Resumen de Advances in the larval rearing of meagre (argyrosomus regius): diet, weaning protocols, and ontogeny of the digestive and innate immune systems
Cindy Patricia Campoverde Vera
Summary Aquaculture production in Europe is dominated by a small group of species, such as salmon, trout, carp, seabass, seabream that limits the number of aquaculture products available on the market, and to some extent, the geographic regions where aquaculture can be done profitably. The market price of these species is often close to or below the minimum cost of production, which has consequent negative influences on the growth of aquaculture in the EU. A sustainable system based on the incorporation of new species could, to a certain extent, reduce the problem by providing a diversified market with increased geographic dispersal of production sites that can reduce transport costs form “farm to table”. Thus, in order to increase aquaculture products in the market new species have been selected taking in account characteristics such as rapid growth, ease of rearing, adaptability to large volumes under intensive rearing, large body size to facilitate automated post-harvest processing and development of value-added consumer products. The purpose of this thesis is to study a species considered promising for Mediterranean aquaculture belonging to the family Sciaenidae and commonly known as meagre (Argyrosomus regius). This species is characterized by its high growth rate (1 kg year-1) and good feed conversion rate. However, as other new species, meagre has some bottlenecks present during its early stages of development that need to be solved. Thus, the general objectives of this thesis aims to evaluate the following aspects: (1) morphology and functional development of the digestive system, based on histological and enzymatic activity analyses, (2) study the effect of different strategies for early weaning onto artificial feed, reduce the use of Artemia to 50% of the amount used in standard production protocols, and evaluate the effects of these diets on development, digestion, growth, survival, and deformations. Two experiments were carried out using different feeding protocols designed for an early incorporation of artificial feed, then evaluate its effects on morphology and functional development using enzymatic markers that reflect the maturity of the digestive tract. Certain problems inherent to the species, in particular, their cannibalistic behavior during the post-flexion phase had effects on survival rate, although early weaning had no major influence on the presence of skeletal deformities. In this sense, we proceeded to the (3) study of fatty acid requirements during larval development (DHA -22:6n-3- and DHA/EPA ratio) in order to examine the effects of different live prey enrichments on larval growth and survival, and evaluate their ability to elongate and desaturate fatty acids from their precursors. Two experiments were designed during larval culture using high, medium and low content of DHA demonstrating that the fatty acids requirements (DHA) are species-specific. The use of hemp oil (rich in 18:3n-6 and 18:4n-3) served to demonstrate the inability of meagre larva to elongate and desaturate fatty acids even when the precursors are offered in the live food.
The immune system is another aspect of great importance during larval rearing, since the success of larval culture is based not only in nutrition, but also on the defense mechanisms that the larva possess against potential pathogens present in the water. The larva depends on a suite of important protective molecules of the innate immune system that deal with potential pathogens until the adaptive or specific system is completely mature. Most of the mortality afflicting industrial aquaculture occurs during the critical stages larval growth, thus limiting the production of juveniles. In this sense, to evaluate the time in which the larva is more susceptible to external factors and its ability to cope with an immune response, we have studied (4) the ontogeny of the innate immune system during larval development, focused on the organogenesis of the major lymphoid organs (thymus, kidney, and spleen), and lymphoid tissues associated with mucosal gut and gills. For this purpose, larval samples were taken in several developmental periods including the critical change in development (metamorphosis) where rapid changes in growth require tissue modification and increased bioenergetic consumption. For this objective, we have used histological techniques and specific stains to detect the main changes in composition and structures in these organs and tissues. A. regius shows similar patterns to most teleosts during organogenesis, but the time of appearance of these structures during their growth is species-specific. In addition, and to complete this study of the development of the immune system (5) the expression of certain genes related to the non-specific immune system was analyzed in order to identify possible markers of immunity during growth. Particularly, we wanted to determine the presence during early developmental periods of significant immune gene transcripts as well changes in expression level in larvae and juvenile tissues, and study whether the relative expression of these genes may be correlated to observed morphological changes seen by histology. The results indicated that the genes under study for this work can be detected during the early stages of development and we found indications that their expression profiles may be influenced by significant dietary changes made during the larval and early juvenile culture.
