Effect of increasing dietary levels of n-3 long-chain polyunsaturated fatty acids on liver composition and histopathology of meagre (Argyrosomus regius, Asso 1801) fingerlings

Abstract

Meagre (Argyrosomus regius) is a promising new aquaculture species, with great potential due to its high growth rate, feed efficiency and easy adaptation to captivity. Essential fatty acids (EFA) are required to sustain growth, development, immune status and survival (Watanabe, 1982; Izquierdo, 2005). EFA requirement of meagre fingerlings were previously estimated (Carvalho et al., 2018). A feeding trial was performed testing the effect of 5 increasing dietary n-3 LC-PUFA levels, below and above the requirement levels (0.8, 1.4, 2.0, 2.6 and 3.6% DM), on liver biochemical and FA composition and histopathology of meagre fingerlings. Fingerlings with an initial body weight of 2.80 g ±0.23 (mean±SE) were allocated in 15 tanks of 200L supplied with filtered seawater. Temperature along the trial was 23ºC± 0.2 (mean±SE). Fish were fed the experimental diets 3 times a day until apparent satiety during 30 days. At the end of the trial, liver samples were collected for biochemical analysis and histological and bacteriological studies. Fish fed the two lowest n-3 LC-PUFA diets (0.8 and 1.4%) showed a higher hepatosomatic index as well as a higher hepatic lipid infiltration. Consequently, these fish showed a more severe steatosis than fish fed 2.0-3.6% n-3 LC-PUFA. FA composition of liver total lipids reflected the dietary composition with the increase of dietary n-3 LC-PUFA increasing SFA, n-3 LC-PUFA (EPA, DHA) and n-6 LC-PUFA (ARA), and decreasing MUFA, LA and LNA. However, meagre FA composition of liver polar lipids was less affected by the diets, n-3 LC-PUFA being preserved. Furthermore, the lowest dietary content in n-3 LC-PUFA (0.8%) also led to a higher incidence of hepatic granulomas. The infectious origin of granulomas was discarded by the absence of symptoms of bacteria, parasites or fungi in visual observations, bacteriological cultures, or specific histological techniques. These results suggest that EFA-deficiency led to a higher incidence of granulomatosis in meagre. Therefore, a minimum of 2.0% to DM n-3 LC-PUFA is required to maintain the EFA profile of hepatic total lipids and normal histomorphology of liver, corroborating the requirement based on growth.