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The intertidal zone is characterized by a sharp vertical gradient of environmental stress, which structures species distribution and their interactions. Few studies, however, have examined the influence of tidal height on host–pathogen interactions. Here, we investigated how the tidal height influence outbreak of the Ostreid herpesvirus type 1 (OsHV-1) affecting the Pacific oyster. A volunteer network composed of 20 oyster growers monitored the survival of 28 batches of oysters during an epizootic event in Southern Brittany, France. Oysters were spat from wild collection or hatchery production. The sampling sites were spread over a 150-km2 area with a tidal height ranging from 0.98 to 2.90 m. Concomitantly, we followed survival of oyster spats in relation with OsHV-1 DNA detection at two sites and conducted risk analysis. We found that tidal height was associated with a lower risk of mortality. This effect was higher for hatchery than for wild oysters probably reflecting differences in health status. Our study opens perspectives for mitigation strategies based on tidal height and emphasizes the value of volunteer science in marine epidemiological studies. [ABSTRACT FROM AUTHOR]

Green seawater discolorations caused by the marine dinoflagellate Lepidodinium chlorophorum are frequently observed during the summer along the southern coast of Brittany, France. Although there is no evidence that L. chlorophorum produces toxins, green seawater discolorations are related to mortalities of filter feeding animals. Here, we investigate the ecophysiological response of the Pacific oyster Crassostrea gigas exposed to L. chlorophorum. Oysters were exposed for 48 h to a low concentration (500 cells mL−1) and a bloom concentration (7500 cells mL−1) of L. chlorophorum and compared to controls fed with the haptophyte Tisochrysis lutea. The direct consequences of L. chlorophorum exposure were assessed through semiquantitative histochemical analysis. Thereafter, a 24 h-recovery phase with a diet based on T. lutea was studied using an individual ecophysiological measurement system. We found that oysters successfully filtered L. chlorophorum cells with increased mucus secretion in all tissues analyzed. Animals previously exposed to a bloom concentration of L. chlorophorum exhibited significantly lower clearance rates shortly after the exposition, probably reflecting the effect of exopolymer particles produced by L. chlorophorum cells, and the subsequent production of mucus in the mantle and gills. However, their absorption efficiency was two-fold higher than control oysters exposed to T. lutea. The increase in absorption efficiency during the recovery phase could be a compensation for a decrease of clearance rate occurred during the exposure phase, interpreted as physiological depletion of C. gigas during green seawater discolorations. For the first time, laboratory experiments showed how high concentrations of L. chlorophorum can affect oyster physiology. • First evidence of the ingestion of L. chlorophorum by the oyster C. gigas. • L. chlorophorum induced mucus production with decrease in clearance rate in oyster. • L. chlorophorum had a negative impact on oyster energy reserves. • Exopolymers produced by L. chlorophorum may cause inflammatory response in oyster. [ABSTRACT FROM AUTHOR]

We used a compiled data set from a monitoring network of oyster production coordinated by IFREMER (the French Research Institute for the Exploitation of the Sea). This network monitors the growth and mortality of the Pacific oyster Crassostrea gigas along French coasts since 1993. The archive, although publicly available, has been challenging to use due to changes in protocols and little information on metadata. Here, we describe data collection for almost 30 years, cleaning and processing. For 13 locations, we modeled growth and mortality of spat (less than one-year-old individuals) and half-grown oysters (between one and two-year-old individuals) as a function of time to cope with changes in data acquisition frequency, and produced standardized annual growth and cumulative mortality indicators to improve data usability. This improved database is expected to be used by ecologists interested in the evolution of life-cycle indicators of a marine species under the influence of climate change. It can also be valuable for epidemiologists because mortality data traces the emergence and spread of a massive epizootic., (© 2022. The Author(s).)

Of all environmental factors, seawater temperature plays a decisive role in triggering marine diseases. Like fever in vertebrates, high seawater temperature could modulate the host response to pathogens in ectothermic animals. In France, massive mortality of Pacific oysters, Crassostrea gigas , caused by the ostreid herpesvirus 1 (OsHV-1) is markedly reduced when temperatures exceed 24°C in the field. In the present study we assess how high temperature influences the host response to the pathogen by comparing transcriptomes (RNA sequencing) during the course of experimental infection at 21°C (reference) and 29°C. We show that high temperature induced host physiological processes that are unfavorable to the viral infection. Temperature influenced the expression of transcripts related to the immune process and increased the transcription of genes related to the apoptotic process, synaptic signaling and protein processes at 29°C. Concomitantly, the expression of genes associated with catabolism, metabolite transport, macromolecule synthesis and cell growth remained low from the first stage of infection at 29°C. Moreover, viral entry into the host might have been limited at 29°C by changes in extracellular matrix composition and protein abundance. Overall, these results provide new insights into how environmental factors modulate host-pathogen interactions., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2020. Published by The Company of Biologists Ltd.)