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.     

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.     

Two epizootic Perkinsus spp. events in commercial oyster farms at Santa Catarina,Brazil

Perkinsus spp. have been detected in various bivalve species from north‐east Brazil. Santa Catarina is a South Brasil state with the highest national oyster production. Considering the pathogenicity of some Perkinsus spp., a study was carried out to survey perkinsosis in two oyster species cultured in this State, the mangrove oyster Crassostrea gasar and the Pacific oyster Crassostrea gigas. Sampling involved eight sites along the state coast, and oyster sampling was collected during the period between January 2013 and December 2014. For the detection of Perkinsus, Ray's fluid thioglycollate medium (RFTM) and histology were used, and for the identification of the species, PCR and DNA sequencing were used. Perkinsus spp. was found by RFTM in C. gigas and C. gasar from São Francisco do Sul. This pathology was also detected in C. gasar from Balneário Barra do Sul both, by RFTM and histology. Perkinsus marinus was identified in C. gigas and C. gasar from São Francisco do Sul and Perkinsus beihaiensis in C. gasar from Balneário Barra do Sul. This is the first report of P. marinus in C. gigas from South America. Results of this preliminary study suggest that both oyster species tolerate the species of Perkinsus identified, without suffering heavy lesions.     

Are strain genetic effect and heterosis expression altered with culture system and rearing environment in the Portuguese oyster (Crassostrea angulata)?

Re‐ranking of strain additive genetic effect and heterosis expression in different culture environments is not well understood in shellfish species. This study was carried out to estimate the strain additive genetic and heterotic effects on harvest body weight in a 3 × 3 complete diallel cross involving three populations of Portuguese oyster (Crassostrea angulata), two populations from Cat Ba (RIA1) and Nam Dinh, northern Vietnam, and an imported stock from China. Strip spawning was applied to produce full‐ and half‐sib families simultaneously of nine cross‐combinations for performance testing at two locations (Cat Ba Island, Hai Phong city and Van Don Island, Quang Ninh province, Vietnam) using two culture systems (‘single seed’ and ‘cultch set’). A total of 7269 individual oysters were examined over a grow‐out period of 270 days. A linear mixed model was used to estimate strain additive genetic and heterotic effects for body weight at harvest. Ranking of strains based on their additive genetic effects did not differ between the locations and culture systems. The non‐additive genetic (heterotic) effects were low and not significantly different from zero for the traits studied among culture systems. There were no significant differences in levels of heterosis for harvest body weight between the two test environments or culture systems. Based on the non‐significant heterotic and large additive genetic effects among strains used for this study, a future breeding programme in this species (C. angulata), setting aside issues of diversity, could be based simply on choice among different strains based on their performance.     

A comparative study of the survival,growth and gonad development of the diploid and triploid Hong Kong oyster,Crassostrea hongkongensis (Lam & Morton 2003)

The Hong Kong oyster, Crassostrea hongkongensis is an economically important and widely cultivated oyster species in southern China. Aiming to improve the aquaculture potentials of C. hongkongensis, a triploid organism was generated by inhibiting the second polar body formation in the fertilized eggs, using 0.5 mg L^?1 cytochalasin B. Triploids exhibited significantly higher growth and were larger than the diploids throughout the life cycle. Much of the triploid growth advantage can be attributed to resource reallocation to structural development due to gonadal sterility. Most of the triploid oysters exhibited polymorphic sterility with atrophic gonads and abnormal gametogenesis. Only few ‘dysfunctional’ sperms were produced by the triploid males, thus ruling out reproductive and genetic contamination of natural oyster populations from triploid cultivations. This advantage in addition to the superior growth of triploids makes it a potent alternative in Hong Kong oyster aquaculture industry. Triploid survival was mostly found to be similar to that of diploids except for day 360, when a significant triploid advantage in terms of survival was noted. Furthermore, we found that growth and survival of Hong Kong oyster is significantly higher at the Zhuhai site in comparison to the Beihai site. Our results indicate a strong environmental effect in Hong Kong oyster cultivation and points to the fact that Zhuhai site was more suitable for oyster farming than the Beihai site. Overall, this study provides valuable information to the aquaculture industry of southern China.     

