Reproductive cycle of invasive alien bivalve Brachidontes pharaonis (Fischer, 1870) in the Gulf of İzmir, Aegean Sea

Context The invasive Lessepsian bivalve Brachidontes pharaonis (P.Fischer, 1870) heavily aggregates on longline mussel farms in the Gulf of İzmir, causing spatial and nutritional competition on the commercially cultivated native mussel Mytilus galloprovincialis. Aims This study evaluated the annual gonad development, gametogenic cycle, and environmental drivers of B. pharaonis to inform management strategies for regional bivalve aquaculture. Methods Monthly sampling was conducted at a commercial longline mussel farm in the Gulf of İzmir between April 2025 and March 2026. Gonad tissues were examined histologically and gonad index (GI) values were correlated with environmental parameters. Key results B. pharaonis exhibited extended reproductive activity throughout the year, peaking in July and September. However, gametogenesis significantly ceased in December, when 90% of the population was in the resting or spent stage. A strong positive correlation was determined between GI values and seawater temperature (r = 0.812; P < 0.01). Female-to-male ratio was 1♀:1.22♂ throughout the study. Conclusions Low winter seawater temperatures limit the reproductive window of B. pharaonis in the northern Mediterranean, resulting in a mandatory winter reproductive resting phase. Implications Shifting mussel spat collector deployment to the December–January period would allow farmers to selectively capture native M. galloprovincialis spats, while effectively avoiding B. pharaonis aggregations.