Photoperiod-Mediated Synchronization of Reproductive Hormones, Follicular Development and Calcium Homeostasis in Common Carp (Cyprinus carpio)

Abstract

Photoperiod is a major environmental regulator of biological rhythms and reproductive processes in teleost fishes. The present study evaluates the effects of altered light–dark regimes on neuroendocrine activity, ovarian development, and calcium homeostasis in the Indian major carp Cyprinus carpio. Adult fish were maintained for 120 days under five photoperiodic conditions: control (L12:D12), long photoperiod (L16:D8), short photoperiod (L6:D18), constant light (LL), and constant darkness (DD). Circulating levels of estradiol, progesterone, follicle stimulating hormone (FSH), luteinizing hormone (LH), and growth hormone (GH) were assessed in relation to gonadosomatic index (GSI), ovarian follicular dynamics, and plasma calcium levels. Fish exposed to the long photoperiod (L16:D8) exhibited significantly elevated estradiol and progesterone levels, increased GSI, and a higher proportion of vitellogenic follicles, indicating enhanced ovarian maturation and reproductive activity. In contrast, short photoperiod and continuous darkness resulted in suppressed gonadotropin secretion, reduced GSI, and increased follicular atresia, reflecting inhibition of the reproductive axis. Constant light and constant darkness disrupted endocrine balance and reproductive homeostasis, suggesting circadian desynchronization under extreme photic conditions. Variations in plasma calcium levels across photoperiod treatments further highlight the association between light regulation, endocrine function, and mineral metabolism. These findings demonstrate that appropriate photoperiod manipulation, particularly long-day regimes, can effectively enhance reproductive performance in C. carpio. The study provides practical insights for the application of chronobiological principles in aquaculture to improve broodstock management and breeding efficiency.