Fundamental study on the behavior of milkfish fry for the evaluation of the efficiency of traditional fry collecting gears in the Philippines

The response of Chanos chanos fry to moving and stationary nets of different mesh size and colour, underwater visibility of the nets and water filtration were studied. Results indicate that milkfish fry may be driven by nets of mesh size larger than that presently used; larger mesh size decreases the net resistance in the water so that collectors may move the equipment easily. The large mesh nets should be of a dark colour, preferably black for effective driving; bowever white mosquito net is best for the core end, since the fry are more easily visible on a white background

Night-time movement of fish in coastal waters of Terengganu

The night-time movement pattern of 15 fish species in Terengganu waters was inferred from the driftnet catch obtained from experimental driftnetting in two monsoon seasons. Nets shot perpendicular to the coast drifted northward or southward parallel to the coast at 0.33-1. 78 km/h. Thirteen out of 15 species caught were enmeshed significantly against the current. Observations on gonad development of some migratory pelagic species showed that these species were on spawning migration to nearl7y known spawning areas. Daytime and long-term movement patterns remain unknown

Sensory systems and feeding behaviour of the giant freshwater prawn, Macrobrachium rosenbergii, and the marine whiteleg shrimp, Litopenaeus vannamei

Information on the sensory basis of shrimp feeding provides the means for assessment of the effectiveness of food items in terms of smell, taste, size, and colour. This chapter summarizes information about the sensory basis of the feeding behaviour of the giant freshwater prawn (Macrobrachium rosenbergii) and the marine whiteleg shrimp (Litopenaeus vannamei). Existing literature on these shrimp species and other decapod crustaceans is reviewed, and unpublished experiments using the selective sensory ablation technique to determine the involvement of vision, chemoreception, and touch sense in the feeding behavior of the juveniles of M. rosenbergii and L. vannamei are also described. To determine the role of vision in feeding, the eyes of the juveniles were painted over (deprived of vision) with white manicure and their feeding response to commercial pellets was compared with those with untreated eyes. The untreated eyed juveniles detected and approached a feed pellet right away, but the specimens blinded by the coating detected a pellet only after random accidental touch with the walking legs while roaming on the aquarium bottom. Juveniles that had learned to feed on pellets showed food search and manipulation responses to a pellet-like pebble without smell and taste. The early larvae (zoeae) of M. rosenbergii already have colour vision (that likely persists through life) and colour preference for blue and white. The adults of L. vannamei discriminated a blue-colored well among seven grey wells in a palette, also showing colour vision in this shrimp. A behavioural experiment with dyed prawn flesh showed that L. vannamei has innate color preference for yellow food over black, red, green, and blue food regardless of the background colours of the aquarium bottom. To disrupt chemoreception, the juveniles of both the species were abruptly transferred to water of drastically different salinity and the osmotic ablation destroyed the chemosensitive sensilla. The osmotically ablated juveniles approached a pellet right away but failed to ingest it; they had learned the visual cue and texture of the pellets and recognized them by vision and tactile sense. To determine the role of sensory appendages in feeding of L. vannamei juveniles, the antennal flagella, antennular flagella, and the pincers of the pereiopods were ablated. The ablated juveniles roamed the bottom, touched a pellet at random, grabbed it with the maxillipeds, and ingested it. Subsequently they learned to lower the head, actively swim forward, sweep the bottom with the maxillipeds, detect a pellet, and ingest it—thus indicating a plasticity in feeding behaviour in L. vannamei

Effect of colored light regimes on the stress response and RNA/DNA ratio of juvenile red sea bream, Pagrus major

We hypothesized that fish with red-sensitive retina would be stressed by red light and thus inhibited in somatic growth. Red sea bream (Pagrus major) juveniles (total length =3 cm) with red-sensitive retina were chosen to test this hypothesis. We examined the effect of different color lights (red with λmax 605 nm; green with λmax 540 nm; blue with λmax at 435 nm; and white with full spectrum) on unfed juveniles in laboratory tanks. Stress level was measured by the plasma cortisol and glucose concentrations, and nutritional status by muscle RNA/DNA ratio. Under red light, plasma cortisol and glucose, and muscle RNA/DNA were significantly higher than under green, blue, or white light. Our hypothesis was partly supported by previous findings on the effects of the color environment and spectral sensitivity of reared fishes. However, the levels of cortisol, glucose, and RNA/DNA in this study were low compared to published values. It seems that hatchery-bred juvenile red sea bream have adapted to red-rich surface light and are able to cope with the stress of living in surface floating cages which is so different from their deep-water habitats

Allometric comparison of the length of the sixth segment in postlarvae and juveniles of the giant freshwater prawn Macrobrachium rosenbergii

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

Shelter colour preference in the purple mud crab Scylla tranquebarica (Fabricius)

Cannibalism a serious problem in the grow-out of mud crabs in ponds. Studies have proven that presence of shelters decreases aggressive behaviour of Scylla spp. in ponds and tanks. In this study, the shelter colour preference was determined in the wild captured purple mud crab Scylla tranquebarica (Fabricius) in an outdoor round black tank. Eight out of 48 PVC pipes were painted either blue, green, red, black or white. Other eight were not painted and were provided as grey shelters. The shelters were presented in different two-colour pairs to a group of 35 crabs; eight shelters of one colour and eight of other colour were arranged alternately on the tank bottom, and the number of occupants was recorded. After emptying the shelters, the shelters rearranged for the next observation. Each colour combination was repeated five times, and total of 15 combinations were tested. The number of occupants against each colour of the different pairs were analyzed by the χ2-test and Thurstone’s law of comparative judgment. The data were converted into mean z-scores and significant biases from zero were noted. Underwater visibility of each colour shelter was visually measured through a waterproof digital CCD camera. The crabs showed a strong bias for blue shelter and white shelter chosen the least under the holding condition. The underwater visibility was highest in blue and white colour shelters among the six colour shelters. The observed colour preference was probably due to colour per se and may be an innate ability, rather than due to relative underwater visibility. During the observations, the shelter occupants were tolerant of intruders; there was no aggressive competition for shelter and two to four crabs shared one shelter. It was hypothesized that the touch stimulus of shelter suppresses the aggressiveness and results in low cannibalism of S. tranquebarica

The relative eye size, visual cells, cone mosaic and retinal tapetum in the spotted barb Puntius binotatus (Valenciennes, 1842)

The relative eye size, types of visual cell and mosaic, and the existence of retinal tapetum in the eyes of the spotted barb Puntius binotatus were determined to gain baseline information on its visual capability. The P. binotatus acquired relatively larger eye size than the other similar sized freshwater fish species with its retina being contained both cone and rod visual cells (cone ellipsoid to outer nuclei ratio = 1: 5.7). Three types of cone were identified (double-, central single- and corner single- cones), arranging in the square mosaic, and the retinal tapetum was determined to be existed. These results evidenced that the P. binotatus has good visual capability as it possessed both the photopic and scotopic visions. Vision can be the primary sense for the P. binotatus. Further study is needed to gain more information on the vision of this species.