Oyster Spat Research Articles

Growth and reproduction of early grow-out hardened juvenile Pacific oysters, Crassostrea gigas in Gamakman Bay, off the south coast of Korea

The southern coast of Korea comprises many small bays where the Pacific oyster, Crassostrea gigas, has been cultivated intensively employing a long-line hanging culture system. Naturally harvested oyster spats used in the long-line culture system are hardened in mid-intertidal area and transplanted to the center of the bay for grow-out usually in May. The May transferred oyster produced considerably smaller quantity of eggs during spawning in September. To enhance the reproductive effort and growth during grow-out, we transferred the hardened oyster juveniles to grow-out sites in January rather than May to allow them to accumulate energy reserves at the time of spring phytoplankton bloom that normally occurs during April and early May. The January transferred oyster juvenile (JTO) showed fast somatic and shell growth during grow-out, reaching a marketable size in August, while the juvenile oyster transplanted in May (MTO) did not reach a market size until November. JTO spawned from June to September, with a single spawning pulse in July, while most of the MTO spawned in September. Quantity of eggs produced by the JTO was 5.7 times higher than MTO, indicating that early grow-out enhanced egg production. The meat weight and levels of tissue carbohydrate and protein of JTO harvested in November were significantly higher than MTO harvested at the same time, suggesting that the meat quality of JTO is superior to the MTO. Overall, our results suggest that transplanted hardened juveniles to grow-out in January increase natural oyster seed harvest in the bay as well as improve quality of the oyster for marketing. Statement of relevanceOyster farming is one of the leading aquaculture industries in the world, producing over 4 million tons of oysters annually world-wide. In the oyster industry, Crassostrea gigas, the Pacific oyster, has been cultured extensively in Asia, and recently in Europe using different modes of culture. In Asia, the Pacific oysters are raised using subtidal hanging long-line system, while they are cultured in intertidal areas using net bags and rack system in Europe.In Korea and Japan, the oyster seeds are supplied mostly from wild, by harvesting oyster larvae during post-spawning season. The newly harvested oyster spats are stunted in intertidal before they are transplanted to the subtidal hanging long-lines, to maximize survival of the seed oysters. This stunting process is called hardening and the hardening of oyster has been practiced for several decades in Korea and Japan. In Korea, the hardening seed oysters are transplanted in May, and the May transplanted oysters are harvested in December as they grow up to 8cm in shell length. The May transplanted oysters produce relatively smaller quantity of eggs during their first spawning, although the small reproductive effort may increase their survival during post-spawning season. The natural oyster spats harvested in late summer or early fall for the oyster culture in Korea are mostly originated from the May transplanted oysters. Currently, the natural oyster spat harvest in Korea suffers from insufficient quantity. In part, this insufficiency is associated to the poor reproductive activity of the May transplanted oyster.Low reproductive effort of May transplanted oysters during their first spawning is, in part, associated to too late transplantation to the bay, and earlier transplantation of the stunted oysters from the hardening ground to the grow-out ground in the bay may enhance the reproductive effort, resulting in subsequent successful larval recruitment and settlement of the larvae. Accordingly, we experimentally transplanted the hardened oysters 4months earlier to the traditional transplanting period (January) and monitored their reproduction and growth. Although the hardening process has been practiced over 50years, no studies have compared growth and reproduction of differentially transplanted oysters in Korea. As the data demonstrated, January transplanted oyster demonstrated earlier gonad maturation and produced 5× more eggs during spawning, compared to May transplanted oysters. No severe mortality was observed from the early transplanted oysters during the course of study, although reproductive activity of January transplanted oysters was relatively intensive, compared to May transplanted oysters.The present study first attempted to evaluate effectiveness of transplantation timing of the hardened oysters, in terms of growth and reproduction. Also, quantity of the eggs produced from differentially transplanted female oysters was first measured and compared in this study using ELISA. As the data indicated, January transplanted oyster produced 5× more eggs, which may enhance the natural oyster spat harvest in the bay, as well as to evaluate the transplantation period and management of oyster farming on the south coast of Korea.

