Kumamoto Oyster Research Articles

Geographical distribution and ecological niche dynamics of Crassostrea sikamea (Amemiya, 1928) in China's coastal regions under climate change

Global climate change drives species redistribution, threatening biodiversity and ecosystem heterogeneity. The Kumamoto oyster, Crassostrea sikamea (Amemiya, 1928), one of the most promising aquaculture species because of its delayed reproductive timing, was once prevalent in southern China. In this study, an ensemble species distribution model was employed to analyze the distribution range shift and ecological niche dynamics of C. sikamea along China's coastline under the current and future climate scenarios (RCP 2.6–8.5 covering 2050 s and 2100 s). The model results indicated that the current habitat distribution for C. sikamea consists of a continuous stretch extending from the coastlines of Hainan Province to the northern shores of Jiangsu Province. By the 2050 s, the distribution range will stabilize at its southern end along the coast of Hainan Province, while expanding northward to cover the coastal areas of Shandong Province, showing a more dramatic trend of contraction in the south and invasion in the north by the 2100 s. In RCP8.5, the southern end retracts to the coasts of Guangdong, whereas the northern end covers all of China's coastal areas north of 34°N. C. sikamea can maintain relatively stable ecological niche characteristics, while it may occupy different ecological niche spaces under future climate conditions. Significant niche expansion will occur in lower temperature. We concluded C. sikamea habitats are susceptible to climate change. The rapid northward expansion of C. sikamea may open new possibilities for oyster farming in China, but it will also have important consequences for the ecological balance and biodiversity of receiving areas. It's imperative that we closely examine and strategize to address these repercussions for a win-win situation.

Growth-related phenotypic and genetic diversity analysis of successive mass selected generations of Kumamoto oyster (Crassostrea sikamea)

The Kumamoto oyster, Crassostrea sikamea, is widely found in China, from Jiangsu to Guangxi, and has high aquaculture potential due to its beautiful shape and unique flavor. The individual size of C. sikamea in China is significantly smaller than that in the United States of America, the main reason should be the lack of artificial breeding. In 2012, a mass selective breeding program for C. sikamea was initiated to improve growth performance of our successive mass-selected C. sikamea generations, with the ultimate aim of improving the growth rate of C. sikamea. At harvest, each selective breeding generation showed superior growth performance compared to the corresponding control line. In the four generations of mass selection, the response to selection (SR), realized heritability (hR2) and genetic gain (GG) for shell height were 0.63–0.82, 0.38–0.51 and 5.54–9.50%, respectively. Meanwhile, the average SR, hR2 and GG of the total weight of C. sikamea after mass selection for four successive generations were 0.61–0.90, 0.36–0.56 and 12.25–15.34%, respectively. The positive selection response for the fast growth of C. sikamea suggested that they have high levels of genetic variation, that can be used in future breeding programs to further improve their aquaculture traits. In addition, we also studied the genetic variability of successive mass-selected generations using microsatellite markers, and the results indicated that the selected line F4 exhibited significant lower genetic variation compared to F0 population. In future, we will use the F4 population to cross with other fast-growing breeding populations from other wild populations to retain the fast-growing traits while restoring its population genetic diversity.

Gonad development and recruitment pattern of the oysters Crassostrea ariakensis and Crassostrea sikamea in a subtropical bay in China

The extent of oyster stocks and their habitat loss experienced on the Chinese coasts suggests that oyster restoration will be necessary to rebuild direct fisheries and recover lost ecosystem services. Limited understanding of oyster biology and ecology is among the potential barriers to successful restoration along the Chinese coasts. At a natural oyster bed in Jiantiao Bay, Sanmen County, China, the study compared gonad development and spawn duration of the Sumonie oyster Crassostrea ariakensis and the Kumamoto oyster Crassostrea sikamea through histological sections from May 2020 to March 2021. Their temporal and vertical recruitment patterns were simultaneously observed along an increasing tidal elevation gradient (0.1 m, 0.6 m, 1.1 m, 1.6 m, 2.1 m). The gonad index did not differ between the oysters (P>0.05), with the maximum values in June–August, but C. sikamea had a longer spawning duration (May–December) than C. ariakensis (May–October). Crassostrea ariakensis recruited to only the lower 3 tidal zones (0.1 m, 0.6 m, 1.1 m), and its recruits varied linearly and significantly as an inverse function of tidal elevation. Significant 2nd degree polynomial fits best described the relationships for C. sikamea recruits and tidal elevation. The greatest recruits were commonly observed at the mid intertidal zones (0.6 m, 1.1 m and 1.6 m). An evident vertical settlement pattern emerged for C. sikamea at both 1-day and 3-day experiments; the greatest settlement rates occurred in the 1.6 m tidal zone, intermediate rates occurred in the 1.1 m and 0.6 m, the lowest rates occurred in the 0.1 m and 2.1 m. The study concluded that C. sikamea sustained longer spawning and recruitment durations than C. ariakensis, and a strong zonation pattern in recruitment emerged; C. sikamea achieved its maximum recruits in the mid-tidal zone, while the recruits of C. ariakensis linearly decreased with increasing tidal elevation.

