Seasonal reproductive dynamics of the freshwater mussel Unio delicatus (Bivalvia: Unionidae) in Lake Gölbaşı, Türkiye

This study investigates the reproductive characteristics of the freshwater mussel Unio delicatus Lea, 1863, with a focus on its gonadosomatic index (GSI). Samples were collected from Lake Gölbaşı (Hatay, Türkiye). To examine seasonal variation, approximately twenty adult individuals were randomly sampled each month between October 2023 and September 2024. Shell length, soft tissue weight, and gonad weight were measured to calculate GSI values. In addition, the relationship between water temperature and GSI was evaluated in relation to environmental conditions.The results indicate a clear seasonal pattern in reproductive activity. In females, GSI values reached their highest levels in February and March (14.65% and 15.22%) and declined during the summer months. Males showed a similar trend, with a peak in March (12.38%). The sex ratio remained close to balanced (1:1.03) and did not differ significantly (P > 0.05). A positive relationship between shell length and gonad weight (R² = 0.162) suggests that larger individuals allocate more energy to reproduction. In contrast, GSI values decreased with increasing water temperature, indicating that reproductive activity is more pronounced during colder periods.The findings show that environmental conditions, particularly temperature, play an important role in shaping the reproductive cycle of U. delicatus . Gonadal development appears to be most active from late winter to early spring. These results may also help in better understanding the species and could support future conservation efforts in freshwater ecosystems.