Katsuyuki Hamasaki

Effects of temperature on the egg incubation period, survival and developmental period of larvae of the mud crab Scylla serrata (Forskål) (Brachyura: Portunidae) reared in the laboratory

Abstract This study examined effects of temperature on the egg incubation period, survival and developmental period of larvae of the mud crab Scylla serrata reared in the laboratory. Pre-mated females were held in tanks in which temperature varied seasonally; their spawning and hatching were recorded daily. The number of days from spawning to hatching and mean rearing temperature were determined for 92 females. The egg incubation period decreased exponentially from 30 to 10 days with increasing mean temperature in the range 20.3–30.0 °C. Larvae were reared in 1-l beakers at seven temperatures (17, 20, 23, 26, 29, 32 and 35 °C) and fed with rotifers and Artemia nauplii. Survival to the first crab stage occurred when larvae were reared at 23–32 °C: the best survival rate was obtained at 29 °C. Also, the number of days from hatching to attainment of each larval stage decreased with increasing temperature. Relationships between mean temperature (T) and egg incubation period and larval developmental period from hatching to reach the megalopal stage (D) were analyzed using the following three equation models: power function, D=aTb; Bělehradeks equation, D=a(T−α)b; and from heat summation theory, D=a/(T−α). For both egg and larval development, the equation of heat summation theory was the best model to characterize the relationship between temperature and the number of days required for development. The biological lower critical temperature represented by parameter α was similar for eggs and larvae: 13.93 and 13.82 °C, respectively, for Bělehradeks equation and 15.70 and 15.08 °C for heat summation theory.

Essential fatty acids for larval mud crab Scylla serrata: implications of lack of the ability to bioconvert C18 unsaturated fatty acids to highly unsaturated fatty acids

Abstract This study was conducted to examine the effect of essential fatty acids (EFA) on the survival, development and bioconversion of fatty acids in mud crab Scylla serrata larvae. Mud crab larvae held in 1-l plastic beakers (30 ind/l) were fed rotifers that had been enriched with oleic acid (18:1n−9, OA), linoleic acid (18:2n−6, LA), linolenic acid (18:3n−3, LNA), eicosapentaenoic acid (20:5n−3, EPA) and docosahexaenoic acid (22:6n−3, DHA) ethyl ester oil during the zoea (Z)1 and Z2 stage. Upon reaching the Z3 stage, rotifers were shifted to unenriched Artemia or Artemia enriched with either EPA or DHA ethyl ester. Further in order to allow comparison of the effect of the enriched rotifers on the fatty acid composition of the Z2 stage larvae, larvae fed enriched rotifers were harvested upon reaching the Z2 stage for fatty acid analysis. The survival of mud crab larvae was more strongly affected by the EFA contained in the Artemia compared to rotifers. Mud crab larvae fed unenriched Artemia showed EFA deficiency signs such as lower survival, longer intermolt period, and a narrower carapace width at the first crab stage, indicating that DHA was superior to that of EPA and followed by LNA and LA. The fatty acid composition of the whole body of Z2 larvae fed rotifers enriched with OA, LA or LNA revealed that the content (g/100 g larvae) of EPA decreased from 1.2 to 0.4–0.8 and DHA decreased from 0.5 to 0.1–0.2, respectively. Moreover, the contents of monoenes increased from 1.4 to 2.5–3.2, indicating dietary EFA deficiency. We conclude that mud crab larvae have a limited or negligible capability to convert C18 unsaturated fatty acids to highly unsaturated fatty acids.

Biocontrol method in aquaculture for rearing the swimming crab larvae Portunus trituberculatus

The aim of this work is to control the waterenvironment for culturing larvae of the swimmingcrab, Portunus trituberculatus, using microorganisms.The bacterial strain PM-4 (Thalassobacter utilis)improved the survival rate of crab larvae andrepressed the growth of Vibrio anguillarum (bacterium)and Haliphthoros sp.(fungus) in seawater. PM-4 wascultured and added daily to seawater during the firstto third zoean growth stage of the crab with diatomsand rotifers. Numbers of PM-4 decreased in culturewater during the first 3 days, because of feeding bythe first zoean stage of larvae. The finalconcentration of PM-4 was 105 to 106 cellsml−1according to the plate count method in larval rearingwater.During 1989 to 1993, we tried seed productions of aswimming crab in 200 m3 containers at TamanoStation, Japan Sea-Farming Association. In 33trials of the biocontrol methods, average survivalrate of crab larvae was 28.3% when the bacterialstrain PM-4 was added. In 42 trials in which the strainPM-4 was not added, average survival rate of crab larvae was15.6%. We conclude that thebacterial strain PM-4 is effective as a biocontrolagent.

