated. Groups of glass eels released at the centre of the arena showed a mean orientation angle significantly biased to the south of 198° in the ambient geomagnetic field, but showed random orientation in the geomagnetic field when its horizontal component was cancelled in a solenoid. Results showed that they use geomagnetic cues for orientation.

The freshwater African catfish Clarias gariepinus is carnivorous and cannibalistic even during the larval and juvenile stages and this behavior causes economic losses in aquaculture. This study examined for the first time the effect of salinity on cannibalism, survival, and growth of African catfish larvae in the hatchery. Larvae (4 days old, median 7.8 mm TL, 2.8 mg BW) of the African catfish were reared for 21 days at nominal salinity 0, 1, 2, 3, 4, 5, 6, and 7 ppt. After 21 days, they grew to 10–39 mm (median 22 mm) and 10–490 mg (median 90 mg), with no significant difference by salinity treatments. Survival ratios were similarly low (24–31%) at 0, 1, 3, and 7 ppt and significantly higher (49–55%) at 2, 4, 5, and 6 ppt. Cannibalism was significantly lower, 15–30% at 4–6 ppt, than the 40–50% at 0–3 and 7 ppt. Size variation was lower at 4–6 ppt and higher at 0–3 and 7 ppt. We recommend hatchery rearing of African catfish at the optimum low salinity of 4–6 ppt rather than in full fresh water at least up to 21 days. This rearing method fosters larval welfare and improves hatchery production.

The addition of artificial shelters of various materials has been used in Macrobrachium rosenbergii tanks and ponds as a means of increasing productivity. The present study investigated the shelter colour preference of M. rosenbergii postlarvae (age 15–18 days after metamorphosis) in the laboratory. Shelter occupancy tests were performed on four groups of 200 postlarvae in four 57-l aquaria, into which were placed shelters made of rigid coloured netting sewn into four-layered cubes open on two sides. The shelters were presented in six colour pairs: black vs dark green, black vs light green, black vs blue, dark green vs light green, dark green vs blue, and light green vs blue. Colour preference was tested six times for each colour pair. The data obtained were analyzed using Thurstone’s law of comparative judgment. The mean z-score was significantly highest (i.e., highest preference) for the black shelter. Vision was involved in the detection of and approach to the shelter. When released in aquaria, intact-eyed individual postlarvae directly approached the shelter straight away, whereas blind postlarvae (paint over the eyes) swam around randomly and occupied the shelter only after accidental contact much later. Postlarvae in open water out of the shelter exhibited frequent aggressive contact, while those in the shelter were quiescent. The use of black shelters in rearing tanks is thus recommended for reducing stress and aggression among M. rosenbergii postlarvae in the hatchery.