Metamorphosis season from megalopa to the first crab stage in snow crab Chionoecetes opilio and red snow crab C. japonicus (Crustacea,Decapoda, Majoidea) in the Sea of Japan,estimated from captive culture

ABSTRACT

Metamorphosis season of megalopae to the first crab stage in snow crab Chionoecetes opilio and red snow crab C. japonicus was inferred by culturing wild-born megalopae collected from the Sea of Japan. Metamorphosis occurred from late June to late July (mainly in July) in snow crab, and from early July to early October (mainly from August to September) in red snow crab. The number of days required from the time of collection to metamorphosis was less than the intermoult period previously reported for snow crab megalopae. However, the developmental period of the megalopae was estimated as substantially longer in red snow crab than in snow crab. Previous studies have shown that the hatching season and the period of the zoeal stage in both two species are similar. These results suggest that a different metamorphosis season between the two species would be due to a difference in their megalopal intermoult period.     

Isolation and characterization of polymorphic microsatellite markers in the red snow crab (Chionoecetes japonicus)

A total of 12 polymorphic microsatellite DNA loci were isolated from the red snow crab, Chionoecetes japonicus (Brachyura: Majidae), one of important fisheries resources in the Far East. The number of alleles observed at each locus ranged from two to 19, with the observed and expected heterozygosities of 0.125-0.875 and 0.156-0.949, respectively, suggesting these loci to be a useful molecular marker for population analysis in this species. Of the 12 loci, seven also were available for genotyping of the snow crab, Chionoecetes opilio, implying these loci as a useful molecular marker in the genus Chionoecetes.     

Evidence for a permanent establishment of the snow crab (Chionoecetes opilio) in the Barents Sea

In the Atlantic the snow crab (Chionoecetes opilio) is naturally distributed on the northwestern side, i.e. eastern Canada and west Greenland. Until recently, there have been no observations of snow crab in eastern Atlantic. However, in 1990s single and occasional reports were made of crabs captured in the eastern part of the Barents Sea, presumably introduced through ballast water. Special attention during the annual bottom-trawl surveys in the Barents Sea during February 2004–2006 were given to include recordings of snow crab to evaluate if the introduced species has succeeded to establish a self-sustaining population in this region. Recordings of snow crabs were systematically noted and biological measurements carried out. The results confirm previous Russian observations of snow crabs in the northern region of Gåsebanken. In addition, a significant number of crabs were also found in the central region of the Barents Sea, mainly in deeper waters from 180 to 350 m depth. The sizes ranged from 14 to 136 mm carapace width. All females above 70 mm were berried with fertilised eggs. A major fraction (31% in 2005; 76% in 2006) of the crabs consisted of juveniles below 50 mm CW, providing evidence for successful recruitment. The small-sized crabs were exclusively found in Gåsebanken, identifying the main recruiting area at present for snow crab in the Barents Sea. The results obtained show that the snow crab is now adapted to the northeast Atlantic.     

New microsatellite markers for the snow crab Chionoecetes opilio (Brachyura: Majidae)

Nine novel microsatellite loci were isolated from Chionoecetes opilio by screening an enriched genomic library using nonradioactive polymerase chain reaction techniques, and the polymorphisms were examined to estimate genetic variability. The genetic variabilities varied depending on the locus. All loci were found to be polymorphic with an average of 9.7 alleles per locus (range 3–25). The observed and expected heterozygosities ranged from 0.80 to 0.98 and from 0.56 to 0.95, respectively. Five loci showed significant Hardy–Weinberg disequilibrium at the P < 0.05 level. The high variabilities revealed in this study suggest that these microsatellite loci should provide useful markers for genetic variation monitoring of C. opilio.     

New microsatellite markers for the snow crab Chionoecetes opilio (Brachyura: Majidae)

Five microsatellite markers were developed for the snow crab (Chionoecetes opilio) and polymerase chain reaction conditions and/or primer sequences of three previously developed microsatellites were adapted for fluorescent labelling analysis. All loci were analysed in 449 individuals from 11 sampling sites in the northwest Atlantic. The high degree of polymorphism exhibited by these microsatellites (mean of 34 alleles per locus and mean observed heterozygosity of 0.76) suggests that they will be suitable for spatial and temporal genetic analysis of C. opilio populations.     

