Influence of the lunar cycle on plasma melatonin, vitellogenin and sex steroids rhythms in Senegal sole, Solea senegalensis

The effect of the moon light cycle on plasma melatonin rhythms was examined in Senegal sole (Solea senegalensis) exposed to natural outdoor or artificial indoor lighting conditions. Furthermore, in a second experiment, the effect of the lunar cycle on vitellogenin and sex steroids (Testosterone, T; Estradiol, E2; 11-ketotestosterone, 11kt) was studied using mature individuals during reproductive season. In the first experiment, during full moon, plasma melatonin peaked at night in covered tanks (deprived of night illumination) from both outdoor (133.2 ± 12.8 pg ml− 1) and indoor (190.6 ± 41.5 pg ml− 1) groups. However, for fish in the open tanks, exposed to approximately 0.3 lx of illumination, nocturnal plasma melatonin was significantly reduced (p < 0.05), approaching to mid-light (ML) values, (79.6 ± 7.1 and 81.8 ± 14.0 pg ml− 1, for outdoor and indoor groups, respectively). During new moon a similar pattern was observed in outdoor group: fish in the covered tank showed higher melatonin values than those in the open tank, which were exposed to the near undetectable night illumination. In the second experiment, plasma sex steroid concentrations were significantly higher during the full moon compared to the new moon. In the case of females, E2 concentration reduced from 2.4 ± 0.6 to 0.4 ± 0.1 ng ml− 1 between full and new moon samplings, while T decreased from 0.3 ± 0.0 to 0.2 ± 0.0 ng ml− 1. Vitellogenin, however, did not show such differences between moon phases. In males, 11kt exhibited a plasma concentration of 14.3 ± 2.1 ng ml− 1 during full moon and 4.7 ± 0.7 ng ml− 1 during new moon while T values were 2.6 ± 0.4 and 1.0 ± 0.1 ng ml− 1 for full and new moon, respectively. In conclusion, these findings pointed out the high sensitivity to moon light of the Senegal sole, which could be using the melatonin signalling to synchronize their reproduction rhythms to the lunar cycle.