Path analysis of desirable traits and evaluation of reproductive performance of Crassostrea sikamea in different ages

Kumamoto oysters (Crassostrea sikamea) are important marine aquaculture shellfish because of their taste and round shell shape. Due to its small size, selective breeding is required to improve its market size, but there are no reports on traits suitable for direction selection and reproductive productivity of Kumamoto oysters of different ages. In this study, path analysis and one-way ANOVA were used to explore the influential indices of oyster body weight and fecundity. The results showed that shell height was the main factor affecting the body weight of Kumamoto oysters at one-year-old and two-year-old, followed by shell length. Body weight was the main factor influencing fecundity, and shell height showed a significant correlation with fecundity. Therefore, shell height should be considered the first parameter for selection when selecting Kumamoto oysters for body weight as the direct selection target. When one-year-old and two-year-old Kumamoto oysters were selected as parents from Xiangshan Bay, hatching parameters and growth parameters of F1 generation larvae were measured. The fecundity and egg diameters were 1.72 × 106 and 46.60 ± 3.62 μm, respectively, which were significantly greater than one-aged oysters (5.86 × 105, 40.26 ± 3.52 μm). The hatching parameters (fertilized egg size, fertilization rate, and hatching rate) and larval growth performance (D-stage veliger and veliger) were similar and did not differ significantly (P > 0.05), and after metamorphosis, there was also no significant difference in shell height size at 40 days of age (P > 0.05). Therefore, there was no significant effect of age on reproduction, larval development, and the early growth of Kumamoto oysters. These results could provide a basis for parental sources and proper traits for Kumamoto oyster selection, which is beneficial to developing and utilizing this excellent germplasm resource.

Changes in Bacterial Communities of Kumamoto Oyster Larvae During Their Early Development and Following Vibrio Infection Resulting in a Mass Mortality Event.

Vibrio and Ostreid herpesvirus 1 are responsible for mass mortalities of oyster larvae in hatcheries. Relevant works have focused on their relationships with the disease when larval mortality occurs. On the contrary, little is known about how the resident microbiota in oyster larvae responds to Vibrio-infected disease causing mortality as the disease progressed, whereas this knowledge is fundamental to unveil the etiology of the disease. Here, we analyzed the temporal succession of the microbiome of Kumamoto oyster (Crassostrea sikamea) larvae during their early development, accompanied by a Vibrio-caused mortality event that occurred at the post D-stage of larval development in a shellfish hatchery in Ningbo, China, on June 2020. The main causative agent of larval mortality was attributable to Vibrio infection, which was confirmed by linearly increased Vibrio abundance over disease progression. Larval bacterial communities dramatically changed over host development and disease progression, as highlighted by reduced α-diversity and less diverse core taxa when the disease occurred. Null model and phylogenetic-based mean nearest taxon distance analyses showed that the relative importance of deterministic processes governing larval bacterial assembly initially increased over host development, whereas this dominance was depleted over disease progression. Furthermore, we screened the disease-discriminatory taxa with a significant change in their relative abundances, which could be indicative of disease progression. In addition, network analysis revealed that disease occurrence remodeled the co-occurrence patterns and niche characteristics of larval microbiota. Our findings demonstrate that the dysbiosis of resident bacterial communities and the shift of microecological mechanisms in the larval microbiome may contribute to mortality during oyster early development.

