Early development and metabolism

This chapter reviews the ontogeny and physiology of larval development in the Sparidae family and provides a comparative view of the larval biology of the species, highlighting those aspects that are relevant for their culture. Despite extensive similarities in larval biology, development and phenotype, species-specific differences exist, partly related to different temperature optima among species. Sparids typically release a large number of small eggs that hatch in an early stage of development. Larvae are very vulnerable during early development requiring relatively strict biotic and abiotic conditions to survive and grow. Sparids spawn buoyant pelagic eggs containing a single oil globule. Fertilized eggs take only a few days to hatch. Decreased hatching, increased larval mortality, and abnormalities occur outside optimal temperature ranges. Newly hatched larvae are small, have a large yolk sac, and typically start feeding within 3–5 days. The early ontogenetic events focus on the development of the organs and structures necessary for growth and survival, including the sensory organs, mouth, trunk and tail muscle mass, and the digestive system. The young larvae have poor visual acuity that improves as the eye diameter increases, and new photoreceptors and structures appear. Skeletal development and ossification have been well studied, in part because malformations are an issue in cultured fish. The pattern of anatomical changes and structural differentiation is related to functionality, behavior, and environmental preferences. The digestive system including gut, gall bladder, liver, pancreas, and gastric glands develops rapidly. The enzymes responsible for digestion of proteins, lipids, and carbohydrates are present at first feeding. The functional ontogeny of the digestive tract and pancreatic enzyme activity patterns are similar to those described in other fish groups. Growth in larvae is influenced by many exogenous factors with temperature and food being the most important. Other key abiotic factors in culture include oxygen, salinity, turbidity, and light. Optimal requirements are species-specific and change during larval ontogeny. The pattern of development is typically from a longitudinally elongated body shape to a longitudinally compressed form often characterized by a large skull and jaws. Sparids grow relatively quickly during early development. The critical stage of initial swim bladder inflation usually occurs soon after complete yolk absorption. The transient physostome larvae inflate their swim bladders by gulping air at the water surface. Factors influencing swim bladder inflation include temperature, salinity, turbulence, light, genetics, egg quality, water quality, and tank hydrodynamics. There is a considerable body of research on Sparid larval nutrition. Highly unsaturated fatty acids (n-3 HUFA) are essential for good growth and survival. Amino acids (AA) are a major energy source during the early life stages. The supply of dietary protein is paramount for optimal larval growth. The metabolism of larvae and juveniles is controlled by micronutrients, such as vitamins and minerals but there is little published literature. The vitamin A concentration of live feeds can be an important determinant for normal skeletal development. The generalized feeding regime for larvae starts with rotifers followed by Artemia and then formulated feeds. On a weight-specific basis, Sparid larvae tend to ingest more than 100% of their own weight in the first days of feeding. The specific ingestion rate decreases with age and more efficient digestion. Experimental microdiets have allowed investigation of early nutritional requirements, sustain survival comparable to those of live feeds, but growth is typically poor. Microdiet attractiveness improves with the inclusion of protein hydrolysates or free AA. Microbound, microencapsulated, and microcoated diets have been used with increasing effectiveness. Further improvements in culture will come from a better understanding of basic nutritional requirements of larval sparids, as well as of their interactions with abiotic factors, such as temperature, light, and hydrodynamics at key developmental stages.