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.
Behavioral characteristics of snow crab Chionoecetes opilio larvae such as phototaxis and geotaxis in relation to their depth regulation were examined in the laboratory. Further, eŠects of agitation and bath treatment with sodium nifurstyrenate (NFS Na) for larval rearing water on survival rates of larvae, rates of larvae which ‰oated in the rearing water, and mortality rates of larvae which sunk to the bottom of the tank were investigated. Theˆrst stage zoeas ‰oated in the upper layer of the rearing chamber showing positive phototaxis and negative geotaxis, however, almost all larvae of the later stages showed no reaction to a light stimulus and sunk to the bottom of the chamber. Larval ‰oating rates and survival rates were improved by agitation and bath treatment with NFS Na for rearing water; especially, survival rates signiˆcantly increased in the tanks with NFS Na bath treatment.
Larval rearing experiments were conducted to examine the potential for mass seed production of the snow crab Chionoecetes opilio using a total of eight 20-kL tanks. Tanks were equipped with agitators, which move the water using a rectangular blade to prevent the zoeas sinking to the bottom of the tank. Larval rearing water was treated with sodium nifurstyrenate once a week to reduce the chance of larval infection by pathogenic bacteria. Zoeas were fed with rotifers and Artemia nauplii. A total of 122 830 megalops and 16 660 first-stage crabs were produced. Thus, the potential for mass seed production of snow crab was determined. Survival rates up to the megalopal stage were high in tanks with a feeding regime that fed rotifers to larvae through an entire zoeal stage. This study also describes the fatty acid composition of snow crab larvae. It revealed that the first zoeas had a high DHA content and DHA/EPA ratio, but these values significantly decreased in the second stage zoeas and megalops. Improving the DHA content and/or DHA/EPA ratio of larvae should be important in studies on mass seed production technology of the snow crab.
Tropical minute rotifer strains (SS‐type) induce mixis at 30–35°C but sexual reproduction and resting egg formation do not proceed well due to rapid environmental change. The present study examined the effect of temperature regulation on rotifer Brachionus rotundiformis (Langkawi strain, SS‐type) resting egg formation in small (500 mL in culture volume)‐ and large‐scale (500 L in culture volume) experiments. Rotifers were cultured at 30°C in 15–17 p.p.t. seawater with an initial density of 1 individual (ind.)/mL. After 4 days, when cultures were in exponential growth stage with active mixis induction, the culture temperature of the experimental rotifers was changed to 25°C. Control rotifers were cultured at 30°C throughout the experiment. Fresh or frozen Nannochloropsis oculata and condensed freshwater Chlorella vulgaris were used as the rotifer diets in the small‐ and large‐scale experiments, respectively. Significantly higher resting egg production was observed with the experimental rotifers (30→ 25°C) versus the control rotifers. In the large‐scale trial, experimental rotifers produced 2.6 × 106 resting eggs during a 9‐day experiment, which was 1.6‐fold more than the control rotifers. Moreover, the efficiency of resting egg formation was found to increase by a factor of 1.8. The present study indicates that decreasing culture temperature from 30 to 25°C after active mixis increased resting egg formation in B. rotundiformis (SS‐type).
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