Gametogenesis, spawning behavior and larval abundance of the Pacific oyster Crassostrea gigas in the Thau lagoon: Evidence of an environment-dependent strategy

International audienceThe Thau lagoon, located at the South of France, provides 10% of the French pacific oyster Crassostrea gigas production. Despite this intensive shellfish production, the larval recruitment of this species within the Thau lagoon is still misunderstood and spat collection remains highly variable; as a consequence, shellfish farmers rely mostly on hatchery”s spat. The present study aimed to describe reproduction features of C. gigas within the Mediterranean, as well as to explore spawning triggers of this species. To achieve this goal, the reproduction cycle from gametogenesis to spawning was followed during two years in several locations within the Mediterranean Thau lagoon. The spawning behavior of C. gigas within the Thau lagoon appeared to be slightly different from the one of the Atlantic Coast, showing several spawning events from June to October. The minimal temperature for massive spawning was 23 °C, which is much higher than temperatures mentioned in the literature for this species. A strong relationship was found between phytoplankton communities and gametogenesis, the latter being improved by a higher diatom/dinoflagellate ratio. Temperature was insufficient alone to explain spawning, highlighting other potential triggers such as the moon or lightning strikes. We made the hypothesis of a hierarchy of events triggering spawning, encompassing locking, synchrony and intensifying triggers. Statement of relevance This paper is filling in a gap in the literature regarding to reproduction of C. gigas in the Thau lagoon, which is always associated to temperature. However, temperature may not be the only factor involved in oyster reproduction, that is what we tried to show in this paper