Bonamia exitiosa in European Flat Oyster (Ostrea edulis) on the Croatian Adriatic Coast from 2016 to 2020

The annual production of European flat oysters (Ostrea edulis) in Croatia is about 50 to 65 tons, and it has a long tradition. All Croatian oyster farms are subjected to the national surveillance program aiming to detect the presence of Bonamia ostreae and Marteilia refringens according to the Council Directive 2006/88/EC. Within the surveillance program, the first findings of the parasite Bonamia spp. occurred in 2016 in two production areas in the north and south of the Eastern Adriatic coast. The repeated findings of the parasite were noted up to 2020 but also on two additional sites in the north. The parasite was detected by cytological analysis of stained heart smears, histological examination, and PCR. PCR positive samples were sequenced for SSU rDNA gene, and BLAST analysis confirmed infection with Bonamia exitiosa. Attempts to prove the Pacific oyster as a putative vector of the parasite failed. The infection prevalence from 2016 until 2020 ranged from 3.3 to 20% in different sites. No mortalities were reported from the infected sites, and it seemed that infection of flat oysters with B. exitiosa did not affect their health. The study has not shown the source and way of infection spread, which imposes the need for more comprehensive molecular and epidemiological studies.

Seasonal Sampling of a Microbial Community in the Sediment of Geoje-Hansan Bay, Republic of Korea

Several oyster farms are concentrated in Geoje-Hansan Bay, Republic of Korea, and there is concern about marine pollution. Hence, we monitored the sediment at this site for a year using 16S rRNA gene sequencing. The predominant phyla were Proteobacteria (69.9 to 79.1%) and Bacteroidetes (8.2 to 10.6%) in all seasons.

Oyster farms are the main spawning grounds of the black sea bream Acanthopagrus schlegelii in Hiroshima Bay, Japan

Understanding the anthropogenic impact of oyster farms is essential for the management and conservation of marine fishes. In Japan, Hiroshima Bay is the region with the most intense oyster farming and thus suitable to study the impact of these farms. Here, we surveyed spherical planktonic eggs of the black sea bream Acanthopagrus schlegelii, one of the most abundant fish in the Bay. Our survey was performed at fourteen stations which included places with oyster farms and historical spawning grounds. We found the highest egg densities in four stations, one with historical spawning aggregations and three with major oyster farms. Besides, surveys at the innermost part of Hiroshima Bay, where two major rivers discharge, showed a low density of eggs indicating that black sea bream avoids spawning in low salinity areas. Our study suggests that oyster farms benefit spawners of black sea bream by providing more food sources than historical spawning grounds for efficient spawning. Yet, whether oyster farms represent a full advantage for the species remains unclear, particularly because they are known to host jellyfishes that prey on eggs and limit water flow that can influence the survival of fish eggs.

Phylogeography of infectious disease: genetic diversity and connectivity of the Ostreid herpesvirus 1 population in France

The genetic diversity of viral populations is a key to understanding ther phylogeographic and dissemination history of viruses, but studying the diversity of whole genomes from natural populations remains a challenge. Molecular ecology approaches are commonly used for RNA viruses harboring small genomes, but have only rarely been applied to DNA viruses with large genomes. Here, we used the Pacific oyster mortality syndrome (POMS, a disease that affects oyster farms around the world) as a model to study the genetic diversity of its causative agent, the Ostreid herpesvirus 1 (OsHV-1) in the three main French oyster-farming areas. Using ultra-deep sequencing on individual moribund oysters and new bioinformatics methodology, we de novo assembled 21 OsHV-1 genomes. Combining whole-genome comparisons with phylogenetic analysis and quantification of major and minor variants, we assessed the connectivity of OsHV-1 viral populations between the three oyster-farming areas. Our results suggest that the Marennes-Oleron Bay represents the main source of OsHV-1 diversity, from where the virus has dispersed to other farming areas, a scenario consistent with current practices of oyster transfers in France. Here, we demonstrate that molecular ecology approaches can be applied to large-genome viruses to determine the extent of their genetic diversity and better understand the spread of viral populations in natural environments.

Investigating the impact of growth kinetics on geochemical records of oyster shells

