Phenotypic trait of Crassostrea hongkongensis♀× C. angulata♂ hybrids in southern China

The Hong Kong oyster (Crassostrea hongkongensis) and the Portuguese oyster (Crassostrea angulata) are important aquaculture species in southern China. To understand the potential feasibility of hybridization between the two species, two‐by‐two factorial cross‐experiments were conducted in Shenzhen (Guangdong province). An asymmetry in fertilization was revealed: C. hongkongensis eggs could be fertilized by C. angulata sperm, but the reciprocal cross was not possible. The fertilization and hatching success in the C. hongkongensis females × C. angulata males (HA) cross was lower than the two intraspecific crosses. Moreover, hybrid larvae had a slower growth rate than those from parental progenies due to genome incompatibility. However, the size of the spat and adult growth were greater than those of the two parental progeny, with obvious heterosis during the grow‐out stage. The survival advantage of hybrids was examined at all times. The surprising finding was that all hybrids were completely fertile, meaning they could produce normal and functional gametes. Genetic analysis by molecular markers was applied to confirm that the HA spat were true hybrids. Our results revealed that the interspecific hybridization between C. hongkongensis and C. angulata is completely successful in one direction. Furthermore, hybrids are viable, fast growing and completely fertile, which provides a promising method for the genetic improvement of oysters as a new aquaculture stock in southern China.     

Growth performance and biochemical composition of the oysters Crassostrea sikamea,Crassostrea angulata and their hybrids in southern China

The Kumamoto oyster (Crassostrea sikamea) and the Portuguese oyster (C. angulata) are important aquaculture species which naturally coexist along the southern coast of China. To understand the potential feasibility of hybridization between the two species, we conducted two‐by‐two factorial cross‐experiments in Beihai (Guangxi province), and also compared the survival and growth of the hybrids to that of the two parental progenies during the grow‐out period from July 2014 to July 2015. Genetic analysis confirmed that the hybrid spats were true hybrids. Additionally, the biochemical composition of the 1‐year‐old oyster progenies was determined. In July 2015, the mean shell height of the hybrids was 42.98 ± 6.29 mm, which was higher than that of the Kumamoto oyster progeny. The cumulative survival rate of the hybrids was 26.37 ± 1.32%, which was higher than that of the progeny of the Portuguese oyster. Mean lipid content of the hybrids was 13.65 ± 1.63% of dry weight, which showed obvious heterosis compared to those of the two parental progenies. Observation of gonads revealed that all hybrids were completely fertile. Furthermore, relative expression of the lipid homeostasis genes, SREBP (sterol regulatory element‐binding proteins), PPARα (peroxisome proliferator‐activated receptor) and INSIG (insulin‐induced gene) were found to vary between parental progenies and the hybrids, thus providing a possible reason for difference in the lipid contents of these experimental groups. Overall, the hybrids were viable, rich in lipid and completely fertile and thus could serve as a promising aquaculture stock for oyster breeding in southern China.     

Effects of controlled air exposure on the survival,growth, condition,pathogen loads and refrigerated shelf life of eastern oysters

The benefits of exposing eastern oysters to air during commercial culture have not been well‐characterized. An adjustable longline system (ALS) designed in Australia and recently adopted by the nascent aquaculture industry in the northern Gulf of Mexico, allows growing oysters at any position in the water column and is perfectly suited to study the benefits of air exposure. Four‐month old diploid oysters were deployed in an ALS and divided into three groups: 1) oysters exposed to air daily for 8–12 hr during low tide, 2) oysters exposed to air weekly (~24 hr once a week), and 3) oysters kept subtidally. Oyster mortality and growth rates, Perkinsus marinus load and condition index were then determined every 3 months over 2 years, while refrigerated shelf life and Vibrio vulnificus load were determined in summer and early fall of the second year. Summer mortalities were delayed, P. marinus infection intensities tended to be lower and condition index was significantly higher in oysters exposed to air daily compared with oysters exposed to air weekly or held subtidally. Shell heights of oysters exposed to air daily were lower for most of the study due to a lower growth rate during the initial sampling interval following deployment. No consistent differences were found in V. vulnificus loads or refrigerated shelf lives between the groups. It is recommended that ALS be set so that oysters are kept subtidally fall through spring to promote growth, and exposed to air daily during summer to delay P. marinus proliferation.     