Prokaryotic and Eukaryotic Microbial Community in Kumamoto Oyster (Crassostrea sikamea) Larvae: Response to Antibiotics in Trace Concentration

Antibiotic treatment is regarded as an emergency measure to avoid disease occurrence of aquatic animals during metamorphosis in an aquaculture system, which is very common in hatcheries of bivalve mollusc larvae. However, it is still unclear how and to what extent the antibiotic addition affects the prokaryotic and eukaryotic microbial communities of marine bivalve larvae. We profiled the community compositions and dominant taxonomies of prokaryotic and eukaryotic microbiota of Kumamoto oyster (Crassostrea sikamea) larvae exposed to seawater with antibiotics in trace concentration. A total of 500,664 16S rRNA and 501,933 18S rRNA gene fragments were selected for classification, resulting in 714 prokaryotic Operational Taxonomic Units (OTUs) and 47 eukaryotic OTUs. Antibiotic exposure altered the structure of larval microbiome and increased the prokaryotic but decreased the eukaryotic microbial diversity. Larval microbiota was sensitive to antibiotics, as evidenced by alternation of the dominant bacterial phyla Proteobacteria, Bacteroidetes, Firmicutes, Chlamydiae, and Actinobacteria, and eukaryotic phyla Streptophyta, Cercozoa, Chlorophyta and Haptophyta. Similarly, a significant effect was observed at the family and genus level, especially the increased bacterial Devosiaceae, Microbacteriaceae, Halieaceae, Vibrionaceae families, and Devosia, Stappia and Vibrio genera, and eukaryotic Isochrysidaceae and TAGIRI1-linage family and Tisochrysis genus. These results indicate that antibiotic treatment may induce a shift in the larval microbiome, which may cause an unstable community structure and in turn affect the oyster health.

A marine probiotic treatment against the bacterial pathogen Vibrio coralliilyticus to improve the performance of Pacific (Crassostrea gigas) and Kumamoto (C. sikamea) oyster larvae

Oyster larvae reared in hatcheries on the U.S. West coast often experience severe Vibrio coralliilyticus-related mortalities early in their development. Current treatment options for these molluscs are either not available or feasible; however, for decades, probiotics have been successfully used in finfish and crustacean shellfish culture. Consequently, the objectives of this work were to 1) isolate marine bacteria from oysters and evaluate their protective activity against Vibrio coralliilyticus infection of Pacific oyster (Crassostrea gigas) larvae, and 2) to determine the long-term effects of probiotic additions on growth and metamorphosis of larval Pacific and Kumamoto oysters (C. sikamea). A combination of three probiotic strains applied once 24 h post-fertilization was more effective in improving survival of larval C. gigas exposed to lethal concentrations of V. coralliilyticus strain RE22, compared with separate additions of individual probiotics. In addition, a single application of the probiotic combination to one-day-old larvae increased the larval metamorphosis success of C. sikamea and both the Midori and Myiagi stocks of C. gigas. These results suggest that probiotics are effective at preventing bacterial infections and can significantly improve performance of oyster larvae, using a single application early in their development.

Bacterial Community Dynamics in Kumamoto Oyster Crassostrea sikamea Hatchery During Larval Development.

Increasing evidence indicates that microbes colonized in early life stages have a long-term effect on animal wellbeing in later life stages. Related research is still limited in aquatic animals, particularly in bivalve mollusks. In this study, we analyzed the dynamics of the bacterial composition of the pelagic larval stages (fertilized egg, trochophore, D-stage, veliger, and pediveliger) and the sessile postlarval stage (spat) of Kumamoto oyster (Crassostrea sikamea) and their relationships with the rearing water bacterioplankton in a hatchery by using Illumina sequencing of bacterial 16S rRNA gene. Both bacterioplankton and larval bacterial communities changed greatly over larval development, and the two communities remarkably differed (r = 0.956, P < 0.001), as highlighted by the differences in the dominant taxa and bacterial diversity. Ecological processes of larval bacterial communities were measured by abundance-unweighted and abundance-weighted standardized effect sizes of the mean nearest taxon distance (ses.MNTD). The unweighted ses.MNTD analysis revealed that the deterministic process constrained the larval bacterial assembly, whereas the weighted ses.MNTD analysis showed that larval bacterial composition was initially governed by stochasticity and then gradually by determinism in the later stages. SourceTracker analysis revealed that the larval bacteria were primarily derived from an internal source, mainly from larvae at the present stage. Additionally, the abundances of larval bacterial-mediated functional pathways that were involved in the amino acid, energy, lipid and carbohydrate metabolisms significantly altered with the larval development. These findings suggest that bacteria assemble into distinct communities in larvae and rearing water in the hatchery system, and the dynamics of bacterial community composition in larvae is likely associated with larval developmental stages.

