“Influence of short-to-continuous and continuous photoperiods combined with elevated temperatures on sexual maturation in lumpfish (Cyclopterus lumpus, L. 1758)”

We tested the photothermal effects on sexual maturation in the broodstock of lumpfish (Cyclopterus lumpus) using short-to-continuous (LDL) and continuous (LL) photoperiods for 24 weeks, with temperature elevation during final maturation in females. The lumpfish sexual maturation was phased differently between the two photoperiods, while temperature elevation enhanced the photoperiod changes. Female gonadosomatic index (GSI), and plasma levels of testosterone (T), 11-ketotestosterone (11-KT) and 17β-estradiol (E2) increased under LDL, and temperature elevation enhanced the sex steroid levels in both photoperiods. While the brain expressions of gnrh2 decreased in females exposed to LDL compared to LL groups, the ovarian expressions of fshr, lhr, vtgr, and cyp19a1 changed significantly at ambient temperatures in LL and LDL groups and at elevated temperature in the LL group. A high proportion of males were sexually mature from the start of the experiment, and a prolonged period with sexually mature males was observed in the ambient temperature LL group. This was revealed by elevated GSI, T, 11-KT, as well as brain expression of gnrh2 at the start of the experiment, with the highest presence of spermatozoa, and pronounced fluctuations in T and 11-KT throughout the experiment with significant changes observed only in LL0T. Brain expressions of gnrh2 and testicular expressions of fshr and lhr declined more rapidly in LDL compared to LL groups. With temperature elevation, gnrh2 expressions were enhanced in both LL and LDL. Despite exposure to different photothermal regimes, gonadal development and selected endocrine factors remained stage-coherent. Ovarian expressions of fshr, lhr, vtgr, cyp19a1 and E2 levels increased up to oocyte maturation and decreased at ovulation, while GSI, T and 11-KT increased until ovulation. Testicular fshr and lhr expressions peaked at spermatocyte stage and decreased subsequently, while plasma levels of T and 11-KT increased until the spermatid stage and decreased at the spermatozoa stage. In this study, we show that while photoperiods phase lumpfish sexual maturation, temperature has an important role in cueing the final stages leading to spawning. Additionally, the gonadal development and endocrine factors remain stage-coherent though at different rates in different photothermal regimes.