Ritmos diários do metabolismo e fisiologia digestiva De larvas de tilápia (oreochromis niloticus) em diferentes temperaturas

Abstract

Larvae have to assimilate the nutrients from the diet to get adequate growth rates and, for that purpose, it is necessary, among other factors, the presence of digestive and metabolic enzymes. Water temperature is one of the main synchronizers of biological rhythms and one of the most influential environmental factors for fish embryonic and larval development. Water temperature can influence the expression of digestive and metabolic enzymes, hormone secretion, growth and survival of animals. The environmental factors display cyclic variations along the day. Among them, high water temperature occurs during the day and low temperatures during the night, generating the thermocycle. Therefore, the objective of the present research was to evaluate the influence of different temperature regimes (thermocycle vs. constant) on the physiology and growth of Nile tilapia (Oreochromis niloticus) larvae. For that purpose, tilapia eggs were fertilized and immediately placed in incubators in two different rearing systems: one system with constant temperature (CTE) and one system with a thermocycle of 31ºC: 25ºC. Both were water recirculation systems equipped with mechanical and biological filters. Photoperiod was set at 12:12 LD (light/dark), with lights on at 9:00h (Zeitgeber time 0 h). Larvae were fed at fixed times, four times a day, with a semipurified diet of 42% of crude protein and 4,100 Kcal/kg of gross energy. Larvae at 4, 8 and 13 days postfertilization (dpf) were collected from both temperature treatments every 4 h along a 24 h cycle for further analyses. The levels of relative expression of all genes were analyzed by qPCR. Larval growth was measured through their standard length (experiment 1). Experiment 1: Larvae maintained at TC showed higher growth (t-test, p <0.05) than larvae kept at CTE. Daily rhythms (Cosinor, p <0.05) in the expression of chymotrypsin, lipase, maltase, isomaltase, cholecystokinin (cck) and neuropeptide Y (npy) were shown in 13dpf larvae reared in TC, with expression peaks close to the feeding phase. Experiment 2: larvae at 13 dpf and maintained at TC showed significant rhythms (Cosinor, p <0.05) of relative expression of the genes alanine aminotransferase (alt), aspartate aminotransferase (ast), malic enzyme, glucose-6-phosphate dehydrogenase (g6pd), pyruvate kinase (pk) and phosphofructokinase (pfk). On the other hand, larvae reared at CTE at the same stage did not present daily rhythms (Cosinor, p >0.05). The acrophase of expression of the enzymes in larvae maintained at TC appeared around mealtime. Therefore, it can be concluded that, under a thermocycle, tilapia larvae improve the maturation of their circadian system, which would thus improve their digestive and metabolic efficiency. These results can be helpful for improving protocols used in tilapia temperature and increasing production performance.