Phenotypic plasticity in oysters (Crassostrea virginica) mediated by chemical signals from predators and injured prey

AbstractPrey organisms reduce predation risk by altering their behavior, morphology, or life history. Avoiding or deterring predators often incurs costs, such as reductions in growth or fecundity. Prey minimize costs by limiting predator avoidance or deterrence to situations that pose significant risk of injury or death, requiring them to gather information regarding the relative threat potential predators pose. Chemical cues are often used for risk evaluation, and we investigated morphological responses of oysters (Crassostrea virginica) to chemical cues from injured conspecifics, from heterospecifics, and from predatory blue crabs (Callinectes sapidus) reared on different diets. Previous studies found newly settled oysters reacted to crab predators by growing heavier, stronger shells, but that adult oysters did not. We exposed oysters at two size classes (newly settled oyster spat and juveniles ~2.0 cm) to predation risk cue treatments including predator or injured prey exudates and to seawater controls. Since both of the size classes tested can be eaten by blue crabs, we hypothesized that both would react to crab exudates by producing heavier, stronger shells. Oyster spat grew heavier shells that required significantly more force to break, an effective measure against predatory crabs, when exposed to chemical exudates from blue crabs as compared to controls. When exposed to chemical cues from injured conspecifics or from injured clams (Mercenaria mercenaria), a sympatric bivalve, shell mass and force were intermediate between predator treatments and controls, indicating that oysters react to injured prey cues but not as strongly as to cues released by predators. Juvenile oysters of ~ 2.0 cm did not significantly alter their shell morphology in any of the treatments. Thus, newly settled oysters can differentiate between predatory threats and adjust their responses accordingly, with the strongest responses being to exudates released by predators, but oysters of 2.0 cm and larger do not react morphologically to predatory threats.

Comparative growth and survival of spat of the Caribbean pearl oyster,Pinctada imbricatacultivated indoor with microalgae diets and outdoor with natural diet

We report the results of survival and growth in size and dry mass of spat of the Caribbean pearl oyster Pinctada imbricata cultivated under outdoor (field culture) and indoor (Laboratory) conditions. Field group fed on environmental seston. Laboratory groups were fed with mono, binary and ternary mixtures of three cultivated algae: Isochrysis galbana (Ig), Tetraselmis chuii (Ig) and the Chaetoceros sp. (Ch-A, isolated from north-eastern Venezuela). After 30 days of trial, fatty acid profiles of spat were determined along with growth in length and height shell, adductor muscle and soft tissue dry mass. During the field grow-out phase (field culture), samplings were performed at days 1, 15 and 30 to measure environmental variables of phytoplankton biomass (chlorophyll a), dissolved oxygen, seston, temperature and salinity. A significant increase in size and soft tissue mass occurred in spat fed the diets including the tropical diatom (Chaetoceros sp.). In contrast, monoalgal diets of Tc and Ig yielded no significant differences in size and mass of spat, compared with the field culture. These results suggest that nutritional requirements of cultivated spat for specific fatty acids of physiological importance for marine bivalves, such as: 16:0, 16:1n-7, 18:2n-6, 20:4n-6, 18:3n-3 and 20:5n-3, were satisfied from microalgal diets with Ch-A, alone or in combination, compared with spat fed from the field culture.

Combined Effects of Temperature and Photoperiod on the Conditioning of the Flat Oyster (Ostrea edulis[Linnaeus, 1758]) in Winter

The flat oyster Ostrea edulis (Linnaeus, 1758) is a commercially valuable species in Europe. The production of O. edulis natural spat has decreased in the past, and it is insufficient to meet demand. Thus, the importance of hatcheries in the production of oyster spat is increasing. The conditioning of O. edulis broodstock may be improved by manipulating several external factors such as temperature, food supply, and photoperiod; however, few studies have reported the effect of these parameters on O. edulis gametogenesis and conditioning. In this report, the combined effects of temperature and photoperiod on broodstock conditioning of O. edulis in winter were studied to provide a protocol that improves flat oyster conditioning. After a resting period in autumn, in winter, oysters' gonads are restructured, and the natural process of gonadal maturation is initiated. Oyster gonads showed follicles in intermediate maturation stages, with the presence of both male and female germinal cells in the same follicles but at different stages of maturation. The combined effect of temperature (TG14-18) and photoperiod on oyster conditioning in winter was evaluated by using direct indexes. Oysters conditioned with a gradient of temperature and a gradient of daylight (PG8-16), a P16 regime or under natural environmental conditions began to spawn 4 wk after the beginning of conditioning. Oysters treated with a P8 regime needed a week more to start spawning. Spawning was a continuous process that lasted for 7 wk. When the total amount of released viable larva was compared, the positive influence of photoperiod was much more evident. Total larval production was two or three times higher in the oysters treated with a gradient of daylight (PG8-16). In summary, using the conditions of temperature and photoperiod described in this study (TG14-18 and PG8-16), it was possible to improve flat oyster conditioning in winter, producing an important amount of viable larvae over a period of 2 mo.

Epizootiology of Perkinsus sp. inCrassostrea gasar oysters in polyculture with shrimps in northeastern Brazil.

