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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.

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.

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.

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.

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.

Many demersal fish species undergo vertical shifts in habitats during ontogeny especially after larval metamorphosis. The visual spectral sensitivity shifts with the habitat, indicating a change in colour vision. Colour vision depends on sufficient ambient light and becomes ineffective at a particular low light intensity. It is not known how fishes see colour in dim light. By means of a behavioural experiment on larval African catfish Clarias gariepinus in the laboratory, we determined colour vision and colour discrimination in dim light. Light-adapted larvae were subjected to classical conditioning to associate a reward feed with a green or a red stimulus placed among 7 shades of grey. The larvae learned this visual task after 70 and 90 trials. A different batch of larvae were trained to discriminate between green and red and then tested for the ability to discriminate between these colours, as the light intensity was reduced. The larvae learned this visual task after 110 trials in bright light and were able to discriminate colours, as light was dimmed until 0.01 lx, the minimal illuminance measurable in this study, and similar to starlight. The retinae of the larvae were found to be light adapted at 0.01 lx; thus indicating cone-based colour vision at this illuminance. For comparison, three human subjects were tested under similar conditions and showed a colour vision threshold at between 1.5 and 0.1 lx. For the larvae of C. gariepinus, the ability of colour discrimination in dim light is probably due to its retinal tapetum, which could increase the sensitivity of cones.

In the retina of the African catfish Clarias gariepinus, the pigment epithelium and the tapetum were formed in newly hatched larvae, the cones developed within 2 days, and the rods within 3 days after hatching. The retinal tapetum shone under surface light under a light microscope; the shine was located in the apical projections of the pigment epithelial cells. Early appearance of the retinal elements enables African catfish larvae to see and feed well even in dim light.

While there is no standard definition of a “juvenile” freshwater prawn, the term is used to refer to the developmental period between postlarva and adult. However, it is important to know the timing of metamorphosis into the juvenile phase when we conduct experiments through the postlarval and juvenile period. In this study, the length of the sixth segment (Ls) and the length of the sixth segment as a proportion (PLs) of the total length (Lt) were measured and then compared between postlarvae and juveniles of the giant freshwater prawn Macrobrachium rosenbergii using an allometric method. A linear regression line for the common logarithm of Ls plotted against the common logarithm of Lt showed a negative allometric relationship. The relationship between PLs and Lt was best represented by two linear regression lines with an intersection at PLs = 12.61 % or Lt = 15.03 mm. We interpret these relationships as an abrupt change in the relative growth of the sixth segment at about the time when M. rosenbergii postlarvae undergo metamorphosis. Thus, the relationship between PLs and Lt can be used to distinguish between larvae and juveniles of M. rosenbergii. The information provided by this work will permit more precise design of experiments focusing on the biological basis for postlarvae and juveniles in M. rosenbergii.

エゾアワビの吻と触角の化学受容器を光学顕微鏡と走査電子顕微鏡で観察した。吻には化学受容器と思われる“味蕾様”の器官が高密度に認められた。頭部触角，上足触角および外套触角のいずれにも先端に繊毛をもつ乳頭突起で覆われていて，これらの触角にマコンブ片を接触させた時のみエゾアワビは補食反応を示した。水槽実験で供試個体はマコンブの匂いによって索餌行動が解発されたが匂源に誘引されなかった。これらより，エゾアワビは触角と吻による味覚と嗅検器による嗅覚をもつが，嗅検器は匂源検出器でないと結論された。

SUMMARY Magnetosensitivity of the Japanese eel, Anguilla japonica, was examined by conditioning and electrocardiography. Marine eels, river eels and farmed eels were conditioned to an imposed magnetic field ranging from 12 663 nT to 192 473 nT parallel to the fish body, which was placed along the earth's west-east axis. Electrocardiograms were recorded with electrodes placed close to the fish body inside a PVC pipe shelter. After 10-40 conditioning runs, all the eels exhibited a significant conditioned response (i.e. slowing of the heart beat) to a 192 473 nT magnetic field and even to a 12 663 nT magnetic field, respectively equivalent to 5.92× and 0.38× the horizontal geomagnetic field (32 524 nT) at our laboratory. The west-east vector of the imposed magnetic field (12 663 nT) combined with that of the geomagnetic field and produced a horizontal resultant magnetic field of 21° easterly. Therefore, Japanese eel are magnetosensitive whether they are at sea, in the river or in the farm. Results of the present study were compared with those of past studies that showed no magnetic sense in the American eel, Anguilla rostrata, and the European eel, Anguilla anguilla.