<p>Sclerochronology provides valuable proxy data for investigating high-resolution paleoclimate dynamics at seasonal/(sub-)annual scale. Nevertheless, the interpretation of these proxy data is often hampered by the interplay between three main factors: (i) paleoenvironmental patterns; (ii) vital (physiological and kinetic) effects related to biomineralization pathways; and (iii) potential alteration during diagenetic modification of skeletal materials.</p><p>Because the interaction between environmental and metabolic factors is, at present, one of the most difficult to quantify, an ideal opportunity is brought forward to better understand the complexity of environment-metabolism interaction in marine biogenic carbonate archives. A project was tailored to investigate the impact of growth kinetics on geochemical proxy data (carbon and oxygen stable isotopes and main and trace elemental data) from oyster shells responding to changes in metabolism due to environmental fluctuations. Motivated by the exceptionally favourable circumstance that oyster farms are located near our Institution in Aveiro (Portugal), these will be used as a natural laboratory. Most interestingly, in Aveiro, oyster growth rates are significantly higher compared to those cultivated in France (Arcachon Bay). This is the case despite the fact, that the oysters grown in Aveiro are imported from France, and this will form the main study site of this project. In order to have a wider range of observational sites, a third oyster station in Southern Portugal (Olhão), influenced by warmer coastal waters will also be sampled. Finally, modern oyster specimen will be compared and contrasted with well-preserved ancient Crassostrea and Ostrea material in an attempt to bridge the gap between the Present and the Mesozoic.</p><p>State-of-the-art petrographic and geochemical research involving both modern and ancient oysters of the same genus will be performed. With reference to recent specimens, this must be performed in combination with a strict biological assessment of oyster metabolic performance, but with focus on carbonate archive research. An international Research Team (Portugal, Germany, France, Spain) was assembled, bringing together experts from a wide range of research fields, including Carbonate Geochemistry, Biomineralization, Sedimentology, Mineralogy, (Micro) Palaeontology, Sclerochronology, Biology, Ecology, Artificial Intelligence, and Data Modelling.</p><p>The goals of this project include: (i) establishing the link between modern environmental seasonal fluctuations, oyster growth rates and impact on the geochemical record of the shell, while additionally understanding non-linear responses (e.g., ontogenenetic evolution, effects of storms or other extreme events); (ii) compile information from a variety of proxies (bio-geochemical, petrographic, mineralogical, ecological), locations and times, aiming to test the best approaches for integration with a coherent framework; (iii) explore the link to ancient shell-archives, distinguishing between the various forcers of their geochemical signals, more specifically the interplay between paleoenvironmental conditions and vital effects.</p>

Oyster growth across a salinity gradient in a shallow, subtropical Gulf of Mexico estuary

An increase in oyster aquaculture as a sustainable method of shellfish production is one response to overharvest and degradation of natural oyster reefs over the past century. Successful aquaculture production requires determining the environmental conditions optimal for oyster growth. In this study, the salinity, temperature, chlorophyll a concentration and the growth of Crassostrea virginica were monitored at four locations within the Mission-Aransas Estuary, Texas (USA), a shallow subtropical estuary influenced by relatively low freshwater inflow. Mean growth of the oyster shell (0.205 mm d–1 and 0.203 g d–1) and soft tissues (3.447 mg d–1) was highest when salinity was low (mean = 15.5) and chlorophyll a concentration was high (8.4 μg l–1). Oyster growth also varied temporally with periods of spawning. In low-inflow estuaries such as the Mission-Aransas Estuary, oyster farms should be sited close to river mouths so that oysters can benefit from freshwater inflows and lower salinities.

Spatial epidemiological modelling of infection by Vibrio aestuarianus shows that connectivity and temperature control oyster mortality

Vibrio aestuarianus infection in oyster populations causes massive mortality, resulting in losses for oyster farmers. Such dynamics result from host-pathogen interactions and contagion through water-borne transmission. To assess the spatiotemporal spread of V. aestuarianus infection and associated oyster mortality at a bay scale, we built a mathematical model informed by experimental infection data at 2 temperatures and spatially dependent marine connectivity of oyster farms. We applied the model to a real system and tested the importance of each factor using a number of modelling scenarios. Results suggest that introducing V. aestuarianus in a fully susceptible adult oyster population in the bay would lead to the mortality of all farmed oysters over 6 to 12 mo, depending on the location in which infection was initiated. The effect of temperature was captured by the basic reproduction number (R0), which was >1 at high seawater temperatures, as opposed to values <1 at low temperatures. At the ecosystem scale, simulations showed the existence of long-distance dispersal of free-living bacteria. The western part of the bay could be reached by bacteria originating from the eastern side, though the spread time was greatly increased. Further developments of the model, including the consideration of the anthropogenic movements of oysters and oyster-specific sensitivity factors, would allow the development of accurate maps of epidemiological risks and help define aquaculture zoning.

Bonamiosis in Australasian aquaculture and in the New Zealand wild capture fishery.

In Australasia, bonamiosis (the disease caused by haplosporidian parasites of the genus Bonamia) was first reported from the New Zealand dredge fishery in the 1980s, and was subsequently reported from oyster farms in both Australia and New Zealand.In New Zealand, Foveaux Strait separates Stewart Island from the South Island. The strait was famous for the quality of the dredge oyster beds that had been fished continuously since the 1860s and were believed to have been carefully managed through a limited effort fishery and a quota.In 1985, fishers reported the occurrence of large numbers of freshly dead oysters on the western central beds. Subsequent examination by aquatic animal health specialists from the Research Division of the Ministry of Agriculture and Fisheries (now the National Institute of Water and Atmospheric Research, Ltd.) revealed that affected oysters were infected with a species of the haplosporidian parasite, Bonamia exitiosus. Monitoring of the beds in the strait over the next 8 years documented the decline in catch and eventually, in 1993, the fishery was closed. Because of the shortage of oysters and the high prices that were anticipated, a number of aquaculture ventures began in New Zealand during the 1990s, mostly using suspended tray culture. However, attempts failed due to the disease. Also in the 1990s there were attempts to farm Ostrea angasi in Australia. Outbreaks of bonamiosis occurred in oyster farms in Tasmania, Victoria and Western Australia. The ongoing mortalities have made farming this type of oyster uneconomic in Western Australia. However, some farming of O. angasi is undertaken in New South Wales where bonamiosis has not been reported. This is one of the few examples of a non-introduced disease agent destroying a wild fishery and aquaculture. Recent research has suggested that the impact of the fishery may have triggered the epizootic in Foveaux Strait as well as causing outbreaks in cultivated oysters. This has lessons for fisheries managers.