First observations of temperature tolerances of adult male snow crab (Chionoecetes opilio) from the Barents Sea population and the effects on the fisheries strategy

This study investigates the effects of temperature on the survival, food intake, oxygen consumption and growth during long-term live holding of captive male snow crab (Chionoecetes opilio) (average?=?0.7?kg). The crabs were held at three different temperatures, 3, 6 and 9°C. The trials were done using groups of snow crabs held in nine land-based holding tanks (three replicates per temperature treatment). The results showed that temperature had a significant effect on survival. The survival rate at 3°C (61%) was significantly higher than at 6°C (33%) and at 9°C (28%). Specific oxygen consumption rates of unfed crab at 6°C were significantly higher than at 9°C and 3°C. In summary, the current study shows that the Barents Sea snow crab have a narrow temperature range in which they thrive compared with the Barents Sea red king crab (Paralithodes camtschaticus). Barents Sea snow crab has similar metabolic and physiological attributes to other major snow crab populations. In conditions when ambient temperatures are approximately 6°C, it may prove beneficial for animal welfare and also be financially advantageous to reduce ambient water temperatures in live snow crab holding facilities on boat or on land.     

Epidemiology of bitter crab disease (Hematodinium sp.) in snow crabs Chionoecetes opilio from Newfoundland, Canada

The parasitic dinoflagellate Hematodinium sp. causes a condition known as bitter crab disease (BCD) in snow crabs Chionoecetes opilio and Tanner crabs C. bairdi. As the name of the condition implies, crabs infected with BCD are unmarketable due to their bitter flavor. We surveyed the distribution of BCD in 3 regions within the snow crab fishery of Newfoundland from 1997 to 2003. Over time, the disease has become firmly established in Conception and Bonavista Bays and persists at low levels on the Avalon fishing grounds. An epizootic occurred within Bonavista and Conception Bays in 1999 and persisted in Conception Bay in 2000, reaching prevalences of over 2% to 9% in trapped and trawled male crabs and from 19 to 26% in trawled and trapped female crabs, respectively. Infections were highest in females and small males, i.e. the unfished and pre-recruit portions of the fishery. In a mortality study, all of the naturally infected crabs died and 50% of the experimentally inoculated crabs died. Patterns in the molting cycle and prevalence of infection indicate that transmission occurs during the post-molt condition, and that overt infections probably develop 2 to 4 mo later with mortalities occurring at least 3 to 4 mo thereafter. The hydrography of this bay may have contributed to the epizootic as infections were centered within the deeper confines of the bay. Analysis of various abiotic factors uncovered a significant positive association between prevalence, depth and mud/sand substrates; the nature of this relationship was not apparent but may be related to diet or alternate hosts. Lastly, given the increase in BCD in snow crabs in Newfoundland, we recommend that fishery management programs for Chionoecetes fisheries employ non-selective gear to monitor for Hematodinium infections in female and juvenile crabs because these under-sampled members of the population may forewarn of impending recruitment declines that might otherwise remain unexplained.     

Biochemical indicators of muscle growth in the snow crab Chionoecetes opilio (O. Fabricius)

This study examined the relationships between muscle growth rate, the activity of metabolic enzymes and the RNA:DNA ratio, in adult snow crabs Chionoecetes opilio. After moulting, crabs were assigned to three feeding rations to attain a range of tissue growth rates. Muscle growth rate, estimated by the variation in dry tissue content per ml of merus of the first walking leg, was positively correlated with changes in muscle cell number, as evaluated by the DNA content per ml of merus. However, no significant correlation was detected between growth rate and the variation in muscle cell size, the latter being estimated by the change in the protein:DNA ratio. This is due to the fact that, in starved crabs, a reduction in the number of cells is partly compensated by a size increment of the remaining ones. This phenomenon also weakened the overall relationship between muscle growth rate and the phosphofructokinase (PFK) capacity per ml of merus. The simple correlation between those two variables was significantly positive for animals which increased their mass of muscle but insignificant for those which were loosing muscle mass. The lactate dehydrogenase (LDH), citrate synthase (CS) and cytochrome c oxidase (CCO) capacity per ml of merus did not match growth rate. The significant simple correlations that were detected between growth rate and the various enzyme activity expressed per g of protein, per μg of DNA and per g of dry mass did not hold when partial correlations were computed. Variations in muscle cell size were related to adjustments in the quantity of RNA per cell, as depicted by the RNA:DNA ratio. Since muscle growth was not correlated with the variation in muscle cell size, it was not correlated with the RNA:DNA ratio either.     

