In an effort to elucidate the causes of early life stage mortality, the histological characteristics of oocyte atresia were examined biweekly in the European common cockle Cerastoderma edule (Linnaeus, 1758) over an 11-mo-period (January–November 2018), at a farmed site on the French Atlantic coast. Gametogenesis was continuous at the population level, with no apparent interindividual synchronicity. Atresia was observed throughout the year, at all stages of oogenesis, characterized by loss of the nucleolus, nuclear and chromatin degradation, and angular cell shape. Both atresic and nonatresic oocytes were observed in the same gonad acini, suggesting that the process was either not propagated or not synchronized. Stereological counts showed that atresic oocytes occupied an annual mean of 30% (range 12%–47%) of the oocyte volume. Estimation of the minimum atresic impact showed that more than 50% of the oocytes, whose fate can be determined from histological sections, were or would become atresic, reducing the fecundity accordingly. Together with previously reported results in other bivalve species, this underscores the need for better recognition, documentation, and integration of this process into models of fecundity, reproductive effort, population dynamics, and production.