Two enigmas may solve each other: the oocyte coat and atresia in the common cockle, Cerastoderma edule (Linnaeus, 1758)

Two co-occurring, enigmatic aspects of bivalve reproduction were investigated in the common cockle Cerastoderma edule: the oocyte coat and oocyte atresia. Qualitative histology and transmission electron microscopy (TEM) of cockles collected on the French Atlantic coast revealed not only the fine structure of the oocyte coat, but also confirmed that it is secreted by the oocyte itself and composed of acid mucopolysaccharides (AMPS), known to be viscous and adhesive. Quantitative histology showed that at the peak of oogenesis, oocyte coats occupy the largest fraction (approx. 40%) of the gonad acinal volume, representing both a significant sacrifice of female gamete capacity, and a non-gamete energetic investment. Potential benefits of the coat include protection from mechanical abrasion, predation, and opportunistic microbes. Atresia (oocyte degeneration) was a known second source of reduced fecundity, with a minimum impact of approximately 50% of the total oocyte volume. It is suggested that this high proportion of atresic oocytes is related to the previously-documented genetic inviability of early post-fertilization life stages. The qualitative histological and TEM observations revealed atresic debris adhering to the exterior surface of the oocyte coats. Such an arrangement would isolate adjacent oocyte coats, enabling the oocytes to be spawned individually, rather than as an egg mass, and therefore to undergo planktonic development and dispersion. Oocyte atresia and the oocyte coat of Cerastoderma edule therefore appear to be linked in the first indication of an adaptive function in bivalves.