Characterization of the bacterium Francisella halioticida isolated from the blue mussel Mytilus sp.

Shellfish farming is a key economic activity along the French coastline, but in recent years it has faced major challenges due to significant and recurring mortality events, particularly affecting oysters and mussels. Unusual mortality in mussels (Mytilus edulis) was first reported in the Charente region in 2014, before spreading to Pays de la Loire, Brittany, and more recently to Normandy and Hauts-de-France. The causes of these mortality events remain undetermined, but the bacterium Francisella halioticida, detected for the first time in France, appears to be a promising lead. This bacterium has previously been identified as responsible for mortality in abalone (Haliotis gigantea) in Japan, and in the Japanese scallop (Pectinopecten yessoensis) in both Japan and Canada. Other bivalve species have also experienced mortality events in France, such as the Manila clam (Ruditapes philippinarum) in autumn 2016 and the great scallop (Pecten maximus) in 2018.

Disease management in mollusks is particularly complex due to the absence of acquired immunity in these organisms and their direct exposure to the surrounding environment. In the absence of effective preventive strategies or disease management plans, the future of the shellfish farming sector in Normandy is at risk. It is therefore essential to understand, explain, and attempt to control these epidemics by developing reliable diagnostic tools and optimized management methods to ensure the biosecurity of bivalve mollusks.

This PhD project is based on two main objectives:

1. Enhance scientific knowledge on diseases affecting bivalve mollusks.
   A priority will be given to studying the pathogenicity of F. halioticida in mussels (M. edulis) under controlled laboratory conditions. The host-pathogen relationship will be investigated using transcriptomic, proteomic, and peptidomic approaches to gain deeper insights into the infection mechanisms and the host’s immune response. A retrospective analysis of diagnostic tools used during past mortality events will also be conducted to identify possible improvements.

2. Establish a prevention and management strategy, and potentially control measures against these diseases.
   Knowledge gained from the first objective could lead to the development of rapid and reliable diagnostic tools. This could also support the creation of a regional surveillance network for shellfish diseases in Normandy.