Aquaculture potential of two Kumamoto oyster (Crassostrea sikamea) populations and their reciprocal hybrids in southern China

The Kumamoto oyster, Crassostrea sikamea, is a potential aquaculture species with a unique shape and delicious taste. It is naturally distributed along the coasts of East Asia, but it was inadvertently introduced into America from Japan, and has formed several geographic populations. In order to evaluate the aquaculture potential of C. sikamea hybrids, a complete diallel cross between two geographical populations (China and America) was conducted by using mixing gametes at an experimental station in Zhanjiang, Guangdong Province in southern China. Two intra-population mating groups (CC and AA) and two reciprocal inter-population crosses (CA and AC) were produced in triplicates. High fertilization rate and hatching rate were observed in all experimental groups, suggesting that there was no sperm-egg recognition barrier between these two geographic populations. Throughout the life cycle, the survival rate of reciprocal hybrids was higher than that of pure breeds. The heterosis in growth of whole weight of reciprocal hybrids in adulthood was significant, which was mainly influenced by mating strategy. In view of the performance difference between the progenies of the two pure populations, the aquaculture potential of the progenies of the American C. sikamea population was higher than that of the progenies of the Chinese indigenous population. The results demonstrated that the crossbreeding between Chinese and American C. sikamea populations can produce considerable heterosis, which can improve the C. sikamea aquaculture industry in southern China.

Molecular Basis of Taste and Micronutrient Content in Kumamoto Oysters (Crassostrea Sikamea) and Portuguese Oysters (Crassostrea Angulata) From Xiangshan Bay.

Oysters are the most extensively cultivated bivalves globally. Kumamoto oysters, which are sympatric with Portuguese oysters in Xiangshan bay, China, are regarded as particularly tasty. However, the molecular basis of their characteristic taste has not been identified yet. In the present study, the taste and micronutrient content of the two oyster species were compared. Portuguese oysters were larger and had a greater proportion of proteins (48.2 ± 1.6%), but Kumamoto oysters contained significantly more glycogen (21.8 ± 2.1%; p < 0.05). Moisture and lipid content did not differ significantly between the two species (p > 0.05). Kumamoto oysters contained more Ca, Cu, and Zn (p < 0.05); whereas Mg and Fe levels were comparable (p > 0.05). Similarly, there was no significant difference between the two species with respect to total amount of free amino acids, umami and bitterness amino acids, succinic acid (SA), and most flavoring nucleotides (p > 0.05). In contrast, sweetness amino acids were significantly more abundant in Portuguese oysters. Volatile organic compounds profiles of the two species revealed a higher proportion of most aldehydes including (2E,4E)-hepta-2,4-dienal in Kumamoto oysters. Overall, Kumamoto oysters contain abundant glycogen, Ca, Zn, and Cu, as well as volatile organic compounds, especially aldehydes, which may contribute to their special taste. However, free amino acid and flavor nucleotides may not the source of special taste of Kumamoto oyster. These results provide the molecular basis for understanding the characteristic taste of Kumamoto oysters and for utilizing local oyster germplasm resources.

Intertidal zonation and recruitment of oysters in the upstream of Jiantiao Bay, Sanmen County, Zhejiang Province

This study aims to explore the intertidal zonation pattern of oysters and examine the effect of breeding site and spat collector on oyster recruitment in the upstream of Jiantiao Bay, which is essential to understand the biology, ecology and resource conservation of oysters. The quantitative field observations identified Kumamoto oyster <italic>Crassostrea sikamea</italic>, Sumonie oyster <italic>C</italic>. <italic>ariakensis</italic> and Hongkong oyster <italic>C</italic>. <italic>hongkongensis</italic> by using the morphological and molecular methods. <italic>C</italic>. <italic>hongkongensis</italic> was first recorded on the coast of Zhejiang Province. <italic>Crasssostrea sikamea</italic> was present at all five tidal zones (1.4 m, 2.2 m, 3.0 m, 3.8m, 4.6 m above the standard level in Jiantao Bay hydraulic station), and its densities gradually declined along a decreasing tidal elevation gradient. The densities of <italic>C</italic>. <italic>sikamea</italic> were the highest at the 4.6 m tidal zone and were significantly greater than those at the 1.4 m and 2.2 m tidal zones (<italic>P</italic><0.05). <italic>Crassostrea ariakensis</italic> and <italic>C</italic>. <italic>hongkongensis</italic> were only present at the deepest two tidal zones (1.4 m and 2.2 m). Results from the oyster recruitment experiments indicated that both recruitment site and spat collector significantly affected the densities and shell height of oyster spat. There were high oyster recruitment in the Jiantiao Bay. The study conlcuded that concrete tiles were better spat collector than plastic ring. In general, the upstream site (S1) should be recommended as the suitable breeding ground for <italic>C</italic>. <italic>sikamea</italic>.