Bivalve culture is of considerable economic and social interest in northeastern (NE) Brazil. The polyculture is an alternative approach to traditional monoculture for reducing the environmental impact of shrimp farming and improving oyster culture. Perkinsus marinus and Perkinsus olseni were found infecting oysters in NE Brazil and can threaten oyster production. This study evaluated Perkinsus spp. occurrence in Crassostrea gasar during all production stages. Oyster spats were produced in a hatchery and grown in shrimp ponds in Rio Grande do Norte state. Perkinsus spp. were surveyed by Ray's fluid thioglycollate medium and confirmed by polymerase chain reaction. Prevalence and intensity of infection were determined in oysters until they reached 7 cm. Results showed that the broodstock was already infected by Perkinsus (60%), but the derived spats were Perkinsus-free. Oyster spats acquired Perkinsus infection when transferred to ponds. The prevalence gradually increased in the seven months following placement in ponds (73%), and then decreased to 17% by the tenth month. The infections were initially mild, but intensity increased at the final growth stage. In conclusion, it is possible to produce Perkinsus-free C. gasar oyster spats from infected broodstock, and their culture in shrimp ponds is feasible.

Transcriptional changes in Crassostrea gigas oyster spat following a parental exposure to the herbicide diuron

The Pacific oyster Crassostrea gigas is the main oyster species produced in the world, and a key coastal economic resource in France. High mortalities affect Pacific oysters since 2008 in France and Europe. Their origins have been attributed to a combination of biotic and abiotic factors, underlining the importance of environment quality. The impact of water pollution has been pointed out and one of the pollutants, the genotoxic herbicide diuron, occurs at high concentrations all along the French coasts. Previous work has revealed that a parental exposure to diuron had a strong impact on hatching rates and offspring development even if spats were not exposed to diuron themselves. In this study, we explored for the first time the transcriptional changes occurring in oyster spats (non exposed) originating from genitors exposed to an environmentally relevant concentration of diuron during gametogenesis using the RNAseq methodology. We identified a transcriptomic remodeling revealing an effect of the herbicide. Different molecular pathways involved in energy production, translation and cell proliferation are particularly disturbed. This analysis revealed modulated candidate genes putatively involved in response to oxidative stress and mitochondrial damage in offspring of genitors exposed to diuron. Complementary measures of the activity of enzymes involved in these latter processes corroborate the results obtained at the transcriptomic level. In addition, our results suggested an increase in energy production and mitotic activity in 5-month-spats from diuron-exposed genitors. These results could correspond to a “catch-up growth” phenomenon allowing the spats from diuron-exposed genitors, which displayed a growth delay at 3 months, to gain a normal size when they reach the age of 6 months. These results indicate that exposure to a concentration of diuron that is frequently encountered in the field during the oyster’s gametogenesis stage can impact the next generation and may result in fitness disturbance.

Hyperspectral remote sensing of wild oyster reefs

The invasion of the wild oyster Crassostrea gigas along the western European Atlantic coast has generated changes in the structure and functioning of intertidal ecosystems. Considered as an invasive species and a trophic competitor of the cultivated conspecific oyster, it is now seen as a resource by oyster farmers following recurrent mass summer mortalities of oyster spat since 2008. Spatial distribution maps of wild oyster reefs are required by local authorities to help define management strategies. In this work, visible-near infrared (VNIR) hyperspectral and multispectral remote sensing was investigated to map two contrasted intertidal reef structures: clusters of vertical oysters building three-dimensional dense reefs in muddy areas and oysters growing horizontally creating large flat reefs in rocky areas. A spectral library, collected in situ for various conditions with an ASD spectroradiometer, was used to run Spectral Angle Mapper classifications on airborne data obtained with an HySpex sensor (160 spectral bands) and SPOT satellite HRG multispectral data (3 spectral bands). With HySpex spectral/spatial resolution, horizontal oysters in the rocky area were correctly classified but the detection was less efficient for vertical oysters in muddy areas. Poor results were obtained with the multispectral image and from spatially or spectrally degraded HySpex data, it was clear that the spectral resolution was more important than the spatial resolution. In fact, there was a systematic mud deposition on shells of vertical oyster reefs explaining the misclassification of 30% of pixels recognized as mud or microphytobenthos. Spatial distribution maps of oyster reefs were coupled with in situ biomass measurements to illustrate the interest of a remote sensing product to provide stock estimations of wild oyster reefs to be exploited by oyster producers. This work highlights the interest of developing remote sensing techniques for aquaculture applications in coastal areas.

