Natural vs hatchery-based selection in common cockle Cerastoderma edule for resilience to Marteilia cochillia

The common cockle ( Cerastoderma edule ) is an ecologically and socioeconomically important bivalve species along the northeast Atlantic coast. Since 2012, cockle beds in Galicia (NW Spain) suffered severe declines due to Marteilia cochillia , particularly in Ría de Arousa. A remarkable recovery in 2018, suggested cockle resilience acquired through natural selection. In this study, we conducted a common garden experiment to check the performance of a 3rd-generation hatchery-selected strain (SS) for parasite resistance, compared with a long-term affected wild strain from Ría de Arousa (AS), and a naïve strain (NS) as control from Ría de Muros-Noia. Offspring from the three origins were obtained using mass spawning of ∼400 breeders per strain. After an outdoor-nursery stage, cockles were exposed to a marteiliosis outbreak in two shellfish beds of Ría de Arousa, Ariño and Sarrido. Mortality, prevalence and infection severity were monitored through the growing-out stage. Forty-seven SNPs previously associated with resilience to M . cochillia were genotyped in 30–50 individuals per strain and sample (before and after exposure) to assess their usefulness for marker-assisted selection. NS cockles rapidly reached high prevalence (above 70%) and massively died in both beds, whereas SS and AS displayed similar lower prevalence (always below 40% in Ariño and below 20% in Sarrido) and mean cumulative mortality (71% both strains in Ariño and 66% for SS cockles and 52% for AS cockles in Sarrido). SS showed the lowest genetic diversity, probably due to unequal parental contribution during mass spawning across generations. Moreover, 16 SNPs consistently differentiated AS/SS from NS, confirming their association with resilience. These findings support the development of a single multiplex SNP-array for assisted selection, avoiding parasite exposure in each generation. The selected strains could contribute to the recovery of newly affected beds reported in Galician rías.