Rearing equipment on the quality aspects and growth performance of Crassostrea gigas (Thunberg, 1793): Implications for marketability

Shell characteristics and consequent marketability of the oyster Crassostrea gigas are influenced by several factors, and most important of which are the rearing methods and gears. In this study, we investigated the effects of two different gears on the quality aspect (shell shape, weight meat percentage, backward shell shape, epibionts and blisters) and growth rate (weight and length) of C. gigas by adopting the innovative Ortac Oyster Farming System, during the field grow‐out phase of production. Aspect and growth of oysters farmed in Ortac and floating bags were compared. After 14 weeks, the aspect of the animals was influenced by the gear, resulting in a lower quality in the Ortac (i.e. worse shell shape, lower weight meat percentage, higher percentage of backwards and blisters) than in the floating bag, while the incidence of epibionts was less in the Ortac. Regarding the growth performance, total wet weight gained in the Ortac was lower (~28.3 g) than in the floating bag (~32.4 g; p < .05). These results highlight the efficiency of the floating bag as oyster rearing system in the Mediterranean basin and suggest the need to alter the Ortac gear to these environmental conditions.     

Genetic parameters of growth and survival in the Pacific oyster Crassostrea gigas

The Pacific oyster Crassostrea gigas is a representative bivalve mollusc that is widely cultured in the world and is the largest molluscan group cultured in China. In order to assess the feasibility of improving survival of C. gigas through genetic selection, the heritability and genetic correlations for growth and survival traits between different life stages were examined. Genetic parameters were estimated based on intraclass correlations of 49 full‐sib families (29 half‐sib families) in larvae (4 and 20 days after fertilization) and spat (140 days after fertilization) stages. The heritability for growth traits in larvae and spat was 0.30–0.86 and 0.53–0.59, respectively, and varied with ages. The heritability of survival was low in larvae (0.13 ± 0.05 and 0.17 ± 0.04, respectively for 4 and 20 days after fertilization) but medium (0.39 ± 0.07) in spat, suggesting that selection for increasing spat survival was feasible. The genetic correlation between growth traits within age was medium to high and positive (ranging from 0.47 to 0.96, respectively, between shell length (SL) and shell height (SH) at 20 days and between SL and SH at 140 days after fertilization), suggesting that selection to improve single growth trait will cause positive response in another growth traits in C. gigas. The genetic correlations between survival and growth traits at 140 days were low but positive (ranging from 0.23 to 0.27, respectively, between survival and SH and between survival and SL at 140 days after fertilization), suggesting that selection for survival may not have a negative response in growth. Overall, this study suggests that survival traits should be taken as improving target of next selection breeding programme in C. gigas.     

Multiplex species-specific PCR identification of native and non-native oysters (Crassostrea) in Brazil: a useful tool for application in oyster culture and stock management

In an effort to develop molecular tools for oyster identification, this study reveals the usefulness of a multiplex polymerase chain reaction (PCR) for the reliable, rapid and low-cost identification of the four oyster species found along the Brazilian coast: Crassostrea gasar, Crassostrea rhizophorae, Crassostrea gigas and Crassostrea sp. Canela originally found at Pará, Brazil. In order to perform a simultaneous identification of these species, we used a set of five primers developed and adapted from the cytochrome oxidase subunit I (COI) and internal transcribed spacer 1 (ITS1) fragments, respectively. Amplification was successful in all four species and PCR products were visualized in agarose gel. A single reaction was capable of distinguishing between the species: C. gigas, with two fragments (236 bp for COI and 718 bp for ITS1); C. gasar, with one fragment (718 bp for ITS1); Crassostrea sp., with one fragment (621 bp for ITS1) and C. rhizophorae with two fragments (377 bp for COI and 718 bp for ITS1). This molecular approach provides a simple and rapid identification of the oyster species from the Brazilian coast, thus increasing the efficient and quality of oyster culture programs by reducing the risk of wrong species identification.     

Insights on the association between somatic aneuploidy and ostreid herpesvirus 1 detection in the oysters Crassostrea gigas,C. angulata and their F1 hybrids