Species-specific responses of resident crabs to vertical habitat complexity on intertidal oyster reefs

Habitat vertical complexity is an important physical feature of marine systems that can influence factors such as predator–prey interactions and recruitment. We examined how vertical structure on intertidal Eastern oyster Crassostrea virginica reefs affected resident crab distributions by deploying trays of varying rugosity at two sites in Charleston, South Carolina. Trays were collected after fifteen weeks and oyster spat and resident macrofauna were counted and measured. Densities of oyster spat and the filter-feeding, invasive, green porcelain crab Petrolisthes armatus were strongly positively associated with rugosity. Flatback mud crab Eurypanopeus depressus densities were positively associated with rugosity and with scorched mussel Brachiodontes exustus densities. The common mud crab Panopeus herbstii exhibited a more even distribution across oyster rugosity treatments and was the only species that commonly occurred in the low rugosity treatment. Differences in crab sizes between sites were attributable to abundances of smaller individuals and may reflect spatial variation in recruitment potential. Differences in crab size distributions among treatments were caused by more relatively large crabs in the high rugosity treatments, where greater vertical structural complexity provided more refuge for larger individuals. The relationships between rugosity and faunal densities were consistent at both sites, indicating that the rugosity metric can be used to estimate changes in spat and associated faunal densities when vertical habitat complexity on oyster reefs is reduced, for instance, through harvesting or sedimentation.

Summer mortalities and detection of ostreid herpesvirus microvariant in Pacific oyster Crassostrea gigas in Sweden and Norway.

The Pacific oyster Crassostrea gigas has recently expanded its range in Scandinavia. The expansion is presumably a result of northwards larval drift. Massive settlements were recorded in many areas along the Swedish west coast and southern Norway in 2013 and 2014. After the spawning season in 2014, the temperature of the surface water peaked at 24-26°C. After this period, high and sudden mortalities occurred in a Swedish hatchery and in wild populations along the Swedish west coast and south coast of Norway. Surveys and collected data showed that mortalities mainly occurred during 3 wk in September. All size classes were affected, and affected populations displayed a patchy distribution with heavily affected and unaffected populations in close proximity. Flat oysters Ostrea edulis and blue mussels Mytilus edulis were unaffected. Ostreid herpesvirus (OsHV) was detected in moribund Pacific oyster spat as well as in surviving adults. The virus was identified as OsHV-1 μvar. This is the first detection of this variant in Scandinavia, showing that OsHV-1 μvar is present in areas with recent establishments of Pacific oysters, and where there is no aquaculture of this species.

How network analysis of oyster movements can improve surveillance and control programs of infectious diseases?

Animal movements are one of the main ways to introduce and spread pathogens. In French oyster farming, many stakeholders and premises are heterogeneously divided over the country and a highly dynamic flow of oysters exists among them. In the context of animal disease surveillance and control, analysis of the animal movement’ network can provide useful information to build adapted surveillance strategies or to develop risk management. Movement network analysis has been widely used in terrestrial production, to evaluate the vulnerability of animal movement network to the spread of a specific disease. In France, since 2008, Pacific oyster spat (Crassostrea gigas) has been affected by massive mortality outbreaks associated with the detection of a newly reported variant of ostreid herpesvirus type 1 (OsHV-1). These mortality events have a direct economic impact causing considerable concern to oyster farmers. A previous epidemiological study has highlighted the potential role of oyster transfers in the spread of these outbreaks. However, neither mandatory database nor reliable data is publicly available concerning oyster transfers in France. In this context, a field study was carried out in the main oyster production area in France, Charente-Maritime bay, to map oyster movements and to characterize the corresponding network structure related to potential disease spread. Seventy-five oyster farmers were randomly selected in Charente-Maritime bay between July and September 2010, according to a stratified sampling design based on farm category regarding production type and location (i.e. spat producers, local farmers, beyond farmers, local farmer-senders, beyond farmer-senders). Data related to the farm characteristics and activities, routine rearing scheme and potential changes in husbandry practices were collected during a face-to-face interview of the oyster farmer, using a standardized questionnaire and a land register. Movement data were spatialized and analyzed using social network analysis. A 85.3% participation rate to the study was obtained. Results showed a seasonal variation of oyster transfers in all the sampling strata, with peaks of transfers in spring and autumn, and different geographical patterns of oyster transfers between strata. Growing sites were shared by farms belonging to different farm category which have different behaviour related to oyster movements in time. These results show heterogeneity of contacts between strata both in time and space. Network measures allowed identifying the growing sites having a high risk of becoming infected, of transmitting diseases to other sites or controlling the flow of oysters from one part of the network to another part. The growing sites at high risk of becoming infected could be targeted for surveillance activities (Figure 1). The growing sites at high risk of transmitting diseases to other sites could be the first ones where implementation of early or enhanced control measures (e.g. quarantine, depopulation) could be focused. The growing sites controlling the flow of oysters could be considered as key players in the initial spread of a disease. If these sites would be removed from the network by early movement restriction, this may contribute to contain the disease spread in a smaller part of the area, providing that the propagation by the marine currents and the effect of environmental factors are considered all together. As a conclusion, movement network analysis may provide valuable information to policy makers in preparedness (surveillance) and response (control) to an epidemic.

