Why are oysters sick more and more often?

Why are oysters sick more and more often?

Violet Field inPlanetDec 31, 2020 7 min read1 views

Discover, every day, an analysis of our partner The Conversation. Today, a researcher in parasitology explains to us why oyster production is threatened today

Hollow oyster, “Crassostrea gigas” - © S.Pouvreau & Ifremer, CC BY-NC-ND
  • Oysters are valuable “sentinels” that reveal imbalances in coastal ecosystems, according to a study published by our partner The Conversation.
  • It is therefore worrying that their populations have been victims of abnormally high mortalities since 2005.
  • The analysis of this phenomenon was carried out by Isabelle Azul, the doctor in parasitology at the French Institute for Research for the Exploitation of the Sea (Ifremer).

Abnormal shellfish mortalities have been observed around the world for about fifteen years now.

In 2008, a particular genotype of the OsHV-1 virus thus appeared; it seems to be at the origin of episodes of massive mortalities affecting young cupped oysters in Europe and particularly in France.

From 2012, adult cupped oysters show mortalities associated with Vibrio aestuarianus bacteria in French oyster farming areas.

Monitoring these diseases, understanding them better, and even anticipating their emergence is a necessity not only to maintain sustainable production but also because these animals are sentinels that reveal imbalances in coastal ecosystems.

The flat oyster Ostrea edulis creating such mini-submarine reefs that support and shelter many other species. Abundant in coastal aquatic ecosystems, shellfish play an essential role in their structure and function.

We must also obviously mention the importance of shellfish farming, which accounts for more than 20% of world aquaculture production. In France, the production of oysters ( oyster farming ) represented, in 2013, 72% of the turnover of the shellfish industry. The Hexagon is the leading producer of oysters in Europe.

A set of imbalances

An imbalance of interactions can favor the appearance of diseases © CC BY-NC-ND

The presence of a pathogenic organism is not always synonymous with disease and mortality in shellfish. In fact, pathogenic organisms tend to develop and induce mortalities when there is an imbalance in the interactions between shellfish, pathogenic organisms, the environment, and cultural practices; this imbalance favors the emergence of diseases.

Because of their mode of production - most often in an open environment - and the lack of antibody production, disease control in shellfish cannot be based on the use of treatments or vaccines. The implementation of surveillance programs and the restriction of animal transfers are the only measures currently available to prevent the introduction of diseases into free areas.

Once installed, it is always possible to minimize their impact on shellfish populations, by proposing appropriate stock management measures and/or by developing disease-resistant animal selection programs.

In 2016, a large European scientific program ( Vivaldi ) was launched in 10 countries to improve our knowledge of shellfish diseases and develop tools to better control their effects.

Monitoring of cupped oysters to study the impact of environmental factors and the presence of different species of macroalgae © Ifremer - CC BY-NC-ND

Identify the "culprits"

The identification and distinction of species of pathogenic microorganisms are necessary to adjust the methods of detection and control of associated diseases.

For a long time, this identification relied on microscopy. For the past twenty years, the use of molecular tools, such as PCR and DNA sequencing, has made it possible to characterize new species and reveal a greater diversity of pathogenic microorganisms in shellfish.

The OSHA-1 virus mentioned above is a member of the herpes virus family that has so far only been detected in bivalves - cupped oysters, flat oysters, clams, and scallops. The complete genome of several OsHV-1 viruses present in cupped oysters and originating from different regions of the world has been sequenced.

Our “Oysters” file

This study (which has not yet been published) shows differences depending on the geographical origin of the samples. She confirms that there was not just one OsHV-1 virus but a "constellation of viruses" within an infected oyster.

Another problem: the “reservoirs”, that is to say, the “compartments” where pathogenic organisms can be present apart from the shellfish. It can be water, sediment, or other marine organisms. Other work carried out in the harbor of Brest has, for example, shown that the parasite Marteilia refringens, which affects the flat oyster, is present in the sediment and water.

Until now, the diagnostic approach has been based on the search for pathogenic organisms in shellfish. Then systems making it possible to detect their presence in water, in particular before the onset of infection, were developed. The fact of being able to detect the OsHV-1 virus in water before the episodes of oyster mortality give us the possibility of using such tools for the early detection of the virus.

Observation of oyster cells by epifluorescence microscopy © Ifremer - CC BY-NC-ND

More resistant individuals

Like mammals, shellfish defend themselves to prevent the development of the disease. The work carried out on the cupped oyster has notably made it possible to show the involvement of autophagy, a mechanism of degradation and recycling of intracellular components, in the oyster's response to the OsHV-1 virus.

Within the same population, some individuals are particularly sensitive to pathogenic organisms and others prove to be more resistant. Scientists have studied the genes that could explain this difference in the cupped oyster.

Identifying key genes could help understand how oyster populations cope with the disease and thus select more resistant animals. However, this selection must not be made to the detriment of genetic diversity or other interesting characteristics (size of the oysters, taste qualities, etc.).

Our "Biodiversity" dossier

Numerical simulations made it possible to define good practices to be implemented in hatcheries, in order to limit the loss of genetic diversity.

Immune memory

Contrary to popular belief, mollusks have a sort of immune memory. This is in the form of patterns in proteins, capable of recognizing pathogenic organisms with which the animal has already been confronted, and of triggering defense mechanisms. Work has made it possible to study ways of stimulating this response.

Oysters seem to defend themselves better against the OsHV-1 virus when they have been previously exposed to a molecule resembling the virus. This phenomenon, called "priming", could pave the way for forms of immunostimulation. Better yet: this ability could be transferable. Initial results seem to show that the offspring of oysters brought into contact with these “stimulating” molecules survive a viral infection better, even if they have never been confronted with it.

Experimental infection of cupped oysters © S. Lesbats & Ifremer - CC BY-NC-ND

The determining role of the environment

The environment plays an essential role in the emergence of diseases in marine molluscs; and the effects of temperature, salinity, acidity, nutrients, and cohabitation with other species were studied.

It has thus been shown that above 29 ° C., the OsHV-1 virus no longer causes mortality in cupped oysters. On the other hand, the pH of seawater does not seem to have an impact on the ability of the virus to induce infection.

In addition, cohabitation with competing species, such as mussels or ascidians, seems to be beneficial for the cupped oyster. Several mechanisms can explain this phenomenon, including competition for food. The oyster has fewer nutrients available, which reduces its development, and may decrease the multiplication of the virus. Additional work is underway to better understand these results.

The microbiota - that is to say all the microorganisms naturally present - in shellfish has also been explored for several years using new sequencing tools. The structure of microbial communities seems to vary depending on the species, their habitat but also the season. The microbiota is unique to each individual and even to each organ. Interestingly, an imbalance of the oyster microbiota has been observed during mortality events: a decrease in microbial diversity is then reported.

All of these observations motivate scientists to continue their research in order to identify profiles indicative of good shellfish health or, on the contrary, indicative of dysfunction.

This analysis was written by Isabelle Arzul, the doctor in parasitology at the French Institute of Research for the Exploitation of the Sea (Ifremer). The original article was published on The Conversation website.