Photoperiod control of sexual maturation of the Atlantic halibut (Hippoglossus hippoglossus): plasma thyroid hormone and calcium levels

Three groups of Atlantic halibut (Hippoglossus hippoglossus) were exposed to simulated natural (n = 225), four-month advanced (n = 225) and four-month delayed (n = 40) annual photoperiod cycles. Of these, 30, 30 and 15 tagged individuals, respectively, were studied for sexual maturation and sampled for blood every four weeks for over three years to obtain repeated annual maturational profiles, plasma thyroid hormone and calcium profiles. Most tagged individuals (28, 24 and 12, respectively) became sexually mature during the study and the male:female ratio was found to be 1:3. The spawning of the control group took place during March, April and May. The photoperiod manipulation significantly shifted the timing of spawning already in the second spawning season, and after three years, spawning of the advanced and delayed groups commenced 114 days before and 130 days after the control group, respectively, in close concordance with the shift in annual photoperiod cycles by 120 days. A distinct annual profile in total plasma calcium levels, reflecting changes in circulating vitellogenin levels, was seen in spawning females in all treatments with highest levels of 4.5 mM observed one to two months prior to spawning and lowest levels of 2.8. mM about a month after the end of the spawning period. Distinctly different patterns of circulating thyroid hormones (TH) were observed in relation to spawning, depending on sex. Under the simulated natural annual cycle photoperiod, male halibut exhibited autumn-winter peaks in plasma triiodothyronine (T3) and thyroxine (T4) (23 ng T3/ml and 28 ng T4/ml) which declined to an annual low-point in late spring (7 ng T3/ml and 12 ng T4/ml), around the time of spawning. The T3:T4 ratio was fairly consistent around one throughout the year. In contrast, females had generally high circulating T3 levels (32 ng T3/ml) and low T4 levels (< 10 ng T4/ml), hence, T3:T4 ratios exceeding 3:1 during the months prior to spawning. This relationship was also present in females on advanced and delayed annual photoperiod cycles, suggesting involvement of T3 in female reproductive processes, possibly by way of increased peripheral monodeiodination of T4.