江苏海门蛎岈山牡蛎礁生态现状评价

PDF HTML阅读 XML下载 导出引用 引用提醒 江苏海门蛎岈山牡蛎礁生态现状评价 DOI: 10.5846/stxb201510102048 作者: 作者单位: 农业部东海与远洋渔业资源开发利用重点实验室 中国水产科学研究院东海水产研究所 上海,农业部东海与远洋渔业资源开发利用重点实验室 中国水产科学研究院东海水产研究所 上海,农业部东海与远洋渔业资源开发利用重点实验室 中国水产科学研究院东海水产研究所 上海,农业部东海与远洋渔业资源开发利用重点实验室 中国水产科学研究院东海水产研究所 上海,农业部东海与远洋渔业资源开发利用重点实验室 中国水产科学研究院东海水产研究所 上海,农业部东海与远洋渔业资源开发利用重点实验室 中国水产科学研究院东海水产研究所 上海,农业部东海与远洋渔业资源开发利用重点实验室 中国水产科学研究院东海水产研究所 上海,江苏海门蛎岈山国家级海洋公园管理处 江苏海门,江苏海门蛎岈山国家级海洋公园管理处 江苏海门,江苏海门蛎岈山国家级海洋公园管理处 江苏海门,江苏海门蛎岈山国家级海洋公园管理处 江苏海门,江苏海门蛎岈山国家级海洋公园管理处 江苏海门,江苏海门蛎岈山国家级海洋公园管理处 江苏海门 作者简介: 通讯作者: 中图分类号: 基金项目: 江苏海门蛎岈山国家级海洋公园管理处项目；中央级公益性科研院所（东海水产研究所）基本科研业务费项目（2014G01，2015M01）；国家公益性行业（农业）科研专项经费项目（201303047）；国家自然科学基金面上项目（31170508） Ecological status of a natural intertidal oyster reef in Haimen County, Jiangsu Province Author: Affiliation: Key Lab of East China Sea and Marine Fishery Resource Exploitation and Utilization, Ministry of Agriculture, East China Sea Fisheries Research Institute, Chinese Academy of Fishery Science, 200090, Shanghai, China,,,,,,,,,,,, Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:基于2013-2014年间的生态调查结果，评价了江苏海门蛎岈山牡蛎礁的生态现状。无人机航拍结果显示，江苏海门蛎岈山分布有750个潮间带区牡蛎礁斑块，总面积约为201519.37 m2；与2003年相比，海门蛎岈山牡蛎礁面积约下降了38.8%。活体牡蛎的平均盖度约为66%，2013年5和9月熊本牡蛎Crassostrea sikamea的平均密度分别为（2199±363）个/m2和（2894±330）个/m2。2013年5月海门蛎岈山熊本牡蛎种群的平均肥满度（CI）和性腺指数（GI）分别为（9.76±0.95）%和（1013±82）mg/g，均显著低于浙江象山港养殖的熊本牡蛎种群（P < 0.05）。海门蛎岈山熊本牡蛎的单倍体多样性和核苷酸多样性指数分别为0.119和0.00028，均高于长江口野生种群和浙江象山港养殖种群。海门蛎岈山熊本牡蛎种群受到尼氏单孢子虫（Haplosporidium nelson）的轻度浸染，其感染率（17.2%）低于浙江象山港养殖群体（47.3%）。泥沙沉积和人类捕捞是江苏海门蛎岈山牡蛎礁面临的主要胁迫因子，今后牡蛎礁恢复的重点是增加附着底物的数量。 Abstract:Oyster reefs are important coastal habitats because they provide ecosystem services such as water filtration, habitat provision, shoreline stabilization, and nutrient retention. The Liyashan oyster reef is located inshore of the Dongzhao Port, Haimen County, Jiangsu Province, and it is a natural intertidal reef. The oyster reef supports abundant species and high fishery production, and it is a major spawning ground for the roughskin sculpin, Trachidermus fasciatus, which is listed as a second-grade state protection animal. However, there is limited information on the ecological status of the natural oyster reef. In this study, we mapped the reef footprint and monitored oyster populations (species, density, biomass, size-frequency, body condition, genetic diversity, and parasite infection) at the Liyashan oyster reef on the basis of field investigations conducted in 2013 and 2014 and compared the metrics to those of farmed oysters. Aerial photos obtained using an unmanned plane indicate that the natural oyster reef is composed of 750 intertidal reef patches, and the area of a patch ranges from 0.98 m2 to 16330 m2. The total area of the intertidal reef is approximately 0.201 km2. However, the aerial footprint of the natural reef has declined by approximately 39% in the past decade because of sedimentation. Mean cover for live oysters was 66%, and mean densities of the oyster Crassostrea sikamea were between 2199 and 2894 ind./m2. The percentage of oyster spats with respect to size-frequency distribution was greater in September than in May, showing high recruitment of oyster larvae in the summer. Mean flesh content, condition index, and gonad index of C. sikamea were (32.15±1.80)%, (9.76±0.95)%, and (600±116) mg/g, respectively. These values were significantly lower at the Liyashan natural oyster reef than at nearby aquaculture sites ((36.50±2.34)%, (15.24±1.04)%, and (1013±82) mg/g, respectively) in Xiangshan Bay, Zhejiang Province. Haplotype diversity and nucleotide diversity index of the C. sikamea population at Liyashan were 0.119 and 0.00028, respectively, and were greater than the values observed at the aquaculture sites. The oyster population had been slightly infected by the parasite Haplosporidium nelsoni, and the infection frequency (17.2%) was lower at the natural reef than at the aquaculture sites (47.3%). The natural oyster reef is rapidly degrading because of heavy sedimentation. The oyster population at the reef had lower body condition, greater genetic diversity, and lower gonad index and parasite infections than those at the aquaculture sites. To protect this important biogenic reef, greater restoration efforts should be initiated to promote the oyster population and reef functions by supplementing hard substrates. 参考文献 相似文献 引证文献

