DAO prepress abstract   -  DOI: https://doi.org/10.3354/dao03756

ABSTRACT: A huge, unprecedented cockle Cerastoderma edule mortality caused by the protist Marteilia cochillia, which had never been detected in Galicia (NW Spain), brought on a cockle fishery collapse in the Ría de Arousa (Galicia) in 2012. Since then, the disease dynamic pattern in the shellfish bed of Lombos do Ulla (at the inner area of that ria) involved an overwhelming annual wave of infections and subsequent cockle mass mortality that caused the near extinction of every cohort recruited to that bed. However, a pattern shift was detected among wild cohorts recruiting since 2016, with progressive declines of marteiliosis prevalence and increments in cockle survival. It suggested two non-exclusive hypotheses, increasing marteiliosis resistance through natural selection and reduced abundance and/or virulence of the parasite. A field experiment was performed to assess those hypotheses by comparing marteiliosis prevalences and severities, as well as mortalities, in cockles that naturally recruited to this bed in 2017 and 2018 with those of naïve cockles collected from a marteiliosis-free area and transplanted into Lombos do Ulla in 2017 and 2018. The marteiliosis prevalence and the cumulative mortality quickly reached very high values among the transplanted cockles, demonstrating that the parasite remained present and virulent in the area. Conversely, marteiliosis prevalence and mortality were much lower in the cockles that recruited to Lombos do Ulla, suggesting increased resistance that may have been driven by natural selection. The young age at which cockles start reproduction and the very high mortality caused by marteiliosis may have enhanced natural selection.