3种无脊椎动物对近江牡蛎Crassostrea ariakensis和熊本牡蛎C. sikamea的捕食研究

3种无脊椎动物对近江牡蛎<i>Crassostrea ariakensis</i>和熊本牡蛎<i>C. sikamea</i>的捕食研究

Seasonal and Temporal Changes in the Kumamoto Oyster Crassostrea sikamea Population and Associated Benthic Macrofaunal Communities at an Intertidal Oyster Reef in China

Oyster reefs (OR) once dominated coastal estuaries but are now one of the most severely impacted marine habitats on Earth. There is little information available regarding the status of natural OR in China. This study examined the seasonal and temporal changes in the Kumamoto oyster Crassostrea sikamea population and associated benthic macrofaunal communities at the largest intertidal OR (Liyashan) of the China Sea in four field surveys (spring and fall in 2013 and 2018). The oyster densities exhibited no evident seasonal variations in either 2013 or 2018 but showed significant temporal changes and declined by 89.66% in spring and 96.27% in fall between 2013 and 2018. The total densities of benthic macrofaunal communities at the reef area of the OR were significantly higher than those at the degraded oyster habitat (DOH) and dramatically decreased by 80.73% in spring and 64.87% in fall between 2013 and 2018. Nonmetric multidimensional scaling ordination indicated the benthic macrofaunal communities differed significantly between the OR and DOH or among the four sampling periods. Analysis of similarity indicated that the benthic communities at the OR had greater seasonal and temporal changes than did those at the DOH, and that temporal changes in benthic communities were greater than seasonal changes at each habitat. The study concluded that the OR had decreased habitat values as the oyster populations and the benthic macrofaunal communities dramatically declined from 2013 to 2018.

Probiotic Bacterium and Microalga Interaction on Rearing Kumamoto Oyster Crassostrea sikamea Spat.

This study assessed in vitro interaction between Bacillus bacteria and microalgae and their posterior in vivo effect on rearing Kumamoto oyster Crassostrea sikamea. The probiotic strains Bacillus licheniformis (MAt32), B. subtilis (MAt43) and B. subtilis (GAtB1) were individually inoculated in triplicate into 250mL flasks containing 1 × 104 colony forming units (CFU) mL-1 of bacteria and 4.5 × 104 cell mL-1 of microalgae (Isochrysis galbana or Chaetoceros calcitrans) to evaluate their growth during a 7-day culture. Single cultures of microalgae or bacilli served as control. Additionally, C. sikamea spat was treated for 28days with four single/combined bacillus treatments in triplicate at a concentration of 1 × 106CFUmL-1 as follows: (a) control, without treatments; (b) combination of two antibiotics (10mg L-1); (c) B. licheniformis; (d) B. subtilis; (e) B. subtilis subtilis and (f) mixed bacilli. The results showed a significantly (P < 0.05) increased growth of Bacillus strains co-cultured with microalgae, while the growth of I. galbana co-cultured with bacteria was not reduced significantly (P > 0.05) compared with the control group. C. sikamea spat treated with Bacillus showed significantly (P < 0.05) higher growth and survival than the control group. In this study, C. calcitrans microalgae were susceptible to the presence of probiotic bacteria. Nonetheless, this reduction in microalgal growth observed in vitro increased growth and survival of C. sikamea spat exposed to probiotic bacteria when compared to spat without probiotics.

