No abstract

Light intensity thresholds for school formation were examined In 3 stages (20, 65, and 120 mm) of striped jack Pseudocaranx dentex juveniles with behavioural and histological approaches. Behaviour under a light intensity of 300 to 1 0 -~ lx was recorded uslng a video camera, and schooling behaviour, swimming speed, and nearest neighbour d~stance were analyzed. The retina of fish adapted to each hght Intensity was histologically examined. The l~g h t intensity threshold for schooling behaviour In 20 and 65 mm fish was 5 X 10-l h, while that of 120 mm fish was 5 X 10-4 Ix. The adaptation ratio, defined as the percentage of cone cell movement, ranged from 30 to 80% corresponding with 10' ' to 1 Ix of Light intensity in 120 mm fish, while 20 mm fish showed a much narrower range of adaptation ratio, i.e. from 50 to 70%. Visual acuity increased exponentially with fish total length from 20 to 120 mm, corresponding with the increase of eye diameter. The higher range of adaptation ratio in 120 mm fish should enable them to recognize other fish even under low light intensity conditions and make their light intensity threshold for schooling lower than that of smaller fish. Establishment of behavioural and histological adaptability to lower light intensity should have relevance to the habitat shift that occurs in this juvenile stage from bright shallow reef pelagic waters to the offshore dark deeper area.

In mass culture of Pacific bluefin tuna Thunnus orientalis, a marked growth variation is observed after they start feeding at 6-7 mm in body length (BL) on yolk-sac larvae of other species, and the growth variation in tuna larvae is a factor leading to the prevalence of cannibalism. To examine the relationship between prey utilization and growth variation, nitrogen stable isotope ratios (d 15 N) of individual larvae were analysed. A prey switch experiment was conducted under two different feeding regimes: a group fed rotifers (rotifer fed group), and a group fed yolk-sac larvae of spangled emperor, Lethrinus nebulosus (fish fed group) from 15 days after hatching (6.87 mm BL). The fish fed group showed significantly higher growth than the rotifer fed group. Changes in the d 15 N of the fish fed group were expressed as an exponential model and showed different patterns from those of the rotifer fed group. The d 15 N of fastgrowing tuna larvae collected in an actual mass culture tank after the feeding of yolk-sac larvae was significantly higher than those of the slowgrowing larvae, indicating that slow glowing larvae depended largely on rotifers rather than the yolk-sac larvae.

The major digestive enzymes in Pacific bluefin tuna Thunnus orientalis larvae were characterized, and the physiological characteristics of the enzymes during early ontogeny were clarified using biochemical and molecular approaches. The maximum activity of trypsin (Try), chymotrypsin (Ct) and amylase (Amy) was observed at pH 6-11, 8-11 and 6-9, respectively. Maximum activity of Try, Ct and Amy occurred at 50 °C, that of lipase (Lip) was at 60 °C and that of pepsin (Pep) was at 40-50 °C. These pH and thermal profiles were similar to those for other fish species but differed from those previously reported for adult bluefin tuna. Enzyme activity for all enzymes assayed was found to decrease at high temperatures (Try, Ct, Amy and Pep: 50 °C; Lip: 40 °C), which is similar to findings for other fish species with one marked exception-increased Try activity was observed at 40 °C. Lip activity appeared to be dependent on bile salts under our assay conditions, resulting in a significant increase in activity in the presence of bile salts. Ontogenetic changes in pancreatic digestive enzymes showed similar gene expression patterns to those of other fish species, whereas marked temporal increases in enzyme activities were observed at 10-12 days post hatching (dph), coinciding with previously reported timing of the development of the pyloric caeca in bluefin tuna larvae. However, complete development of digestive function was indicated by the high pep gene expression from 19 dph, which contradicts the profile of Pep activity and previously reported development timing of the gastric gland. These findings contribute to the general knowledge of bluefin tuna larval digestive system development.

-Since 1990, mass mortality of fertilized eggs and hatched larvae of the leopard coral grouper Plectropomus leopardus has occurred repeatedly during its seed production in Okinawa Prefecture. A hitherto unknown protozoan parasite multiplied in the yolk sac in great numbers. The yolk sac of fertilized eggs and hatched larvae eventually burst and parasites were released to sea water. The development of the parasite was briefly described, which is very similar to that of the dinoflagellate Ichthyodinium chabelardi, known to infect yolk of several marine fish larvae in Europe. Based on the similarities of, and minor differences in, the mode of infection, development and SSU rDNA sequence, the parasite is tentatively designated as Ichthyodinium sp. PL (= PL for the abbreviated form of the scientific name for the host). Although the life cycle of the parasite is unknown, the disease outbreak was controlled by rearing the broodstock and incubating fertilized eggs in oxidant-treated seawater. The PCR diagnosis using specific primers designed from part of SSU rDNA sequence of the parasite revealed that disease outbreaks in different years were caused by a single species of parasite. This is the first report of Ichthyodinium infection in tropical fish from the Asia-Pacific region.