The histological characteristics of oocyte atresia were examined in the Manila clam Tapes philippinarum (Adams and Reeve, 1850), at Croisic Traict on the French Atlantic coast, over 4- and 5-month gametogenic periods (May to August 2015 and April to September 2016). Atresia was found at all stages of gametogenesis, as well as in residual oocytes, and was characterized by several characteristics: loss of the nucleolus, nuclear degradation, altered cytoplasmic staining affinities, cytoplasmic retraction, and finally the loss of all cellular content. Histological observations indicated that T. philippinarum partially spawned repeatedly over the gametogenic period. Stereological counts showed that at least 15% of the oocyte volume was occupied by atresic oocytes (AO) at the onset of gametogenesis before any spawning activity; this increased to 30% in the middle of the gametogenic period (including both pre- and postspawning oocytes) and 80% at the end of the gametogenic period (postspawning atresia). Of all oocytes whose fate could be determined during active gametogenesis, nearly half were atresic. Similar observations were made for smaller sample sizes of clams from two other sites in nearby Bourgneuf Bay over a 26-mo period. Both AO and nonatresic oocytes were observed in the same gonad acini, suggesting that the process was either not propagated or not synchronized. The considerable proportion of oocytes affected by atresia underscores the need for better recognition, documentation, and integration of this process into models of reproductive effort and fecundity in this species. In particular, condition indices based on tissue : shell weights should be interpreted as estimations of reproductive investment, not as indications of potential reproductive outcome.
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