Growth and Developmental Processes Alternate During Larval Development of Atlantic Herring.

During their development, fish pass through a series of developmental processes advancing, for example, their physiological and locomotive abilities. In particular, many marine fish larvae often hatch at an (semi-) embryonic developmental stage, and existential processes, such as digestion and respiration, and structures, such as muscle and skeleton, must form and/or change during the larval development. In this study, we analyzed the gene expression of factors associated with myogenesis, skeletogenesis, and growth within the different larval life stages of Atlantic herring. We evaluated these results in relation to length and stage ratio and performed histological analysis of cross-sections of herring larvae in different stages. Overall, the length per stage ratios showed that there are two major growth periods in larval herring development, the first occurring during the dorsal fin development phase and the second during the transition from caudal fin to pelvic fin development phase. This is consistent with the histological analysis, as an increase in muscle fibers was observed in both phases. The gene expression data also showed that factors responsible for muscle cell lineage determination and fiber development were highest before a period with increased growth. Combining our results with other studies on skeletogenesis, organogenesis, and the development of neural and sensory systems in herring, it becomes evident that other energetically costly developmental processes tend to occur in periods when growth is less prominent. It can therefore be concluded that growth and developmental priority periods alternate during larval development.