Effects of nonnative invertebrates on two life stages of the native eastern oyster Crassostrea virginica

Since their recent introductions into Florida waters, three sessile invertebrates [Perna viridis (Asian green mussel), Mytella charruana (charru mussel) and Megabalanus coccopoma (pink titan acorn barnacle)] have expanded their range along the Atlantic coast in estuarine waters. Little research has been done to understand how these nonnative species interact with the ecologically and economically important eastern oyster Crassostrea virginica. To assess the potential effects of P. viridis, M. charruana and M. coccopoma on C. virginica, the following questions were addressed in manipulative experiments. (1) Does the presence of nonnative species decrease oyster larval settlement? (2) Do oyster larvae avoid settling on nonnative species? (3) Do nonnative species decrease survival of juvenile oysters (spat)? (4) Do nonnative species hinder spat growth? We included two controls: absence of nonnative species and presence of the native mussel Geukensia demissa. The nonnative species influenced settlement, growth and survival of C. virginica in different ways. M. coccopoma and P. viridis negatively influenced larval settlement, whereas M. charruana had no influence on the total number of settled larvae. Oyster larvae avoided settling on all three nonnative species and the native G. demissa. Both nonnative mussels negatively affected survival of juvenile oysters but only M. charruana also reduced spat growth. The native mussel, G. demissa, had no negative impacts on total settlement, survival and growth of C. virginica; in fact, it increased larval settlement in some trials. These three nonnative species should be classified as invasive because all had negative effects on native C. virginica.

Detection and distribution of ostreid herpesvirus 1 in experimentally infected Pacific oyster spat

High mortality rates are reported in spat and larvae of Pacific oyster Crassostrea gigas and associated with ostreid herpesvirus 1 (OsHV-1) detection in France. Although the viral infection has been experimentally reproduced in oyster larvae and spat, little knowledge is currently available concerning the viral entry and its distribution in organs and tissues. This study compares OsHV-1 DNA and RNA detection and localization in experimentally infected oysters using two virus doses: a low dose that did not induce any mortality and a high dose inducing high mortality. Real time PCR demonstrated significant differences in terms of viral DNA amounts between the two virus doses. RNA transcripts were detected in oysters receiving the highest dose of viral suspension whereas no transcript was observed in oysters injected with the low dose. This study also allowed observing kinetics of viral DNA and RNA detection in different tissues of oyster spat. Finally, viral detection was significantly different in function of tissues (p<0.005), time (p<0.005) with an interaction between tissues and time (p<0.005) for each probe.

Restoring degraded European native oyster, Ostrea edulis, habitat: is there a case for harrowing?