Population genetic analysis and origin discrimination of snow crab (Chionoecetes opilio) using microsatellite markers

Major habitats for the snow crab Chionoecetes opilio are mostly found within the northwest Atlantic and North Pacific Oceans. However, the East Sea populations of C. opilio, along with its relative the red snow crab (C. japonicas), are two of the most important commercial crustacean species for fisheries on the east coast of the Korean Peninsula. The East Sea populations of C. opilio are facing declining resources due to overfishing and global climate change. Thus, an analysis of population structure is necessary for future management. Five Korean and one Russian group of C. opilio were analyzed using nine microsatellite markers that were recently developed using next-generation sequencing. No linkage disequilibrium was found between any pair of loci, indicating that the markers were independent. The number of alleles per locus varied from 4 to 18 with a mean of 12, and allelic richness per locus ranged from 4.0 to 17.1 across all populations with a mean of 9.7. The Hardy–Weinberg equilibrium test revealed significant deviation in three out of nine loci in some populations after sequential Bonferroni correction and all of them had higher expected heterozygosity than observed heterozygosity. Null alleles were presumed in four loci, which explained the homozygosity in three loci. The pairwise fixation index (F _ST ) values among the five Korean snow crab populations did not differ significantly, but all of the pairwise F _ST values between each of the Korean snow crab populations and the Russian snow crab population differed significantly. An UPGMA dendrogram revealed clear separation of the Russian snow crab population from the Korean snow crab populations. Assignment tests based on the allele distribution discriminated between Korean and Russian origins with 93 % accuracy. Therefore, the snow crab populations around the Korean Peninsula need to be managed separately from the populations in Bering Sea in global scale resource management. Also, this information can be used for identification of snow crab origin which is problematic in worldwide crab trade.     

Prevalence of bacteria in the spermathecae of female snow crab, Chionoecetes opilio (Brachyura: Majidae)

The spermathecal contents of primiparous, multiparous and barren female snow crabs were observed by a scanning electron microscope. Bacteria colonies were observed with a significantly higher frequency in the spermathecae of primiparous and old barren females. Bacteria infect the spermathecae and destroy the spermatophores and spermatozoa inside. These observations suggest that bacteria in the spermathecae do not exclude opportunistic microbes by modifying pH of the medium as suggested in the literature. The prevalence of bacteria in primiparous and old barren females suggests that they infect individuals with a weak anti-microbial protection. The absence of bacteria in the highly acidic seminal fluid derived from males upon copulation suggests that it may provide anti-microbial protection. Bacteria do not seem to be able to survive in an anaerobic environment for a long period.     

A review of factors contributing to the decline of Newfoundland and Labrador snow crab (Chionoecetes opilio)

The Newfoundland and Labrador (NL) snow crab resource, presently the basis of the most important commercial fishery in the region, is in decline. Short-, mid-, and long-term recruitment prospects are deemed poor in most areas. Fishery declines have been most apparent in the north, beginning in the mid- to late 2000s, but are expected to begin in the more productive southern areas in the near future. A multitude of emergent theories to explain the resource decline have been hypothesized as contributing factors, including fishing, trawling impacts, seismic activities, disease, predation, and increasing temperature. This study comprehensively reviews and qualitatively relates the results of recent research and literature on each of these factors. We find that several factors may be contributing to a lack of recruitment in the stock, but diminishing productivity resulting from a warming oceanographic regime is the primary cause of the resource decline. Further, we postulate that trends occurring in the snow crab stock are indicative of a broader-scale ecological regime shift occurring along the NL shelf.     

Cholinesterase activity in tissues of some crab species from the Japan Sea

The comparative study of the cholinesterase activity in some crab species was carried out for the first time with use of a set of thiocholine substrates. The substrate specificity was studied in stellar nerve, heart, and hemolymph of three crab species. The crab hemolymph was shown to be characterized by the highest enzyme activity. The enzyme from various crab organs has different structure o substrate specificity. Properties of crab enzymes were compared with acetylcholinesterase (AChE) of human blood erythrocytes, butyrylcholinesterase (BuChE) of horse blood serum, enzyme of squids and bivalve molluscs. The obtained data allow the conclusion to be made on differences in properties of enzymes both at the interspecies and at the tissue levels.     