Cytogenetic abnormalities associated with viral infections, including from viruses of the Herpesvirales order, have been reported in vertebrate species. Ostreid herpesvirus 1 (OsHV‐1) has been detected worldwide during mortality outbreaks of the Pacific oyster Crassostrea gigas. On the other hand, a high proportion of aneuploid cells in somatic tissues have been observed in C. gigas. In this study, we analysed the putative association between aneuploidy levels and the detection of OsHV‐1 in gills of C. gigas, the Portuguese oyster C. angulata and their F1 hybrids cultured in Ria Formosa (Portugal). OsHV‐1 was detected by PCR in 5.4% of the total of oysters analysed (n = 111) namely in 11.1%, 8.0% and 1.7% of C. gigas, C. angulata and F1 hybrid respectively. Sequencing analysis of a viral fragment amplified with the C2/C6 primer pair revealed a high similarity with the OsHV‐1 reference type. Moreover, in situ hybridization confirmed the presence of OsHV‐1 in gill tissue. Oysters where OsHV‐1 was detected had a significantly higher mean percentage of aneuploid cells (25%) than the ones where the virus was not detected (18%). However, the overall low percentage of positive samples contrasted with the high mean percentage of aneuploidy observed, with 50% of the oysters analysed showing a percentage of aneuploid cells between 20% and 30%. We hypothesize that somatic aneuploidy may adversely affect oysters making them more prone to OsHV‐1 infection, but the virus is unlikely to be the cause of somatic aneuploidy.     

Different responses between orange variant and cultured population of the Pacific oyster Crassostrea gigas at early life stage to temperature‐salinity combinations

Although breeding of rare shell colour variants has drawn widespread attention from shellfish breeders, the potential disadvantages of their adaptive capacity have been ignored in practice. To explore the difference in adaptive capacity between orange shell variant (OSO) and commercially cultured population (CPO) of the Pacific oyster Crassostrea gigas at early life stage, the development to D‐larvae and larval survival and growth (just 23 and 30°C for larval experiment) of them were compared under different temperature (16, 23 and 30°C) and salinity (17, 25 and 33 psu) combinations. In this study, at 23°C and 25 psu, for both OSO and CPO there was no difference in fertilization rates and survival (p > .05) (mean percentages of D‐larvae after fertilized 40 hr ≥ 95.00%; mean larval survival rates on day 10 > 80.00%). However, the percentage of D‐larvae of CPO at 40 hr was significantly (p < .05) higher than OSO at temperatures of 16 and 30°C and 25–33 psu and 17 psu at 23°C. Similarly, CPO has a better larval survival on day 10 and growth than OSO at salinities of 17 and 33 psu at 23°C. Overall, our results indicate that OSO can have an equally good performance like CPO at early life stage under optimal condition (23°C; 25 psu), but the potential disadvantages in adaptive capacity will be shown at suboptimal conditions. These findings can guide future hatchery breeding of OSO, and suggest the potential disadvantages in adaptive capacity in rare colour variants need more attention in further breeding.     

Perkinsus mediterraneus infection induces oxidative stress in the mollusc Mimachlamys varia

Perkinsus mediterraneus is a protozoan parasite that can cause marine mollusc diseases known as perkinsosis being a serious threat for clam cultures worldwide. The aim of the present study was first to determine the Perkinsus species infecting the variegated scallop Mimachlamys varia and then to evaluate the existence of oxidative stress in gills of M. varia according to different degrees of infection. DNA sequencing confirmed that P. mediterraneus was the species infecting M. varia. ROS production was progressively increasing with the degree of infection although the differences were only significant in the high-infected group. Low degree of infection significantly increased superoxide dismutase (SOD) and glutathione S-transferase (GST) activities and nitrite levels with respect to the control group. In the high-infected group, a significant increase was evidenced in all analysed enzymes, catalase, SOD, glutathione reductase and GST. Non-significant differences in MDA levels were observed between the control and low-infected groups; however, a significant increase in MDA levels was observed in the high-infected group. In conclusion, the infection by Perkinsus mediterraneus in M. varia induces oxidative stress and an antioxidant response directly related to the infection degree that can contribute to the pathogenicity of the infection.     

长牡蛎第三代选育群体生长性状的选择效应

为了培育长牡蛎的高产抗逆新品种,实验采用群体选育方法构建了中国、日本和韩国3个种群的快速生长选育系,2007年至今已连续进行了6代选育。本实验对长 牡蛎选育F₃代壳高和活体体质量的增长、选择反应和遗传获得等遗传参数进行了分析。结果表明,从第120日龄开始,中国、日本和韩国3个选育群体的壳高和 活体体质量均显著高于对照组,在420日龄时,平均壳高较对照组分别高13.4%、10.1%和10.5%,平均活体体质量较对照组分别重18.5%、13.4%和11.6%;壳高的平均现实遗传力分别为0.447±0.226、0.471±0.297和0.367±0.167,表明适于对壳高的生长速度进行进一步的选育。长牡蛎中国、日本和韩国选育F3代活体 体质量的遗传获得平均值分别为16.01%±3.82%、15.03%±5.21%和11.57%±5.15%,表明对长牡蛎壳高的生长速度进行选育时,活体体质量的生长速度也得到了明显提高。本研究结果可以为长牡蛎快速生长品系的连续选育提供依据。     