Population genetic structure and variability of Pacific herring Clupea pallasii in the stocking area along the Pacific coast of northern Japan

The genetic diversity of wild and hatchery-released Pacific herring Clupea pallasii collected from three brackish lakes and two bays in Honshu and Hokkaido, Japan was examined with five microsatellite loci. All loci showed high genetic variability with expected heterozygosities ranging from 0.815 to 0.945. Significant differences in genotypic and allelic distributions were detected among all locations except for between the two bays in Honshu Island. Pairwise population analysis based on the FST> values showed close genetic relationships among the locations in Hokkaido Island, and the hierarchical analyses of molecular variance showed significant genetic difference between the two islands. Those results suggest the existence of subpopulations due to natal homing. In addition, stocked fish showed as much genetic diversity as the wild fish. The pairwise population analyses also showed close relationships between the hatchery fish and the wild fish in respective stocking areas, showing that no effects of stocking programs on genetic diversity of wild populations were detected.

Potential of Stock Enhancement for Decapod Crustaceans

To evaluate the potential of stock enhancement for decapod crustaceans, we analyzed stock enhancement programs for 4 species: the penaeid prawns (Penaeus japonicus) in Japan and (Penaeus chinensis) China, the European lobster (Homarus gammarus) in Norway, and the mud crab (Scylla paramamosain) in Japan, for which stocking effectiveness has been intensively examined. For each species, we summarized the life cycle, fishery, and stocking programs and then comparatively analyzed the recapture rates, the yield from the released individuals (YPR), and the economic efficiency. Scylla paramamosain had some advantages in stock enhancement, including restricted living areas such as bays and estuaries, a large body size, and high values of the ratio of price per landed individual to the cost per released juvenile. These characteristics promised higher values of YPR and economic efficiency, even at lower recapture rates. Therefore, hatchery releases affect areas with small stocks of S. paramamosain. This scenario might also be inferred for H. gammarus, which had the highest YPR and does not generally migrate from the release site, although the economic efficiency was low due to the high cost of juvenile production. On the other hand, fishery productions of the penaeid prawns, which had a lower YPR, could not be augmented without a huge number of released juveniles and the maintenance of nursery habitats. The restocking is a more cost-effective measure for sustainable fishery production than only a sea ranching (put and take) operation. Therefore, hatchery releases for decapod crustaceans should be applied to rebuild wild stocks (restocking) with appropriate fishing regulations and habitat restoration. In the future, selection of target species for stock enhancement programs should take into account the necessity for restocking and their life history characteristics.

Genetic effects of hatchery fish on wild populations in red sea bream Pagrus major (Perciformes, Sparidae) inferred from a partial sequence of mitochondrial DNA

Variation in the mitochondrial DNA transcriptional control region sequence was investigated in wild and hatchery-released red sea bream Pagrus major from Kagoshima Bay, where an extensive hatchery-release programme has been conducted for >30 years. The programme has successfully augmented commercial catches in the bay (released juveniles have been produced from the captive broodstock, repeatedly used over multiple generations). Samples were also obtained from outside the bay, where limited stocking has occurred. Genetic diversity indices measured as number of haplotypes, haplotype richness, haplotype diversity and nucleotide diversity were lower in hatchery-released fish than in wild fish. Genetic differences in wild fish from the bay, especially in the inner bay, compared with fish from outside the bay were detected in terms of decreased genetic diversity indices and changed haplotype frequencies. Unbiased population pair-wise F(ST) estimates based on an empirical Bayesian method, however, revealed low genetic differentiation between samples from the bay and its vicinity. Mixed stock identification analyses estimated the proportion of hatchery-released fish in wild populations in the inner and central bays at 39·0 and 8·7%, respectively, although the precision of the estimates was very low because of the small genetic differentiation between populations and relatively small sample sizes. Hence, the long-term extensive hatchery release programme has affected the genetic diversity of wild populations in the bay; however, the genetic effects were low and appeared to remain within the bay.

Improvement of larval rearing technique for mass seed production of snow crab Chionoecetes opilio

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. Zo eas 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.

