Response to selection for increasing resistance to the spring mortality outbreaks in Mytilus edulis occurring in France since 2014

Abstract

Massive spring mortality outbreaks have been reported in Mytilus spp. in France since 2014. The main areas affected are the Pertuis Charentais Sounds and the coast of the Pays de la Loire, which cultivate mainly M. edulis, and the putative causal agents remain unknown. We report the results of the first generation of mass selection focused on survival and resistance to the spring mortality in M. edulis. Two stocks of mussels were sampled in the Pertuis Charentais Sounds in 2014; one of these went through a spring mortality outbreak and the survivors were used to produce the selected stock, while the second stock was M. edulis considered to be naïve against the putative causal agents of the mortality outbreak and was used to produce the control stock. A first cohort was produced in 2015 and tested at one site in the Pertuis Charentais Sounds. In April and May 2016, a spring mortality outbreak was observed when seawater temperature ranged from 10 to 15 °C. In October 2016, the selected stock had a lower mortality (44%) than the control stock (78%). To confirm this result and investigate genotype by environment interaction, a second cohort was produced in 2016 and tested at six sites. Although no significant mortality was reported at the four sites in the Pertuis Charentais Sounds in 2017, the selected stock had a lower mortality (17-27%) than the control stock (61-74%) at the two sites along the coast of the Pays de la Loire. The survival had increased of 34-48% after one generation of mass selection and the realized heritability was high, ranging from 0.55 to 1.15, but further generations of selection are required to obtain a better estimation of the heritability. Our study showed a positive response to selection in three environments that were different to the site from which mussels were selected, suggesting the absence of strong genotype by environment interaction in sites where spring mortality outbreaks occurred. Selection to enhance M. edulis survival of the spring mortality should be efficient and should be capable of easy implementation through mass selection. Apparently, selection to enhance the survival of M. edulis did not affect the mussel growth in comparison with the controls, but automatically improved the yield of M. edulis in sites affected by the spring mortality outbreaks. Highlights ► Mass selection to enhance the resistance to the spring mortality outbreak in M. edulis was successful. ► Significant genetic improvement after one generation of selection (+34-48%) ► No significant strong genotype by environment interaction ► Most of the mortality occurred at temperature ranging from 10 to 15 °C

High mortality of mussels in northern Brittany – Evaluation of the involvement of pathogens, pathological conditions and pollutants

“…Results showed that a 'mortality window' opened in all sites during the spring season; similar observations also have been reported recently in France by other researchers (Benabdelmouna et al, 2018;Dégremont et al, 2019). This period corresponded to the spawning season of mussels.…”

“…Similarly, Lokmer & Wegener (2015) argued that, in addition to the presence of pathogens, environmental factors have a strong impact on disease efficacy and mortality. In addition, it seems that genetics could also have an impact on mortality (Dickie et al, 1984;Mallet et al, 1987;Dégremont et al, 2019).…”

Experimental infection of Mytilus edulis by two Vibrio splendidus‐related strains: Determination of pathogenicity level of strains and influence of the origin and annual cycle of mussels on their sensitivity

“…Mallet and Haley (1984) indicated that mortality variation from one origin to another evidences the differences between populations of M. edulis; this is also what Dickie et al (1984) observed thanks to the significant differences in genetic constitution among the stock used in their experiments. Following this, Mallet et al (1986) showed that viability is highly heritable in M. edulis populations, and very recently, Dégremont, Maurouard, Rabiller, and Glize (2019) revealed that selected populations of French M. edulis were significantly more resistant to mortality outbreaks than natural unselected populations. Mytilus edulis and M. galloprovincialis are native species to the French coast, and numerous different natural populations exist and are capable to hybridize with each other.…”

Observations on recent mass mortality events of marine mussels in the Oosterschelde, the Netherlands

“…Further experimental evidence for a genetic base comes from a selective breeding experiment of mussels for increasing resistance to the spring mortality. Progeny of survivors showed less mortality in the following outbreak, indicating a significant genetic improvement for the selected trait, while the putative agent remains still unknown (Dégremont et al 2019). We do not know the extent to which genetic factors affected the mortality outbreaks in the Oosterschelde.…”

“…MMEs can severely affect the commercial production of bivalve species and give rise to socio-economic vulnerabilities (Krause et al 2019). Since 2014, MMEs of blue mussel stocks have been reported on a nearly yearly basis from the west coast of France (Dégremont et al 2019). Mussels are an important aquaculture product in France and tools for reducing the economic consequence are limited (Avdelas et al 2021).…”