Morphological and biochemical changes in oocytes during final oocyte maturation in Japanese anchovy (Engraulis japonicus)

Abstract

Japanese anchovy (Engraulis japonica) is an important marine species for commercial fisheries in addition to being a key part of coastal ecosystems around Japan. The species is a multiple spawner having oval-shaped pelagic eggs. Based on its morphological changes and observation of spawning, this study shows that Japanese anchovy completes final oocyte maturation (FOM) within eight hours, and the process can be classified into five stages: stage-I (St-I), post-vitellogenic oocyte (PVO); stage-II (St-II), germinal vesicle migration (GVM); stage-III (St-III), germinal vesicle breakdown (GVBD); stage-IV (St-IV), late hydration; and stage-V (St-V), ovulation under photoperiod of 14L:10D with water temperature 19.5 to 20.9℃. The average wet weight of St-I oocytes was 69 μg with a longitudinal diameter of 728 μm and 75% water content and then increased to 244 μg, 1,386 μm, and 94%, respectively, at St-V. Gel chromatographic analysis indicated a change in peak position of native lipovitellin from 430 to 170 kDa. Moreover, SDS-PAGE analysis indicated proteolytic degradation during the latter stages of FOM. The free amino acid (FAA) content in the St-I oocytes (2 nmol/individual) became approximately 11-fold greater in St-V eggs (22 nmol/individual), suggesting that the FAAs are produced from the proteolytic degradation of the yolk proteins and regulate the buoyancy of the eggs through osmotic effector generation in the Japanese anchovy.