Effects of salinity and dietary n-3 highly unsaturated fatty acids on the survival, development, and morphogenesis of the larvae of laboratory-reared mud crab Scylla serrata (Decapoda, Portunidae)

We investigated the effects of feeding rotifers containing various levels of n-3 highly unsaturated fatty acids (n-3HUFA) to Scyllaserrata larvae at different developmental stages on their survival, development, and morphogenesis when they were cultured at six salinity levels. The first-, third-, and fifth (last)-stage zoeae and megalopae were reared to first-stage crabs at salinities of 10, 15, 20, 25, 30, and 35‰, with three different feeding regimes of rotifers containing different levels of n-3HUFA. The larvae successfully developed to the subsequent stages at 20–35‰ salinity. The highest survival rates to first-stage crabs were recorded at 20–25‰ salinity. The morphological features of the megalopa observed in the last-stage zoeae, represented by the ratio of the chela length to carapace length, tended to advance with increasing salinity, indicating higher assimilation efficiency at higher salinities. The megalopal features of the last-stage zoeae were enhanced when the larvae were fed rotifers containing higher amounts of docosahexaenoic acid (DHA). As reported previously, final-stage zoeal larvae with advanced megalopal features often experienced moult death syndrome (MDS). These results show that when larvae are fed rotifers with high DHA under high-salinity conditions, morphogenesis is accelerated, resulting in MDS. Therefore, to evaluate the effects of salinity on larval survival, it is necessary to examine larval morphogenesis in terms of MDS. In conclusion, we recommend that not only survival but also larval morphogenesis should be examined when evaluating the results of rearing experiments with S. serrata larvae.

Allozyme variation of littleneck clam Ruditapes philippinarum and genetic mixture analysis of foreign clams in Ariake Sea and Shiranui Sea off Kyushu Island, Japan

Allozyme variation of the littleneck clam Ruditapes philippinarum was evaluated in four samples from Nameishi and Matsuo in the Ariake Sea, Ryugatake and Ushibuka in the Shiranui Sea off Kyushu Island, Japan, and in one sample from Jinzhou, China, in the Bohai Sea. A Ruditapes bruguieri sample imported from the Korean Bay off Nampo, North Korea was also studied. Among the R. philippinarum samples, heterozygosity varied from 0.265 to 0.301 and Fis estimates indicated significant homozygosity excess in 15 of 40 loci analyzed. Deviations from Hardy-Weinberg equilibrium were significant in all samples (P<0.05). Pairwise FST estimates indicate that genetic differences between the Chinese and Japanese samples were very low, but significantly different from zero. Mixture proportions with 95% confidence intervals of Chinese R. philippinarum in Nameishi and Matsuo were estimated at 0.4098 [0.2512, 0.5705] and 0.4899 [0.3262, 0.6540], respectively. However, genetic invasion of stocked Chinese R. philippinarum into wild populations in the Ariake Sea remains uncertain due to the low precision of the estimates caused by the high similarity of allele frequencies between Jinzhou and the Ariake Sea.

Effects of Temperature on Snow Crab (Chionoecetes opilio) Larval Survival and Development under Laboratory Conditions

ABSTRACT To understand more fully the larval dispersal and settlement of the snow crab Chionoecetes opilio in natural habitats, we tested the effects of temperatures ranging from ∼1–20°C and ∼1–18°C on the survival and developmental period of snow crab larvae in the zoeal and megalopal stages, respectively, through laboratory experiments. The survival rates of second zoeae and megalopae were significantly greater at 5–16°C and 5–14°C, respectively. There were no statistically significant differences among the survival rates of megalopae reared at 3–16°C, although greater survival rates were observed at 5–14°C. The mean numbers of days from hatching to second zoeae and megalopae and from megalopae to reach first crab instar were significantly shorter at warmer temperatures. Moreover, the relationships between mean temperatures and larval periods were well described by the heat summation theory equations. The threshold temperatures for larval development were estimated to be -2.24–0.63°C; they decreased with larval development as an adaptation for deeper vertical distributions in later larval stages. On the basis of larval distribution with respect to water temperature in natural habitats as well as the heat summation theory equations, the entire larval duration from hatching to first crab instar was estimated to be 74.4–123.4 days, which is similar to that in natural habitats inferred on the basis of the time lags in the occurrence of peak abundance between each larval stage.