Endocrine (plasma cortisol and glucose) and behavioral (locomotor and self-feeding activity) circadian rhythms in Senegalese sole (Solea senegalensis Kaup 1858) exposed to light/dark cycles or constant light

Abstract

The existence of daily rhythms under light/dark (LD) cycles in plasma cortisol, blood glucose and locomotor and self-feeding activities, as well as their persistence (circadian nature) under constant light (LL), was investigated in Senegalese sole (Solea senegalensis). For the cortisol and glucose rhythms study, 48 soles were equally distributed in 8 tanks and exposed to a 12:12 LD cycle and natural water temperature (experiment 1). After an acclimation period, blood was sampled every 3 h until a 24-h cycle was completed. Blood glucose levels were measured immediately after sampling, while plasma cortisol was measured later by ELISA. In experiment 2, the fish were exposed to LL for 11 days, and after this period, the same sampling procedure was repeated. For the study of locomotor and self-feeding rhythms (experiment 3), two groups of sole were used: one exposed to LD and the other to LL. Each group was distributed within 3 tanks equipped with infrared photocells for the record of locomotor activity, and self-feeders for feeding behavior characterization. The results revealed a marked oscillation in cortisol concentrations during the daily cycle under LD, with a peak (35.65 ± 3.14 ng/ml) in the afternoon (15:00 h) and very low levels during the night (5.30 ± 1.09 ng/ml). This cortisol rhythm persisted under LL conditions, with lower values (mean cortisol concentration = 7.12 ± 1.11 ng/ml) and with the peak shifted by 3 h. Both rhythms were confirmed by COSINOR analysis (p < 0.05). The synchronizing role of temperature and feeding schedule, in addition to light, is also discussed. Diel rhythms of glucose were not evident in LD or LL. As to locomotor and self-feeding activity, a very marked rhythm was observed under LD, with higher activity observed during the night, with acrophases located at 2:14 and 3:37 h, respectively. The statistical significance of daily rhythms was confirmed by COSINOR analysis. Under LL, both feeding and locomotor rhythms persisted, with an endogenous period (τ) around 22.5 h. In short, our findings described for the first time the existence of circadian cortisol and behavioral circadian rhythms in flat fish. Such results revealed the importance of taking into account the time of day when assessing stress responses and evaluating physiological indicators of stress in fish.

Are cortisol and melatonin involved in the immune modulation by the light environment in pike perch Sander lucioperca?

“…Such night increase in plasma cortisol was already described in salmonids . However, cortisol peaks happen during photophase at 15:00 in Senegalese sole and at light onset in goldfish and Eurasian perch . A previous experiment in pike perch also showed a cortisol peak at 10:00 during photophase while humoral immune activity peaks also occurred during the night as in the present experiment.…”

“…Cortisol, considered as a major stress marker in fish, plays a crucial role in several biological functions including stress response, growth, immune functions, and energy metabolism . In the present experiment, the increase in plasma cortisol at day 1 under a 100‐lx light confirmed that the exposure to high light intensities is stressful for pike perch.…”

“…Such observations may suggest a habituation to stress and/or a rapid metabolism and clearance of cortisol indicating the interest for using various stress indicators to account for the stress responsiveness in percids, such as brain neurotransmitters . Additionally, plasma level of glucose strongly depends on cortisol (through the regulation of neoglucogenesis during a stress event), but also on feeding (diet composition, feeding habits) . Glucose responses did not follow the same pattern as plasma cortisol (no day‐night variation and no increase at day 1 under a 100‐lx white spectrum) pointing the importance of the feeding habits.…”

Thermal Preference of Juvenile Dover Sole (Solea solea) in Relation to Thermal Acclimation and Optimal Growth Temperature

“…Whether this significantly influenced our results is unclear and demands further research. As diurnal activity and endocrine (cortisol) rhythms in Senegal sole persisted under constant light conditions [24], we do not expect that results of our preference test were influenced by conducting them under constant light.…”

“…Senegal sole ( Solea senegalensis ) display a clear daily activity rhythm, with the highest activity during night hours [24]. Daily activity rhythms have not been experimentally assessed in Dover sole but these could be similar to Senegal sole as for the nocturnal nature of both species [25], [26].…”