Habitat degradation is a contributory factor to poor recruitment and sustainability of the European native oyster, Ostrea edulis. Bed cleaning (harrowing) is a widely referenced but little studied habitat management measure aimed at exposing clean shell for oyster larvae to settle upon. This study carried out a large-scale field experiment in Lough Foyle on the border of Northern Ireland and Ireland over 3 years aimed at investigating the effects of harrowing on oyster spat settlement, substratum condition, suspended particulate matter and associated faunal assemblage. The results demonstrated that O. edulis spat settlement was higher in unharrowed areas and there was no significant difference in bivalve settlement between the two treatments. Harrowing had no significant effect on percentage cover of fouling organisms, but there was a significant difference between assemblages in harrowed and unharrowed treatments. This study concluded that harrowing is not suitable for all oyster production areas and should only be employed with caution.

The Chilean black urchin, Tetrapygus niger (Molina, 1782) in South Africa: gone but not forgotten

It is important to keep lists of invasive and alien species up to date, but it can be difficult to determine when species should be removed from such lists. One such example is the urchin, Tetrapygus niger (Molina, 1782), which is believed to have been unintentionally introduced to Alexander Bay on the west coast of South Africa with oyster spat imported for aquaculture (the only recorded alien population of T. niger globally). This species is a kelp grazer in its native range, capable of converting healthy kelp populations to barren landscapes. This study involved the re-survey of two aquaculture dams in September 2014, which previously contained the urchin, as well as intertidal and subtidal transects of the surrounding coast. Tetrapygus niger were absent from all sites, despite the presence of a healthy population of native urchins Parechinus angulosus (Leske, 1778). There was also little indication of urchin grazing on kelp stipes. As a result of this absence record, it is recommended that T. niger be removed from the South African list of introduced marine species, and from global lists of invasive species.

Comparison of Substrates for Eastern Oyster (Crassostrea virginica) Spat Settlement in the Loxahatchee River Estuary, Florida

The eastern oyster [Crassostrea virginica (Gmelin, 1791)] is an important epibenthic species in estuarine and coastal marine ecosystems, providing habitat for commercially valuable species and enhancing ecosystem function. One way to assess oyster population structure and the potential suitability of oyster restoration sites is through deployment of adult oyster shells or other substrates, and quantifying oyster spat settlement. The suitability of travertine tiles versus axenic adult oyster shells for C. virginica settlement was compared by deploying shellstrings with tiles and shells in four different locations across two seasons (fall or spring) in the subtropical, Loxahatchee River estuary, FL. There was no significant difference in spat densities on oyster shells compared with tile tops and bottoms, although there was significant spatial and temporal variation in spat settlement. Spat were slightly more abundant on the top of deployed tiles compared with the bottom, which differs from typical C. virginica settlement behavior. One possible explanation may be the presence of other fouling organisms on the bottom of tiles which decrease oyster settlement rates. Results show that oyster spat settlement was indistinguishable between travertine tiles and oyster shells and thus suggest that travertine tiles are preferable to axenic oyster shells because spat settlement can be precisely quantified per unit area.

Stability of Ostreid herpesvirus-1 (OsHV-1) and assessment of disinfection of seawater and oyster tissues using a bioassay

Microvariant genotypes of Ostried herpesvirus-1 (OsHV-1 μVar) have recently emerged as a cause of epizootic mortality in Pacific oysters (Crassostrea gigas) in many countries. Measures that can reduce the spread of the virus are required to decrease the incidence and distribution of the disease at the local and global scales. Disease management strategies and biosecurity plans require data describing the stability of OsHV-1 in the environment and the methods required for effective disinfection of the virus. A bioassay using intramuscular injection of 10month old oyster spat had a limit of detection of 3.6×103 copies of OsHV-1 genome per oyster. This was 10-fold more sensitive compared to immersion challenge of 5month old spat, even though the oysters were exposed to a greater volume of inoculum over a period of 2h. OsHV-1 remained infectious in seawater for 2days at 20°C and in wet or dry, non-living oyster tissues for at least 7days at 20°C. OsHV-1 was inactivated by: commercial multipurpose disinfectants used according to label directions (Virkon-S, Dupont; quaternary ammonium preparation, Livingston); sodium hydroxide (20g/L 10min), iodine (0.1% 5min) and formalin (10% v/v 30min); and physical measures including heating to 50°C for 5min and exposure to a high dose of ultraviolet light. Ineffective disinfectant treatments were: heating to 42°C for 5min, and alkaline detergent (2000ppm, 10min) (Pyroneg, Johnson Diversey). Sodium hypochlorite (50ppm available chlorine, 15min) inactivated OsHV-1 in relatively clean seawater, but this treatment was not effective after addition of protein, 10% v/v foetal bovine serum. A concentration of 200ppm available chlorine for 15min did not inactivate OsHV-1 in oyster tissue. This study provides information that will assist in modelling the disease risk posed by OsHV-1 and data that are necessary to devise effective biosecurity strategies to control the spread of OsHV-1 in a variety of situations. The potential role of fomites in the spread of the virus, and recurrence of disease in endemically infected areas were highlighted by the persistence of infectivity in non-living oyster tissues. Statement of relevanceData on the duration of infectivity and effective disinfection of OsHV-1 in filed relevant preparations is needed for improved disease control.