Unraveling concordant and varying responses of oyster species to Ostreid Herpesvirus 1 variants

The Ostreid herpesvirus 1 (OsHV-1) and variants, particularly the microvariants (μVars), are virulent and economically devastating viruses impacting oysters. Since 2008 OsHV-1 μVars have emerged rapidly having particularly damaging effects on aquaculture industries in Europe, Australia and New Zealand. We conducted field trials in Tomales Bay (TB), California where a non-μVar strain of OsHV-1 is established and demonstrated differential mortality of naturally exposed seed of three stocks of Pacific oyster, Crassostrea gigas, and one stock of Kumamoto oyster, C. sikamea. Oysters exposed in the field experienced differential mortality that ranged from 64 to 99% in Pacific oysters (Tasmania>Midori = Willapa stocks), which was much higher than that of Kumamoto oysters (25%). Injection trials were done using French (FRA) and Australian (AUS) μVars with the same oyster stocks as planted in the field and, in addition, two stocks of the Eastern oyster, C. virginica. No mortality was observed in control oysters. One C. virginica stock suffered ~10% mortality when challenged with both μVars tested. Two Pacific oyster stocks suffered 75 to 90% mortality, while one C. gigas stock had relatively low mortality when challenged with the AUS μVar (~22%) and higher mortality when challenged with the French μVar (~72%). Conversely, C. sikamea suffered lower mortality when challenged with the French μVar (~22%) and higher mortality with the AUS μVar (~44%). All dead oysters had higher viral loads (~1000×) as measured by quantitative PCR relative to those that survived. However, some survivors had high levels of virus, including those from species with lower mortality. Field mortality in TB correlated with laboratory mortality of the FRA μVar (69% correlation) but not with that of the AUS μVar, which also lacked correlation with the FRA μVar. The variation in response to OsHV-1 variant challenges by oyster species and stocks demonstrates the need for empirical assessment of multiple OsHV-1 variants.

Intertidal Zonation of the Suminoe Oyster Crassostrea ariakensis and the Kumamoto Oyster Crassostrea sikamea on the Coast of the Northern East China Sea

The Suminoe oyster Crassostrea ariakensis and the Kumamoto oyster Crassostrea sikamea coexist and are habitat-forming ecosystem engineers in the Yangtze River estuary and its adjacent coasts. To date, little quantitative information is available on the intertidal zonation patterns of these two oysters within their native range or on the related mechanisms generating these spatial patterns. This study first carried out field surveys and laboratory experiments to explore the vertical zonation patterns of the two oysters in the intertidal habitats of northern Hangzhou Bay, assess the roles of abiotic (aerial exposure stress) and biotic (larval distribution, recruitment, and competition) factors in regulating vertical zonation and understand their driving mechanisms. At the study site, C. sikamea dominated the higher intertidal zone (approximately 6.7 h and 56% emersion/12 h tidal cycle), whereas C. ariakensis occupied the three deeper intertidal zones (mid, mid-low, and low intertidal). Mean aerial exposures of 56% and 42% represented the optimal growth zones and the lower limits of C. sikamea, respectively. The growth boundary of the two oysters was located in the intertidal zone that was exposed 42% of the time. The area with the highest C. ariakensis densities (the optimal growth zone) was the mid-low intertidal (mean: 3.0 h and 25% emersion/tidal cycle). In a laboratory 72-h aerial exposure experiment, the percentage of water loss was significantly greater for C. ariakensis than for C. sikamea after 24 h (P 0.05). Twenty-two individuals among the 30 oyster recruits (73%) in the high intertidal region were identified as C. sikamea, and six recruits (20%) were identified as C. ariakensis. All 36 recruits (100%) in the low intertidal region were identified as C. ariakensis. It was concluded that the differential larval distribution, settlement, and post-settlement mortality along the intertidal elevation gradients served as a mechanism for vertical zonation of these two oyster species.

Effect of Seawater Acidity on the Initial Development of Kumamoto Oyster Larvae Crassostrea sikamea (Amemiya, 1928)

The oceans have absorbed more than 40% of the carbon dioxide (CO2) generated by anthropogenic activities, causing a decrease in the average pH of 0.1 units in seawater since preindustrial times. This phenomenon has been called “ocean acidification.” This change poses serious threats to the cultivation of oysters and especially to larval and spat production, activities carried out in coastal and estuarine areas, where pH levels are currently below the IPCC scenario for the year 2100 of pH = 7.8. The goal of the present work was to experimentally evaluate the effect of simulated acidification (pH 7.39 ± 0.04) on the culture in a short-term trial of Kumamoto oyster larvae Crassostrea sikamea taking the current ocean pH conditions (pH 8.116 ± 0.023) as a reference. The evaluation was carried out in an experimental system with continuous water flow and pH manipulation by CO2 bubbling. Veliger larvae (6-day-old postspawn) were cultured at a density of six larvae mL–1 and fed with a monoalgal diet based on Isochrysis galbana at 30,000 cells mL–1. Mortality (%) and growth (shell length in µm) were evaluated, and damage to larval morphology (determined using scanning electron microscopy) and Ca2+ contents in the shells (%) were quantified by X-ray fluorescence. The results show a high sensitivity of C. sikamea veliger larvae to low pH levels with negative impacts on growth and survival, decreases in the Ca2+ concentrations of the shells, and the presence of morphological anomalies during the prodissoconch I stage, which were observed after the first 24 h of cultivation in experimental conditions and became progressively more evident, especially by the sixth day of culture. Acute exposure to a low pH and a saturation of aragonite (Ωar) <1 caused poor calcification in C. sikamea larvae, causing negative effects on larvae, such as shell lesions during development, smaller larvae, and higher mortality and relation to a control pH.

