Diel rhythms of serum metabolites and thyroid hormones in red porgy held in different photoperiod regimes

Diel rhythms in serum glucose, lactate, cholesterol, triglycerides and thyr oid hormones were studied in red porgy, Pagrus pagrus, held under different photoperiod regimes (constant darkness - DD, 8L:16D,12L:12D), at a constan t temperature (17.1-18.7 degrees C) and fed with commercial pellets, by mea ns of a self-feeder. A clear diurnal rhythm in feeding activity, regardless of the photoperiod regime was demonstrated. All serum components showed si gnificant diel rhythms, although they were not necessarily consistent or ci rcadian in periodicity. As well as this, differences in the average values among the varying treatments were observed. Fish held under the 12L:12D pro tocol displayed significantly higher average T-4, T-3 and lactate levels du ring the day rather than at night. Maximum glucose values occurred 8-12 h a fter dawn and maximum lactate concentrations at 06:00 or 14:00 h. Diel vari ations in cholesterol were evident only in the DD group with peak values in versely correlated with the rhythm of food intake. Triglycerides displayed a similar pattern of changes. Significant diel fluctuations in T-4 serum le vels were observed only in fish exposed to the 12L:12D protocol, with peak values at dawn. A clear diurnal peak (10:00 h) in T-3 concentrations was ob served in fish subjected to the 12L:12D regime, while fish exposed to the 8 L:16D protocol showed two peaks: one in the photophase (10:00 h) and anothe r in the scotophase (02:00 h). The light-dark alternation and the general a ctivity rhythm of fish seem to be the main synchronizers of the diel rhythm s observed in this study.