Survival and growth of reciprocal crosses between two stocks of the Hong Kong oyster Crassostrea hongkongensis (Lam & Morton, 2003) in southern China

The Hong Kong oyster, Crassostrea hongkongensis, is one of the most economically important and well‐known oyster species in southern China. To explore the possibility of improving the performance of the Hong Kong oyster, complete diallel crosses between two oyster stocks, the Zhuhai stock (Z: fast growth line F4) and the Maowei Sea stock (M: resistant line F3), were conducted using pooled gametes. Three replicates, each consisting of two pure stocks (ZZ and MM) and two reciprocal crosses (ZM and MZ), were successfully generated. High fertilization rate and hatching level were observed among all the experimental groups, suggesting that there was no sperm–egg recognition barrier between geographic populations. Reciprocal crosses had higher survival rate compared to the pure stock crosses and the rate of survival increased with progeny growth. Growth heterosis became obvious both in larval and adult stages, and was primarily influenced by the egg origin and mating strategy at the larval stage. Also, the phenotypic traits of all progeny differed amongst the culture sites, suggesting a significant environmental effect. The Zhuhai site was more suitable for oyster aquaculture than the Maowei Sea site. Thus, our results demonstrated that crossbreeding between Zhuhai and Maowei Sea stocks of Hong Kong oyster to produce superior heterosis represents a promising means to improve the fishery yield of this species in southern China.     

Broodstock conditioning of the Portuguese oyster (Crassostrea angulata,Lamarck, 1819): influence of different diets

The Portuguese oyster Crassostrea angulata shows great potential in oyster farming. The conservation of pure populations of this species is important for production diversification and biodiversity preservation. In this way, the zootechnological development for seed hatchery production is extremely important. Broodstock conditioning is a key step in the process of rearing bivalves in a hatchery. Many factors regulate the reproductive cycle, being food one of the most important ones. To evaluate the effect of different diets on C. angulata reproductive performance, broodstock were conditioned with different food regimes formulated fundamentally by flagellates (Diet 1 – Pavlova lutheri and Isochrysis galbana clone T‐ISO; Diet 2 – P. lutheri, T‐ISO and Skeletonema costatum) and constituted fundamentally by diatoms (Diet 3 – S. costatum and Chaetoceros calcitrans; Diet 4 – P. lutheri, S. costatum and C. calcitrans). During conditioning, samples of oysters were collected to evaluate condition index, gonadal development and biochemical composition. At the end of the conditioning period, oysters were induced to spawn to evaluate reproductive output (fecundity, fertilization rate and D‐larvae development). The diets had an impact on the gametogenesis process, energy storage and reproductive output performance, being the best results those obtained in broodstock fed with the diatoms‐predominant diets. However, those fed with diets majority flagellates had an unsuccessful performance. Holistic approaches incorporating all results in this study reveal and reinforce the idea that the diatom species used presented the nutritional requirements to C. angulata broodstock, being essential in the conditioning phase.     

太平洋牡蛎与近江牡蛎的种间杂交

为了评估太平洋牡蛎与近江牡蛎能否产生远缘杂种优势,于2010年5月,以成熟的两种牡蛎亲本为材料开展了2×2远缘杂交研究,由长牡蛎自繁组GG(Crassostrea gigas♀×C.gigas♂)、近江牡蛎自繁组AA(C.ariakensis♀×C.ariakensis♂)、正交组GA(Crassostreagigas♀×C.ariakensis♂)、反交组AG(C.ariakensis♀×C.gigas♂)4个实验组组成。分析了子代早期表型性状和杂种优势,并对杂交子代进行了遗传鉴定。结果表明:GA杂交组与AG组的受精强度具有不对称性。幼虫浮游期间,表型性状的中亲杂种优势几乎为0,GA组生长与存活性状表现出积极的单亲杂种优势,而AG组则具有明显的远交衰退现象;幼虫早期表型性状受到母本效应影响,而后减弱。变态期间,GA组变态率较高,得到了大量杂交稚贝;而AG组变态率极低,仅获得了72个杂交稚贝。稚贝培育期间,稚贝表现出中亲生长劣势与存活优势;GA组具有明显的单亲生长与存活优势,而AG组则表现出生长劣势并具有一定程度的存活优势。利用复合COI及ITS2鉴定了种间杂交子结果表明:正反交组杂交子均为真正意义上的两性融合杂交子。实验获得了具有显著杂种优势的GA组杂交子,为现有牡蛎的遗传改良提供了新的方向。     

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.     