The allometry of oysters: spatial and temporal variation in the length–biomass relationships forCrassostrea virginica

We examine the relationship of biomassBand lengthLin the eastern oysterCrassostrea virginicaby focusing on the scaling exponentbin the allometric equationB=aLbusing four datasets: Delaware Bay, Chesapeake Bay, Galveston Bay and a regionally extensive compilation from the NOAA Mussel Watch Program. The average value of the scaling exponent in Delaware Bay and Chesapeake Bay is about 2. For Galveston Bay, the value is distinctly higher, near 2.6. Over all Mussel Watch sites, the value is again near 2. Within Delaware Bay, the salinity gradient exerts an important effect. Shells are longer for their meat weight at lower salinities. The range of scaling exponents revealed by Mussel Watch data is exceedingly large (b&lt; 1 to &gt;3). Scaling exponents below 2.5 are unusual in bivalves. Among bivalves, only other oyster taxa have comparably low scaling exponents averaging near 2. We propose that oyster biomass routinely scales nearer the square of the length rather than the cube and that this is a constraint imposed by the exigency of carbonate production for reef maintenance and accretion in the face of high rates of taphonomic degradation. The adaptation as a reef builder requires the formation of carbonate that rapidly breaks down, thus requiring that carbonate produced be maximized. A biomass-to-length scaling exponent of 2 provides a mechanism to maximize shell production relative to biomass, while at the same time providing maximum surface area for the all-important settling of oyster spat to maintain the population.

Interaction between toxic dinoflagellate Alexandrium catenella exposure and disease associated with herpesvirus OsHV-1 μVar in Pacific oyster spat Crassostrea gigas

Blooms of toxic dinoflagellates can co-occur with mass mortality events associated with herpesvirus OsHV-1μVar infection that have been decimating Pacific oyster Crassostrea gigas spat and juveniles every summer since 2008 in France. This study investigated the possible effect of a harmful dinoflagellate, Alexandrium catenella, a producer of Paralytic Shellfish Toxins (PSTs), upon the oyster spat–herpesvirus interaction. Oyster spat from a hatchery were challenged by cohabitation with oysters contaminated in the field with OsHV-1μVar and possibly other pathogens. Simultaneously, the oysters were exposed to cultured A. catenella. Infection with OsHV-1μVar and PST accumulation were measured after 4 days of experimental exposure.Exposure to Alexandrium catenella modified the host–pathogen interaction by reducing prevalence of OsHV-1μVar infection. In addition, oysters challenged with OsHV-1μVar and possibly other pathogens from the environment accumulated smaller amounts of PSTs than unchallenged oysters. Three possible mechanisms are suggested by these results: (i) possible direct interactions between A. catenella and herpesvirus (or associated pathogens) could reduce viral transmission and algal availability for oyster consumption; (ii) oyster feeding behavior or digestive functions may have been altered, thus decreasing both uptake of viral particles and consumption or digestion of toxic algae and consequent toxin accumulation; (iii) immuno-activation by A. catenella could enhance defense efficiency against OsHV-1μVar infection. These findings suggest further research on relationships between OsHV-1μVar and toxic dinoflagellates and their combined effects upon disease transmission and proliferation processes, as well as on oyster physiological and immunological involvement in this complex, tripartite interaction.

Dual analysis of host and pathogen transcriptomes in ostreid herpesvirus 1-positive Crassostrea gigas.

Ostreid herpesvirus type 1 (OsHV-1) has become a problematic infective agent for the Pacific oyster Crassostrea gigas. In particular, the OsHV-1 μVar subtype has been associated with severe mortality episodes in oyster spat and juvenile oysters in France and other regions of the world. Factors enhancing the infectivity of the virus and its interactions with susceptible and resistant bivalve hosts are still to be understood, and only few studies have explored the expression of oyster or viral genes during productive infections. In this work, we have performed a dual RNA sequencing analysis on an oyster sample with a high viral load. High sequence coverage allowed us to thoroughly explore the OsHV-1 transcriptome and identify the activated molecular pathways in C. gigas. The identification of several highly induced and defence-related oyster transcripts supports the crucial role played by the innate immune system against the virus and opportunistic microbes possibly contributing to subsequent spat mortality.