Estimating the maximum sustainable yield of snow crab (Chionoecetes opilio) off Tohoku,Japan via a state-space stock assessment model with time-varying natural mortality

A maximum sustainable yield (MSY) obtained by maintaining or restoring fish stock levels is a tangible benefit of ecosystem services. Snow crab (Chionoecetes opilio) off Tohoku, Japan has been managed by a total allowable catch since 1996, although the abundance has not increased. Surprisingly, there was no increase after 2011, when fishing pressure was greatly reduced because of the Great East Japan earthquake. This implies that some of the crab's biological characteristics, such as recruits, natural mortality coefficient (M) and terminal molting probabilities (p), might have changed. We developed “just another state-space stock assessment model” to estimate the MSY of the snow crab off Tohoku considering interannual variations in M and p. The multimodel inference revealed that M increased from 0.2 in 1997 to 0.59 in 2018, although it did not vary according to instars, sex or terminal molt. The parameter p also increased by 1.34–2.46 times depending on the instar growth stages from 1997 to 2018. We estimated the MSYs in three scenarios, which changed drastically if M and p were set as they were in the past or at the current values estimated from this study. This result indicated that the MSY of snow crab would also vary with time based on their time-varying biological characteristics.     

First record of the decapod-egg predator Ovicides paralithodis (Nemertea,Carcinonemertidae) from the snow crab Chionoecetes opilio (Decapoda,Brachyura)

The carcinonemertid monostiliferan Ovicides paralithodis Kajihara and Kuris, 2013 was originally described as an egg predator of the red-king crab Paralithodes camtschaticus (Tilesius, 1815) in the Sea of Okhotsk, the Bering Sea, and the Gulf of Alaska. In the present study, several carcinonemertid specimens were obtained from the egg mass of the snow crab Chionoecetes opilio (O. Fabricius, 1788) in the Sea of Japan. Partial sequences of the mitochondrial cytochrome c oxidase subunit I gene (COI) determined from two specimens of the carcinonemertid were identical with a barcode sequence from the holotype of O. paralithodis, indicating that the host range of the species covers at least the two decapod species, P. camtschaticus and C. opilio.     

The moulting cycle and changes in body density in larvae of the snow crab Chionoecetes opilio (Brachyura: Majoidea) under laboratory conditions

The moulting cycle and the time course of changes in body density from hatching to the end of the megalopal stage in snow crab (Chionoecetes opilio) larvae were investigated in laboratory-reared specimens. Morphological changes in the epidermis and cuticle were photographically documented to characterize the moult-cycle stages: A–B (postmoult), C (intermoult), D (premoult) and E (ecdysis). Moult-stage characteristics were based on a microscopical examination of integumental modifications, particularly of the telson. During stages A–C, the larval cuticle changed from a spongy structure to become conspicuously thicker and more solid in appearance. In stage D, the epidermis retracted from the cuticle and new setae and appendages were formed. The body densities of larval snow crabs were lowest just after moulting; they increased greatly during stage C, and then gradually increased to reach a plateau at 1.0897–1.0931 g cm^?3 during stage D. Over the whole larval period, they have a density greater than that of seawater. These observations will assist in understanding of larval distribution and transport in snow crabs in their natural habitat, and provide a useful tool to determine the developmental stages of larvae sampled from the plankton and from larval cultures.     

Fungus invasion of legs of the tanner crab, Chionoecetes bairdi

Histological examination of tissues from the legs of tanner crabs, Chionoecetes bairdi, infected with black mat fungus, Trichomaris invadens, showed that hyphae penetrated the cuticle of the crab, extensively invaded the epidermis, and extended into the muscle tissue.     

Fungus invasion of legs of the tanner crab, Chionoecetes bairdi.

Histological examination of tissues from the legs of tanner crabs, Chionoecetes bairdi, infected with black mat fungus, Trichomaris invadens, showed that hyphae penetrated the cuticle of the crab, extensively invaded the epidermis, and extended into the muscle tissue.     

Pattern and causes of a temperature-dependent gradient of size at terminal moult in snow crab (Chionoecetes opilio) along West Greenland

Geographic variation in size of male and female snow crabs (Chionoecetes opilio) was investigated along the west coast of Greenland to test the hypothesis that size at terminal molt (=adulthood) is temperature dependent. A total of 81,490 snow crabs were collected in small-mesh traps in Disko Bay (68–69°N) and six sites near Sisimiut (66–67°N) in May and June from 2000 to 2005. Average bottom temperature over the study period ranged from −0.8 to 3.2°C across the sampling sites. Mean carapace width was positively correlated with temperature in both sexes, a pattern often described as a converse Jame’s cline. We infer that temperature per se is the causative factor and discount season length, food availability or density as ultimate causes of the cline. Temperature effects on body size of crabs apparently result from a change in the number of instars before terminal molt. This interpretation is supported by size frequency analysis showing that in general crabs were larger at instar in a colder than in a warmer site. We briefly discuss the implications of our findings for population reproductive potential and the effectiveness of a fixed legal size limit in protecting some adult males from exploitation at different temperature regimes.