Siltation increases the susceptibility of surface‐cultured eastern oysters (Crassostrea virginica) to parasitism by the mudworm Polydora websteri

Mudworm (Polydora websteri) parasitism is known to result in unsightly mud blisters in eastern oysters (Crassostrea virginica), resulting in reduced product quality and increased vulnerability to illness and environmental stress. While typically a concern only for bottom‐grown oysters, an abnormal severe outbreak of P. websteri in surface‐cultured oysters in New Brunswick, Canada, was reported in 2013, along with an anecdotally reported concurrent increase in siltation. Although heavier loads of silt are reported to increase P. websteri infestations in oysters, studies report mixed effects of siltation to this regard. Here, we report the results of a field experiment testing the effect of siltation on P. websteri recruitment in surface‐grown oysters at an aquaculture site in Richibucto, New Brunswick. Live oysters of similar size were deployed at the aquaculture site and were left to collect P. websteri recruits under one of two siltation treatments (high vs. low) for 50 days. Results suggested that P. websteri recruitment correlated with metrics of oyster size (shell weight, length, width, depth and surface area). When P. websteri counts were standardized for oyster shell morphometry, P. websteri recruitment was significantly higher in oysters exposed to the high siltation treatment, accumulating approximately 1.5× as many P. websteri as oysters exposed to the low siltation treatment. Our results suggest that higher amounts of siltation on surface‐cultured oysters can result in increased rates of P. websteri parasitism. Enhanced cleaning regimes may help to alleviate the impacts of P. websteri in surface‐grown oysters, although other mitigation strategies exist.     

Growth of the oyster Crassostrea corteziensis (Hertlein, 1951) in Sonora, Mexico

This is the first evaluation of growth and survival of spat of the Cortez oyster Crassostrea corteziensis (Hertlein) produced under controlled conditions in a coastal area in the state of Sonora, Mexico for aquaculture purposes. A suspended culture technique, used for the Pacific oyster C. gigas, was used. The Cortez oyster has an isometric shell growth during the first 13 months, reaching 71.3±1.9 mm length, 52.6±1.3 mm thickness and 25.1±0.8 mm width. Allometric growth was found between total weight and length, thickness and width (survival was 70%). The relationships between particulate organic, inorganic material, chlorophyll a and environmental parameters with growth are described. Growth rates of C. corteziensis were affected by temperature with retardation at less than 18°C. For aquaculture purposes, it is recommended that spat be sowed after winter, and oyster harvest occur at the end of autumn. According to the von Bertalanffy equation, Cortez oysters would reach the traditional exploitation size of 65 mm (mean length) at harvest. Finally, the results of this study have shown that C. corteziensis is a good candidate for aquaculture projects in this region.     

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.     

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.     

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.     

Tetraploid induction in tropical oysters,Crassostrea belcheri (Sowerby) and Crassostrea iredalei (Faustino)

Tetraploid induction has been conducted on temperate oysters but not on tropical oysters. In this study, different heat shocks (32, 35 and 38°C) and cold shocks (1, 4 and 7°C) were used to induce tetraploidy in two tropical oyster species, Crassostrea belcheri and Crassostrea iredalei, through meiosis I inhibition. Temperature shocks were applied on the newly fertilized eggs at 8–10 min post fertilization and terminated when second polar bodies began to form in the control eggs. The ploidy of the larvae and spat was determined via direct chromosome count. The percentage of larval survival until Day 20 was low (between 0.4% and 42.9%) for both temperature shocks and oyster species. No surviving larva was recorded for induction at 1, 4 and 38°C. Tetraploid spat was only recorded in C. iredalei but the percentage is low through heat shock induction of 32 and 35°C. This study shows that the tetraploid induction success rate was slightly higher in C. iredalei compared to C. belcheri. No surviving tetraploid spat were recorded for both oyster species through the cold shock method. This study shows that heat shock can be used to inhibit meiosis for the production of tetraploids but more experiments need to be conducted to determine the optimum temperature when dealing with tropical oysters.     