Gametogenesis From the Early History Life Stages of the Kumamoto Oyster Crassostrea sikamea and Their Breeding Potential Evaluation.

The Kumamoto oyster, Crassostrea sikamea, is native to Southeast Asia, including China, Japan and Korea, and is an important traditional wild fishery resource. Although this oyster’s early gametogenesis was reported in Mexico, no related research was found on the breeding potential using early forming gametes. We re-examined the gametogenesis of C. sikamea during early life history in southern China and further divided it into three phases: sex differentiation (1 month old, shell height 2–3 mm), physiological maturity (2 months old, shell height 3–5 mm) and functional maturity (3 months old, shell height 9–12 mm). The breeding potential was evaluated using four sets of gametes from parent oysters of different ages (2, 3, 6, and 15 months old). The physiologically mature gametes were not suitable for artificial hatchery due to the low production of eggs, and yielding a high deformity rate of D larvae (95.47 ± 1.25%) and heavy larval morality (90.23 ± 1.84%) post-fertilization. However, progeny from functionally mature gametes grew significantly faster than those of other age groups, with no significant differences in fertilization, hatching level or survival of progeny among them. This study clearly demonstrates that the first batch of functionally mature gametes can develop normally and produce viable progeny, suggesting that artificial hatchery of C. sikamea is completely feasible using parent oysters from 3 months old and onward. Furthermore, this hatchery method can effectively shorten the breeding cycle and accelerate the breeding process.

First report of black-heart disease in Kumamoto oyster Crassostrea sikamea spat caused by Polydora lingshuiensis in China.

A black-heart disease caused by polydorid infestation is reported for the first time in Kumamoto oyster Crassostrea sikamea Amemiya, 1928 spat in a pond at Beihai city, Guangxi province, China, with a prevalence of 100% and a cumulative mortality rate of 50% within 2 mo. In heavily infected oyster spat, blisters extended toward the center of the inner shell surface, around the adductor muscle scar area to form a large black area occupying approximately 50% of the area of the inner shell surface. Morphological analysis identified the pathogen as Polydora lingshuiensis Ye et al., 2015, which was reconfirmed by comparison of its corresponding 18S rRNA and mitochondrial CO1 gene sequences with those in the GenBank database. The mean abundance of mud blisters was significantly higher in live spat than in dead spat, suggesting that P. lingshuiensis preferentially infests live oyster spat. Additionally, P. lingshuiensis larvae were detected in the inlet near the dam, which suggests that the source of P. lingshuiensis larvae infecting the spat may be larvae entering the ponds through the water current from the sea.

Comparative studies of the growth, survival, and reproduction of diploid and triploid Kumamoto oyster, Crassostrea sikamea

Triploid oysters have been used for farming to improve growth but have not been created in the Kumamoto oyster, Crassostrea sikamea, which is one of the crucial aquaculture species on the southern coast of China. In the present study, triploids were created using cytochalasin B to inhibit polar body II release in C. sikamea, with the untreated oysters as controls. Triploidy rates of 87 and 57.67%, on average, were obtained in larvae and adults, respectively. Larval growth and survival of the triploid were significantly lower than that of the controls (p &lt; 0.05). In contrast, the triploid postlarvae and adults had a significant growth advantage over the controls (p &lt; 0.05) during the period of 180 (December) to 450 days (September of the next year). Moreover, the triploids clearly exhibited significant sterility in the reproductive season. The glycogen and triglyceride contents in the gonad, adductor muscle, mantle, and gill were higher in triploids than in controls from 180 to 450 days. As a result, high physiological energy supply was strongly correlated with superior growth and reduced reproduction in triploid C. sikamea. The triploid C. sikamea is an excellent oyster species and can be used to improve growth for C. sikamea farming.