Effects of temperature regime (daily thermocycles versus constant) on growth and the ontogeny of the daily rhythms of expression of digestive factors of Nile tilapia (Oreochromis niloticus)

Abstract

Environmental factors present cyclic variations during the day. Of them, high temperatures occur in the light phase, whereas low temperatures take place at night, which is known as thermocycle. Although applying thermocycles to fish larvae improves growth in some species, nothing is known about the impact during the early development of the digestive system. The aim of the present research was to investigate the effect of different temperature regimes, cycling versus constant, on the daily rhythms of digestive factors and growth of Nile tilapia (Oreochromis niloticus) larvae. For this purpose, fertilized eggs were divided into two groups: one under a thermocycle (TC) of 31ºC:25ºC day:night and other group at constant temperature of 28ºC (CTE). Photoperiod was set at 12:12h light/dark cycle. Larvae length was measured on 4, 8 and 13 days post-fertilization (dpf). Larvae samples were collected every 4 h during a 24-hour cycle on 4, 8 and 13 dpf. The expression levels of pepsinogen, chymotrypsinogen, trypsinogen, lipase, maltase, isomaltase, npy and cck were analyzed by qPCR. The results showed that larval growth was greater when larvae were reared at TC than at CTE. Moreover on 13 dpf, most analyzed genes (chymotrypsinogen, lipase, maltase, isomaltase, npy and cck) displayed daily rhythms in the TC group, but not in CTE, with most acrophases located around mealtime. These rhythms could explain the higher growth rate observed in the TC larvae due to improved feed digestion and utilization. The results can be useful for improving the rearing protocols used in larviculture and to enhance production performance.