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This work focused on the reproductive cycle and cleaning process of follicles in Mytilus galloprovincialis and aimed to extend knowledge of the reproductive cycles of Mytilidae. Biometric and histological measurements were taken monthly over 12 months from mussels at a polluted site, the port of Oran in Algeria. Environmental parameters were monitored concomitantly. Mytilus galloprovincialis reproduced throughout the year, with a main spawning period between November and February and a second between March and June. Several follicle cleaning processes were observed throughout the reproductive cycle. They occurred under two circumstances. First, in the absence of reserve tissues, mature gametes were degraded. This happened when spawning was about to end and corresponded to the last stage of reproduction. Second, atresia, gamete degeneration and a cessation of spawning occurred whatever the stage of the gonad development and whatever the environmental parameter values. These disturbances of reproduction may have resulted from pollution in the port of Oran and increased when temperature exceeded critical thresholds for gametogenesis. To conclude, gamete degeneration and spawning cessation because of coastal pollution and global warming could threaten M. galloprovincialis recruitment, and ultimately the shellfish economy, and could distort biomonitoring strategies using mussels.