Relationship between egg specific gravity and egg quality in the Japanese eel, Anguilla japonica

Eggs from maturation-induced Japanese eels, Anguilla japonica, often sink in seawater immediately after artificial insemination and do not hatch. In the present study, the specific gravity of unfertilized eggs from individual females was measured in isotonic (about 310 mOsm/kg) and hypertonic (about 875 mOsm/kg) saline solutions and the relation of specific gravity to egg quality was examined. Egg specific gravity under isotonic conditions showed a significant negative correlation with egg fertility, hatchability and water content, suggesting that inadequate hydration of oocytes during final maturation, which leads to insufficient egg buoyancy, is one of the causes of poor egg quality. Some of the eggs that showed lower specific gravity than seawater under isotonic conditions exhibited higher specific gravity than seawater under hypertonic conditions, indicating that the buoyancy acquired by the eggs is sometimes lost after their transfer to seawater. Only eggs that retained low specific gravity under hypertonic as well as isotonic conditions exhibited high fertility and hatchability. Taken together with data from other studies, these results suggest that the poor quality of eggs that sink immediately after artificial insemination is attributable to at least two causes: the failure of oocytes to acquire sufficient buoyancy during maturation and the loss of buoyancy in seawater because of the inability of egg osmoregulation.