Synchronization to light and mealtime of daily rhythms of locomotor activity, plasma glucose and digestive enzymes in the Nile tilapia (Oreochromis niloticus)

The light-dark cycle and feeding can be the most important factors acting as synchronizers of biological rhythms. In this research we aimed to evaluate synchronization to feeding schedule of daily rhythms of locomotor activity and digestive enzymes of tilapia. For that purpose, 120 tilapias (65.0 ± 0.6 g) were distributed in 12 tanks (10 fish per tank) and divided into two groups. One group was fed once a day at 11:00 h (zeitgeber time, ZT6) (ML group) and the other group was fed at 23:00 h (ZT18) (MD group). The fish were anesthetized to collect samples of blood, stomach and midgut at 4-hour intervals over a period of 24 h. Fish fed at ML showed a diurnal locomotor activity (74% of the total daily activity occurring during the light phase) and synchronization to the feeding schedule, as this group showed anticipation to the feeding time. Fish fed at MD showed a disruption in the pattern of locomotor activity and became less diurnal (59%). Alkaline protease activity in the midgut showed daily rhythm with the achrophase at the beginning of the dark phase in both ML and MD groups. Acid protease and amylase did not show significant daily rhythms. Plasma glucose showed a daily rhythm with the achrophase shifted by 12 h in the ML and MD groups. These results revealed that the feeding time and light cycle synchronize differently the daily rhythms of behavior, digestive physiology and plasma metabolites in the Nile tilapia, which indicate the plasticity of the circadian system and its synchronizers.