长牡蛎生长性状遗传力、遗传相关和表型相关分析

为了查清乳山(RS)和崆峒岛(KTD)海域长牡蛎生长性状各指标的遗传力、遗传相关和表型相关,本实验通过部分因子交配设计和人工授精的方法建立家系,并在乳山和崆峒岛海域进行养殖。用混合线性模型分别计算RS和KTD海域188日龄、338日龄和474日龄长牡蛎生长性状各指标的遗传参数。结果发现:不同日龄长牡蛎生长性状各指标,在乳山海域的遗传力是0.28~0.55,为中高遗传力,在崆峒岛海域的遗传力是0.34~0.63,为高遗传力;与乳山海域相比,崆峒岛海域各指标的遗传力偏高。运用亲本模型,将日龄和地点作为固定效应,计算得到壳高、壳长、壳宽和湿重的遗传力分别为0.25±0.08、0.29±0.09、0.14±0.05和0.26±0.09。不同海域、不同日龄的遗传相关和表型相关结果各不相同,但各指标间均呈现正相关;总体来讲,性状间的遗传相关大于表型相关。实验结果有助于掌握山东半岛南北两侧长牡蛎生长性状的数量遗传学参数,为制定长牡蛎在该海域的选育技术路线提供数据支持。     

中国牡蛎帕金虫的检测与鉴定

对中国沿海23个地点的太平洋牡蛎（Crassostrea gigas）、近江牡蛎（Crassostrea ariakensis）、褶牡蛎（Ostrea plicatula）进行样品采集。利用液体巯基醋酸盐培养基培养法、组织切片法、聚合酶链反应法,对样品进行帕金虫含量检测。结果表明在三种牡蛎体内春、夏、冬季都有帕金虫寄生,夏季帕金虫感染率最高可达100%,其他季节感染率较低。利用分子生物学方法鉴定帕金虫种类,结果表明,在中国沿海的三种牡蛎体内主要寄生两种帕金虫：Perkinsus.sp.n（EU068107）与Perkinsus olseni,这两种帕金虫在中国沿海分布具有一定地域性,Perkinsus.sp.n（EU068107）主要分布在中国南海的海南省、广西壮族自治区、广东省沿岸,而Perkinsus olseni分布在中国东海的浙江省沿岸。     

Resistance of Dermo in eastern oysters, Crassostrea virginica (Gmelin), of North Carolina but not Chesapeake Bay Heritage

Growth, intensity of Perkinsus marinus (Levine) infection, and survival of synchronously spawned North Carolina (NC) and Chesapeake Bay‐heritage (CB) oysters, Crassostrea virginica (Gmelin) were evaluated under standard tray culture conditions at several sites in both regions (Wye River, Maryland; Mobjack Bay, Virginia; Pamlico River, NC and Bogue Banks, NC). Infection prevalence reached 100% in oysters held at all high‐ and moderate‐salinity sites, at which time the CB strain ceased to grow. Shortly after growth ceased, CB oysters exhibited mortality that rapidly progressed to 100%. Unlike the CB strain, growth continued in the NC strain despite high P. marinus prevalence. When mortality did occur in the NC strain, at a reduced rate of 37–40%, it was associated with higher intensity of P. marinus than the infection intensity correlated with death of CB oysters. At the low‐salinity site in NC, P. marinus infection persisted at low weighted prevalence throughout the latter portion of the culture period but was not associated with mortality of either strain. These trends in growth and disease resistance for the two strains demonstrate that aquaculture performance is related to the level of disease resistance in oyster strains, salinity of water in growing areas and virulence of P. marinus.     

Ostreid herpesvirus OsHV‐1 μVar in Pacific oysters Crassostrea gigas (Thunberg 1793) of the Wadden Sea,a UNESCO world heritage site

The Wadden Sea is an extensive wetland area, recognized as UNESCO world heritage site of international importance. Since the mid‐1990s, the invasive Pacific oyster Crassostrea gigas (Thunberg 1793) population in the area has grown exponentially, having a distinct impact on the ecosystem. The recent spread of the emerging oyster pathogen Ostreid herpesvirus OsHV‐1 μVar worldwide and specifically in the oyster culture areas in the south of the Netherlands raised the question whether the virus may also be present in the Wadden Sea. In the summer of 2012 juvenile Pacific oysters were collected from five locations in the Dutch Wadden Sea. The virus was shown to be present in three of the five locations by real‐time PCR and sequencing. It was concluded that OsHV‐1 μVar has settled itself in Pacific oyster reefs in the Wadden Sea. These results and the recent discoveries of OsHV‐1 microvariants in Australia and Korea indicate that OsHV‐1 μVar and related variants might be more widespread than can be deduced from current literature. In particular in regions with no commercial oyster culture, similar to the Wadden Sea, the virus may go undetected as wild beds with mixed age classes hamper the detection of mortality among juvenile oysters.     