The improving technology in artificial propagation of aquatic animals is having a direct effect on increasing aquaculture fishery production of many aquatic food species throughout the world. High-valued fish species are being produced en masse and released into the wild to augment populations depleted due to over-fishing and habitat degradation. This study was conducted to determine the effect of procedural conditions in mariculture hatcheries on the olfactory systems of fish in Kagoshima Prefecture, Japan, and to compare the results with fish from wild stocks. Red sea bream (Pagrus major) and black sea bream (Acanthopagrus schlegeli) were suitable candidates for this study because of their economic importance and history in mariculture production, especially ongoing restocking of natural populations via stock enhancement programs in Japan. An extensive survey revealed a high percentage of fish with an abnormal single olfactory nasal opening. The single abnormal nasal opening contrasted strongly with a pair of natural nasal openings, an anterior and a posterior nares, in normal fish of both species whereby olfactory water is sampled when the fish swims. Ultrastructural observation of lamellar topography demonstrated not only a substantial variation in sensory epithelial distribution patterns in red sea bream of cultured stock but also showed that the sensory epithelial area in abnormal cultured fish was significantly smaller (P

孵出直後のアカウミガメとアオウミガメは特定の波長の光に走光性を示し,アオウミガメの錐体は3種の感光色素をもつことが知られており,これらはウミガメが色覚をもつ可能性を示す。筆者らは弁別学習実験でアカウミガメの色覚を確認した。孵出後23日目の稚ガメと3才の個体を,それぞれ緑パネルと赤パネル,緑球と黒球の弁別を学習させ,学習完成後に赤パネルと黒球を明度の異なる3段階の灰色と置換して移調試験を行った。移調試験で3才個体は絶対選択反応(正選択率80-100%)を,稚ガメは移調色に関わらず一貫性のない絶対選択反応とランダム選択反応(正選択率40-100%)を示したことより,両者は色覚をもつが稚ガメの色覚は未熟であると結論された。

感光色素を一種類しかもたないタコ類は色盲とされる。これを行動実験で検証するために, スナダコ5個体とマダコ7個体を用いて色覚を確かめる学習実験を行った。供試個体を負刺激球(白球あるいは灰色球)と同時呈示した青球(直径25mm, 反射スペクトルのλmax=460nm)に触れてから餌を摂るよう条件付けた後, 負刺激球を明度が段階的に異なる灰色球に換えて移調試験を行った。移調試験では, スナダコは灰色球の明度にかかわらず有意に高頻度で青球を選択し, 色覚をもつ可能性が示された。マダコは灰色球の明度によって選択球が変わり, 色盲であると結論された。

タコが何色の隠れ場を好んで選択するか, また選択する色は背景色の違いによって変化するかを調べることを目的とし, 供試個体にマダコおよびスナダコを用いて水槽内行動実験を行った。高照度下で黒, 赤, 橙, 黄, 緑, 青, 白の7色, 低照度下で黒, 黄, 青の3色を背景色とし, 円筒形の隠れ場8色(黒, 赤, 橙, 黄, 緑, 青, 白および透明)を設置し, 30分おきのタコの状態を目視観察, 記録した。背景色にかかわらず黒, 赤, 橙の隠れ場が両種のタコに共通して高頻度で選択された。マダコとスナダコで異なる行動結果も得られたが, スペクトル感度の違いによるものと考えられ, 両種とも, 暗さを好む傾向があると結論された。

The high visual resolution of a fast moving object or an instantaneous image by animals requires that they have a low optic critical duration. The freshwater bluegill showed the ability to detect an image displayed for 3.2 μs at contrast 67.1. The same image was undetectable by the human eye. The optic critical duration and the contrast threshold in the bluegill were found to be 1 55 that of man. In terms of time scale and contrast, the visual ability of the bluegill seems far beyond that of a human, and it is more able to see approaching divers and fishing gears in the scattering medium of freshwater lakes.

To clarify the catching mechanism of cuttlefish basket traps for Sepia esculenta, the catches of 1, 373 cuttlefish basket traps were analyzed for sex composition, and 917 cuttlefish were assessed for sexual maturity. Females were caught in significantly greater numbers (58%) than males, and the sex ratio (female/male) tended to increase between February and May. The observed frequency of traps with only male or only female catch did not follow Poisson's distribution. From one to a maximum of four females appeared in each of 292 traps, and 1-5 males in each of 114 traps. All males and the majority of the females were sexually mature from December to June, except for two females captured in December and January. Most but not all females carried spermatophores in the buccal area. Cuttlefish preferred to lay eggs at the front side of branches placed outside the traps rather than on the inside. These results indicate that spawning and mating behavior are not the only factors governing the entry of cuttlefish into traps, but that there are probably some other factors which are still unknown.

