Morphology of glochidia and early developmental processes in Unio delicatus (Bivalvia: Unionidae)

In this study, embryonic development in the gonad and glochidia development in the gill marsupium of Unio delicatus (Bivalvia: Unionidae) were examined using morphological, visual, and statistical analyses. The study was conducted in Gölbaşı Lake (Hatay Province) from January to December 2024, with twenty individuals sampled monthly. Gonad and gill tissues were examined under a light microscope, and oocyte development and glochidia incubation were described. Oocyte diameters were measured using ImageJ and analyzed using descriptive statistics and one-way analysis of variance (ANOVA), supported by boxplot and heatmap visualizations. The results revealed a clear and statistically significant sea sonal reproductive pattern (p < 0.05). Oocyte development began in autumn, progressed through winter and spring, and peaked in late spring–early summer, followed by depletion of gonadal tissue in summer and the initiation of a new cycle. In the gill marsupium, glochidia development started in February, with mature glochidia observed in April and a release period in May–June; gills were empty by August. SEM observations indicated that hook-like structures at the shell apex function in attachment to host fish tissue. Overall, the findings demonstrate that gonadal embryogenesis and marsupial incubation in U. delicatus are functionally integrated processes forming a well-defined seasonal reproductive cycle, of fering new insights into the gonad–marsupium transition and supporting conservation strategies for freshwater mussels.