Using new protein sources in feed for gilthead seabream (S. aurata) broodstock: what happens to the offspring?

Constant intensification and growth of aquaculture production are dependent on the development of sustainable protein sources that can replace the most used protein sources in aquafeeds: fishmeal and soybean meal [1]. In this study, gilthead seabream (S. aurata) broodstock was given two diets where vegetable protein sources were replaced with alternative ones, such as algae, duckweed, insects, and microbiomes (bioflocs). Two inclusion levels (V2 and V3) were compared with a group receiving a control diet (V1). The effect of the diet was evaluated on larval development during endogenous feeding, which is a high vulnerability period, due to the small size of the larvae, incomplete development, and low energy reserves [2]. After hatching, larvae were reared at 19 °C and were sampled at hatch (T0) and the complete yolk sac absorption stage (T1). For both time points, larvae were evaluated in terms of 1) total length (TL); 2) gut development, by analyzing serial hematoxylin-eosin and AB-PAS stained sections; 3) muscle development, by whole-mount immune-fluorescence; 4) skeletogenesis, using a double staining whole-mount technique with Alcian Blue and Alizarin for cartilage and bone tissues respectively. The fish handling procedures and sampling methods used in the trial followed the E.U directive 2010/63/EU guidelines on the protection of animals used for scientific purposes. TL was similar among groups at any time point. In all groups at T0, the gut appeared undifferentiated, consisting of an AB-PAS negative cubic cells monolayer, located dorsally to the yolk sac. At T1, in all groups, it was already possible to observe the gut folds and a PAS-positive layer. At the end of T1, the mouth was open and the yolk sac was absent. The lateral muscle was striated with 5 layers at T0 and 7 layers at T1 demonstrating a growth mechanism of hypertrophy and hyperplasia. Moreover, larvae from V1 showed a higher degree of ossification especially at the head level and at the distal portion of the notochord, at the future caudal fin position at T0, with no differences at T1. larvae development, even if further studies are necessary to assess the impact of these diets on larvae after the onset of exogenous feeding.