The substance 2-methylisoborneol (MIB) often affects aquatic organisms and causes problem of off-flavours in fish. In order to know how fish become tainted with MIB, response and chemical sensitivity of three freshwater fishes to MIB were examined behaviorally and electrophysiologically. The electrocardiographic tests showed that the threshold of detection for this compound was 4.8 × 10−5 ngl−1 for the Nile tilapia and 2.1 × 10−5 ngl−1 for the rainbow trout. Fish whose olfactory rosettes were removed also showed the cardiac response to MIB, but the threshold increased by 3 and 5 orders of magnitude in the Nile tilapia and rainbow trout respectively. The recordings of neural response of the olfactory tract of the carp to MIB solutions showed the olfactory threshold at a concentration of 4 × 10−8 ngl−1. In behavioral tests in tanks, MIB solutions did not attract these fishes and evoked no avoidance in these fishes. It was concluded that, while freshwater fishes are extremely sensitive to MIB, they would not escape from waters contaminated with MIB and thus easily be tainted with MIB.

The morphology of the sense organs of the larvae and juveniles of the ribbonfish Trichiurus lepturus in Wakasa Bay was examined to obtain insights on changes in behavior and habitats with growth of the fish. Development of the sense organs proceeded in accord with the rest of the morphological development of the larvae and juveniles. Sense organs required for feeding were formed at the onset of exogenous feeding. The taste buds first appeared at the begining of the postlarval period and increased in number in association with the pharyngeal teeth and gill arches. During the late phase postlarval period, the lateral line canal on the head and the olfactory rescue were formed. The retinae became duplex and developed horizontal cells in three layers; this is inferred to be advantageous for a piscivorous feeding habit. The presence of well developed sense organs synchronized with the active migration of postlarvae from offshre to coastal waters.

The development of the optomotor reaction (OMR) in milkfish (Chanos chanos), from the larval, through the metamorphic, to the juvenile stage was observed. The period from the appearance of the pelvic fins until the complete disappearance of the finfold was named ”metamorphic stage”. While the larvae showed strong rheotactic responses, their OMR was somewhat weak. It was clear that the OMR underwent a big change through the metamorphic stage, and became strong and almost perfect in the juveniles.

Ontogenetic development of the sense organs of the flounder, from newly hatched larvae to 72-day old juveniles was examined by light- and scanning electron-microscopy and correlation with changes in behaviour was made. The larvae are capable of horizontal orientation when the otic epithelium is ciliated at 6 hours from hatching. Newly hatched larvae have at least two types of olfactory receptor cells and a pair of free neuromasts with well developed cupula. At 9 hours from hatching, the larvae avoid obstacles by mechanoreception. At 3 days from hatching, eyes are deeply pigmented, the visual system is morphologically complete, and the larvae initiate feeding and are positively phototactic. Differentiation of the taste buds occurs 9 days from the onset of feeding, at this time, food preference is exhibited by the larvae in the sea. Retinal rod cells differentiate at 25 days and twin cones form at 29 days from hatching. Positive phototaxis disappears at 30 days and the larvae become nocturnally active at 33 days from hatching. The shif from pelagic to benthic habit occurs at this stage.

The development of sensory systems and the coincident acquisition of associated behaviors were examined in two paternal brooders, largemouth bass Micropterus salmoides and Nile tilapia Tilapia nilotica. In newly hatched eleutheroembryos of largemouth bass, the eyes are unpigmented, the nares are closed, and the free neuromasts and taste buds are not developed. Six hours after hatching (when the epithelium of the otic vesicle is ciliated), the eleutheroembryos are able to aggregate by responding to the movements of neighboring siblings. Thereafter, four correlations between sense organ development and initiation of fish behavior were observed: appearance of the first free neuromasts on the head and positive rheotaxis; morphological completion of the larval-type retina and positive phototaxis; formation of twin cones in the retina and optomotor reaction; and appearance of taste buds in the oral cavity and selective feeding. The ontogenetic development of the sense organs of Nile tilapia reflects ad...