Effect of ploidy on the mortality of Crassostrea gigas spat caused by OsHV‐1 in France using unselected and selected OsHV‐1 resistant oysters

The effect of ploidy on the mortality of Crassostrea gigas spat caused by the ostreid herpesvirus (OsHV‐1) genotype μVar was investigated at five sites along the Atlantic coast in France in 2011. Sibling diploids and triploids were produced using either unselected or selected OsHV‐1‐resistant oysters. No significant interactions were found between the factors of environment, genotype and ploidy at the endpoint dates. The mean mortality rates at the sites were 62% and 59% for diploids and triploids, respectively, and the two rates were not significantly different. The mean mortality rates were 33% and 32% for sibling diploids and triploids, respectively, when OsHV‐1‐resistant parents were used and 91% and 85%, respectively, when unselected parents were used. The results were confirmed through other broodstocks tested in 2013. Our study is the first to clearly show that mortality related to OsHV‐1 is similar between diploids and triploids in C. gigas when the same germplasm is used for both ploidy. Furthermore, OsHV‐1 resistance was not substantially altered by triploidization, indicating that the achieved selective breeding of diploid oysters for OsHV‐1 resistance can be translated into improved survival in triploids.     

Stress Gene Expression in Crassostrea gigas (Thunberg, 1793) in response to experimental exposure to the toxic dinoflagellate Prorocentrum lima (Ehrenberg) Dodge, 1975

Harmful algal blooms (HABs) are natural phenomena with different effects on the aquatic environment that affect both human economy and health. Several genomic studies have been done to characterize the effects of contaminants on Crassostrea gigas. However, oysters’ molecular response to HABs exposure needs to be studied more. In this study, we challenged C. gigas with Prorocentrum lima, a diarrhoetic toxin producer, under controlled experimental conditions considering dinoflagellate density and exposure time (acute and sub‐chronic) as variables. The expression profile of six stress response genes was analysed by semiquantitative RT‐PCR: Glutamine synthetase (GS), Glutathione S‐transferase (GST), heat shock protein 70 (HSP₇₀), and heat shock protein 90 (HSP₉₀), CuZn superoxide dismutase (SOD) and melanogenic peroxidase (POX). The results revealed that these gene expressions depend on exposure time and cell concentration. In general, an increased expression was observed in all tested cell densities as immediate response to exposure (0–3 h); whereas lower expression levels than control were observed after 6 h. Treatment with 3,000 cells mL^‐1 promoted higher expression than control on most genes after 336 h of exposure. This is the first report providing information at molecular level on C. gigas response to dinoflagellate blooms.     

Enhancing lipid nutritional quality of oysters by hybridization between Crassostrea gigas and C. angulata

In the present study, hybrid and inbred oysters were produced by factorial hybridization between two closely related species, the Pacific oyster Crassostrea gigas and the Portuguese oyster C. angulata. Tissues samples were used for the analysis of total lipid content (TLC) and fatty acid composition (FAC). In general, FAC but not TLC in hybrid oysters was significantly different from inbred oysters. Hybrid oysters have significantly lower ∑SFA composition but higher ∑PUFA composition than inbred oysters, with heterosis for ∑SFA, ∑MUFA and ∑PUFA of ?6.03%, ?6.53% and 14.31% respectively. Moreover, hybrid oysters have significantly higher EPA + DHA content and n3/n6 PUFA ratio than inbred oysters, indicating higher lipid nutritional quality than inbred oysters. Among them, the hybrid ♀ C. gigas × ♂ C. angulata appears to be the most nutritious. These results indicated that the lipid nutritional quality of oysters can be improved by hybridization.     

广东近岸海域贝类寄生派琴虫流行病学调查研究

贝类一般在开放式海域养殖,这使得贝类病害防控工作很难开展,因此调查病原流行感染规律对贝类病原的发生、发展和防治尤为重要.文章采用常规PCR技术,调查了广东近岸6个养殖海域5种重要养殖贝类寄生派琴虫的感染情况,分析了贝类寄生派琴虫感染的时空变化情况.结果显示,共检测到2种派琴虫,其中北海派琴虫(Perkinus beih...