Exposure of larvae to daily thermocycles affects gonad development, sex ratio, and sexual steroids in Solea senegalensis, kaup.

The effect of water temperature during the development of fish larvae on sex differentiation is well known, but not so well known is the impact of the daily thermocycles. Our aim was to investigate the effect of early exposure of Senegal sole larvae to different temperature cycles on gonad development, sex ratio, and sex steroid (11-ketotestosterone (11-KT); estradiol (E(2) ); and testosterone, (T)) content in muscle extracts of juveniles. From 1 to 97 days posthatching (DPH) fish larvae and post-larvae were subjected to three temperature regimes: Thermophase-Cryophase (TC), Cryophase-Thermophase (CT), and constant temperature. In fish exposed to TC, sex determination occurred earlier, because 90% of soles were males/females at 110 DPH, whereas 45% of fish under CT were undifferentiated at that time. Fish under TC showed the highest growth rates, followed by fish under constant temperature and by fish under CT, the differences being statistically significant between the TC and CT groups. Regarding sex ratio, juveniles exposed to TC showed a higher proportion of females than fish under CT or constant temperature. Under TC, fish showed the highest concentration of E(2) , whereas 11-KT concentration was highest in fish under CT and constant temperature. Fish under constant temperature and CT showed higher T levels than those under TC. These results provide the first insights into the effect of daily thermocycles on sex differentiation in fish, and underline the key role of natural environmental cycles on the control of sex ratios during larval development, which may be applied to the manipulation of sex ratio in aquaculture.

Effects of continuous light on the reproductive system of European sea bass gauged by alterations of circadian variations during their first reproductive cycle.

The European sea bass is a short-day breeder, a characteristic that is highly valued in aquaculture. A high percentage of males of this species mature precociously before reaching commercial size, resulting in economic losses for fish farmers. We investigated the effects of continuous light (LL) on the circadian variations of several reproductive hormones in males of this species in order to understand how the presumed absence of the melatonin rhythm caused by LL affects their daily profile. The study was conducted during four critical stages of the sea bass reproductive cycle: pre-spermatogenesis (PSpg), spermatogenesis (Spg), spermiation (Spm), and post-spermiation (PSpm). Every 3 h during a complete 24 h cycle, six fish kept under a natural photoperiod (NP), and another six fish kept under LL were anaesthetized, measured, weighed, and bled. The pituitary was removed and frozen at -80 degrees C. The pituitary content of sea bream gonadotrophin-releasing (sbGnRH) and luteinizing hormone (LH), as well as plasma content of LH, testosterone, and 11-ketotestosterone (11-KT) were analyzed by ELISA. The percentage of spermiating males (precocity) per group was determined by periodic abdominal massages of the animals. Our results confirm that LL treatment, maintained from the early stages of development onward, effectively reduces the percentage of precocious male sea bass. As has already been described for caged sea bass, plasma LH showed a clearly marked nocturnal rise near midnight during Spg and Spm during NP, but which was absent under LL. Pituitary sbGnRH and LH content and plasma LH concentration, under both NP and LL, increased during the second half of the reproductive cycle, while sexual steroids were higher at the beginning of the cycle. LL inhibited steroid secretion, especially testosterone secretion, during Spg. In summary, without photoperiod cue, as accomplished by continuous exposure to LL, circadian variations of reproductive hormones appeared altered, causing irregularities in the reproductive process of male sea bass. These findings may have a practical application in aquaculture, namely by applying LL treatment in an effort to reduce the presence of precocious males in a stock.

Both pineal and lateral eyes are needed to sustain daily circulating melatonin rhythms in sea bass.

This study investigates the role of the pineal organ and lateral eyes (the two most important sources of melatonin in vertebrate species) on daily melatonin rhythms of sea bass, a fish exhibiting reversed melatonin profiles, as well as their contribution to circulating melatonin levels. To this aim, the pineal and/or the eyes were surgically removed (Exp. 1), the optic nerve sectioned and retinal dopaminergic neurons damaged with injections of 6-hydroxydopamine (Exp. 2), and the pineal or the eyes covered with aluminium foil (Exp. 3). The results show that plasma and ocular melatonin display opposing profiles. In Experiment 1, pinealectomized fish displayed lower nightly plasma melatonin levels (66+/-22 pg/ml) than intact or sham-operated groups (131+/-14 pg/ml), as it occurred in ophthalmectomized fish (64+/-12 pg/ml). Fish that were both pinealectomized and ophthalmectomized showed a further decrease in plasma melatonin levels (1.4+/-0.4 pg/ml), which approached daytime levels. In Experiment 2, plasma melatonin levels in both optic nerve-sectioned and ophthalmectomized fish were lower than control levels, while injection of 6-hydroxydopamine did not modify plasma melatonin concentrations. In Experiment 3, covering only the pineal made melatonin drop after a light pulse at MD, and covering only the eyes had a similar effect. In conclusion, these findings suggest that even though sea bass eyes do not directly contribute to plasma melatonin, the pineal organ, which unlike that of mammals is a direct photoreceptor in fish, requires light information from the lateral eyes to normally secrete melatonin into the bloodstream.