Hybridization between Crassostrea hongkongensis and Crassostrea ariakensis at Different Salinities

To evaluate the survival and growth performance of hybrid larvae and juveniles, we conducted hybridization experiments between Crassostrea hongkongensis and Crassostrea ariakensis at three salinities (15, 23, and 30 ppt). Hybrid fertilization occurred in one direction (C. hongkongensis ♀ and C. ariakensis ♂), and the fertilization rates did not differ significantly among the three trial salinities. Molecular genetics analysis revealed that hybrid progenies were true hybrids. Hybrid juveniles survived better than C. hongkongensis juveniles at a salinity of 30 ppt. Hybrid larvae and juveniles showed intermediate growth between C. hongkongensis and C. ariakensis progenies. The growth of hybrid juveniles was significantly greater than that of C. hongkongensis juveniles at a salinity of 30 ppt. Overall, C. hongkongensis has a high economic value but favorable hatchery and farm of this species is restricted to lower salinity sites. Because the hybrids were shown to have greater growth and survival characteristics at high salinity, hybrids may be a more desirable candidate to be cultured in those areas of China with higher salinities, such as the Bohai Sea of China where the salinity is usually 30 ppt.     

Is fertility of hybrids enough to conclude that the two oysters Crassostrea gigas and Crassostrea angulata are the same species?

The distinction of the two cupped oysters Crassostrea gigas (Thunberg, 1793) and Crassostrea angulata (Lamark, 1819) into two species was chiefly due to their differing geographical distributions, C. gigas being present in Asia and C. angulata in Europe. Today it is commonly accepted that C. angulata and C. gigas are a single species according to morphological, genetic and F1 hybridization data. However, the demonstration of the fertility of their hybrids and the absence of any reproductive isolation remained to be investigated. Consequently, we studied the fertility of hybrids and sperm competition by performing three different experiments and producing G1 and G2 hybrid progenies between wild populations of C. angulata and C. gigas. Progenies showed very close developmental yields, at 24 hours after fertilization, according to dam taxa suggesting a strong maternal transmission of oocyte quality, but no reproductive isolation was observed between the two taxa. Significant decreases of developmental yields were noticed in C. angulata females with sperm competition, most probably due to early larval mortality. The fertility of hybrids C. angulata × C. gigas was demonstrated, which is further evidence that they are the same species. To definitively state the precise taxonomic classification of C. angulata and C. gigas, further studies are needed to (i) identify geographical zones where these taxa are in contact and (ii) assess their level of hybridization in these zones.     

Effects of temperature and photoperiod on the conditioning of the flat oyster (Ostrea edulis L.) in autumn

The production of the flat oyster Ostrea edulis (L.) natural spat in Europe has decreased almost by 60% in the past ten years. Thus, the importance of the production of oyster spat in hatcheries is evident. One of the critical steps in hatchery production is broodstock conditioning, especially difficult in autumn, when gonadal development is in resting period. Conditioning is influence by temperature, photoperiod and nutrition. In this work, the effects of two temperature and three photoperiod regimes on the conditioning of O. edulis were studied for three years by stereological analyses and registering number and dates of spawning and larval yield. Temperature had a positive effect on the gonadal development of O. edulis during conditioning. The percentages of germinal cells in oysters conditioned with a gradient of temperature (14–18°C) were double compared to oysters conditioned at 15°C. Oysters conditioned with longer photoperiods showed higher percentages of germinal cells. There was no interaction between temperature and photoperiod. Spawning was observed in the oysters treated with daylight (8–16 h) ten weeks from the beginning of conditioning. Flat oysters conditioned with 8 h and 8–12 h of daylight delayed the first spawning for a month. Total larval production was higher in the oysters treated with the longest daylight gradient. Gonadal and gametogenic development was a non‐synchronic process and the spawning extended for around two months. A protocol for flat oyster broodstock conditioning in autumn by using both a gradient of temperature (14–18°C) and daylight (8–16 h) is proposed.