Shigeki Dan

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.

Larval development and emigration behaviour during sea-to-land transition of the land hermit crab Coenobita brevimanus Dana, 1852 (Crustacea: Decapoda: Anomura: Coenobitidae) under laboratory conditions

To determine the early life history of the land hermit crab Coenobita brevimanus Dana, 1852, larvae were cultured individually in the laboratory. The zoeal and the megalopal stages are described and illustrated. The larvae developed through four planktonic zoeal stages to the megalopal stage. The major differences in the zoeal characters between C. brevimanus and other described Coenobita species were found in the armature of the pleomeres, whereas the character of pleomeres of C. brevimanus zoeae is the same as that of the coconut crab Birgus latro, a different genus in the same family. Morphological similarity was also found in segmentations of antennules and antennae in megalopae between C. brevimanus and the coconut crab. Megalopae of C. brevimanus were cultured in containers holding seawater and a hard substrate. These crabs migrate from the sea to land after developing a habit of acquiring gastropod shells.

Species Composition, Reproduction, and Body Size of Mud Crabs, Scylla spp., Caught in Urado Bay, Japan

ABSTRACT Species composition, reproduction and body size of mud crabs, genus Scylla de Haan, 1833, were investigated using gill nets from October 2008 to October 2009 in Urado Bay, Japan. Three mud crab species were identified in the area, with S. paramamosain Estampador, 1949 being the dominant species (74% of the catch), followed by S. serrata (Forskål, 1775) (23%) and S. olivacea (Herbst, 1796) (3%). We found temporal changes in species composition with abundances of S. serrata increasing towards the summer season. Ovigerous females of S. paramamosain and S. olivacea occur between January and October, peaking in the warm (May-July) and rainy (June-July) seasons, but we found no berried females of S. serrata. The female-biased sex ratios of S. paramamosain and S. serrata, and the larger body size of females during the period from autumn (November) to early spring (April), suggest that large gravid females may have migrated offshore before the spawning season. The mean body size of females and males of S. paramamosain and S. serrata tended to increase and the mating activity was high between May and October, showing the moulting and growth season.

Evaluation of the effects of probiotics in controlling bacterial necrosis symptoms in larvae of the mud crab Scylla serrata during mass seed production

We aimed to evaluate the practicability of probiotics in controlling larval necrosis symptoms and mass mortality occurred during seed production of the mud crab Scylla serrata. We targeted a bacterium in the family Flavobacteriaceae that causes necrosis symptoms of mud crab larvae. Five candidate probiotic bacterial strains, which showed inhibitory effects against the causative bacterium but were innocuous to larvae, were screened from the seed production environment. Sterile culture medium supernatants of the candidate strains inhibited the causative bacterium through the secretion of extracellular factors. These inhibitory effects were largely reduced when the strains were inoculated into sea water, suggesting medium-dependent production of inhibitory factors. The strain that exhibited the highest inhibitory effect on larval necrosis in 7-day larval rearing experiments was selected as a probiotic strain. In large-scale larval rearing experiments, the probiotic treatment significantly suppressed larval necrosis symptoms and improved larval survival in the early rearing period until the fourth zoeal stage. However, the effect disappeared after the fifth (last) zoeal stage, corresponding to a decrease in inoculated probiotic bacterial counts in the larval rearing water. Almost all larvae had died by the first crab stage. The effectiveness of probiotics developed in vitro has not yet proved practical in commercial-scale seed production. The continuous proliferation of probiotic bacteria and extracellular production as inhibitory factors in larval rearing water are the key factors in developing probiotics for commercial-scale seed production.

Effects of n-3 highly unsaturated fatty acid content in Artemia on survival and development of laboratory-reared horsehair crab Erimacrus isenbeckii larvae

The effect of varying levels of dietary n-3 highly unsaturated fatty acid (HUFA) content in Artemia sp. on survival, the number of days required to reach each larval stage, and the growth and morphogenesis of larvae of the horsehair crab Erimacrus isenbeckii were examined. Artemia enriched with materials of six types [condensed Nannochloropsis sp., ethyl oleate, two enrichment levels of eicosapentaenoic acid (EPA), and those of docosahexaenoic acid], and unenriched Artemia were fed to larvae. Newly hatched larvae were reared in groups of 50 individuals in two 2-l beakers until they reached the first crab stage. No significant difference was found in survival rate, growth, or morphogenesis of larvae, but fewer days tended to be required to reach the first crab stage in the group enriched with high EPA compared to the other groups. Results show that, although the amount of n-3 HUFA in unenriched Artemia might meet the demands of horsehair crab larvae for their survival, growth, and morphogenesis, Artemia should be enriched with high EPA to improve the rate of larval development.

Ontogenetic change of body color patterns in laboratory-raised juveniles of six terrestrial hermit crab species

We examined the ontogenetic change of body color patterns in the laboratory-raised juveniles of six terrestrial hermit crab species, including Birgus latro, Coenobita brevimanus, C. cavipes, C. purpureus, C. rugosus, and C. violascens, which commonly occur in the southern islands, Japan. The body color patterns of coenobitid juveniles were species-specific. The diagnostic features of body color patterns enable identification of juveniles of coenobitid crab species in the wild, thereby helping to understand the precise habitats of each coenobitid species.

Improved method for culturing the swimming crab Portunus trituberculatus larvae to prevent mass mortality during seed production

Larvae of the swimming crab Portunus trituberculatus are traditionally cultured with rotifers, Artemia, and phytoplankton. However, abnormal morphology of last-stage zoeae caused by excessive nutritional intake, abnormal morphology of megalopae caused by indirect ingestion of phytoplanktons via rotifers during zoeal stages, and lack of essential nutrients due to starvation of Artemia cause mass mortality. To develop techniques addressing these issues simultaneously, we evaluated methods to prevent megalopal abnormalities while avoiding starvation of Artemia (experiment 1). Then, we evaluated the effects of Artemia feeding schedules on the occurrence of last zoeal abnormalities (experiment 2). In experiment 1, megalopal abnormality was prevented by rearing larvae with enriched Artemia supplemented with digestible Nannochloropsis, but without rotifers. However, the developmental rates were variable among individuals because of the large size of enriched Artemia for early zoeae, and last zoeal abnormality was induced because of excessive Artemia feeding, resulting in low survival. In experiment 2, newly hatched Artemia were only supplied during the first zoeal stage, and larvae were reared with low density Artemia and/or low salinity. These measures achieved high survival rate while suppressing the last zoeal abnormality. Our results suggest that P. trituberculatus larvae can be successfully cultured using only Artemia.

Efficiency of stagnant water larviculture using disinfected amictic eggs of the rotifer Brachionus plicatilis in Japanese flounder Paralichthys olivaceus

There is a need to develop low-cost methods for larviculture that emphasize disease prevention. We evaluated stagnant water larviculture in flounder Paralichthys olivaceus using rotifer amictic eggs treated with glutaraldehyde. Growth and survival of the larvae were similar to in larvae reared by standard running water larviculture with daily feeding of rotifers (control). In the experimental group, the n-3 highly unsaturated fatty acid content of rotifers in the rearing water was higher than in the enriched rotifers fed to the control group. We also observed a similar pattern for the fatty acid composition of flounder larvae. The viable bacterial counts associated with rotifers in stagnant water were lower than those associated with enriched rotifers, and there were fewer bacterial genera. However, the viable bacterial count was the same or higher in the experimental rearing water compared with that in the control group. The α-Proteobacteria and Cytophaga–Flavobacterium–Bacteroidetes group dominated the bacterial community structure after larval hatching. To our knowledge, this is the first time that stagnant water larviculture, using only disinfected amictic eggs, has been successfully demonstrated on a mass production scale. This method appears to offer benefits including improved nutrition, more favorable bacterial communities, and lower cost.

Chela asymmetry in a durophagous crab: predominance of right-handedness and handedness reversal is linked to chela size and closing force.

ABSTRACT The swimming crab Portunus trituberculatus is a durophagous brachyuran. Right-handed crabs are predominant, but left-handed crabs are also found in nature. Left-handedness may arise from loss of the right crusher. We examined whether heterochely (morphology) was correlated with differences in closing force (physical property) and handedness (behaviour). The closing force was stronger in larger chela with greater apodeme height and handedness resided in the chela with stronger closing force. With loss of the right chela (autotomy), handedness transitioned from the right to left chela, and all crabs were left-handed thereafter. Reversed handedness was accompanied with a reduction of size and closing force in the regenerated right chela, and growth of the original left chela. After handedness reversal, dentition on the left dactylus of the newly-converted crusher was close to that of the original right crusher, but did not attain the same shape, even after 10 moults. Left-handed crabs were significantly worse than right-handed crabs at crushing hard-shelled prey. Chela formation was symmetrical in the zoea, and heterochely and right-handedness started in the megalopa, regardless of maternal handedness. Since the left chela is capable of being the crusher, heterochely may be caused by differences in morphogenetic velocity between the right and left chelae, under a signal discriminating right from left. Right-handedness is an attribute of P. trituberculatus, that would be inheritable across generations. It is probable that right-handedness was used in the earliest durophagous crabs, and this trend has been succeeded to extant species. Summary: Handedness in the swimming crab resides in the claw that is larger with greater apodeme height and has the stronger closing force.