Seasonal changes in glycogen contents of the Iwagaki oyster, Crassostrea nippona from the coast of Toga Bay, Konoura, Tomari, and Oki Islands in the Sea of Japan

In general, oysters accumulate glycogen in the connective tissues for energy storage and utilize it during gonadal development. However, little is known about the glycogen accumulation in the Iwagaki oyster, Crassostrea nippona, an important species for the coastal fisheries in the Sea of Japan. In the present study, seasonal changes in the glycogen contents of the tissues of Iwagaki oysters from the coast of Toga Bay, Konoura, Tomari, and Oki Islands in the Sea of Japan were examined. Glycogen contents gradually decreased from May to September at the four sampling sites concomitantly with spawning. On the other hand, in the Pacific oyster Crassostrea gigas, it is known that glycogen contents are low even in July. This is probably due to the difference in spawning season between both species: August-September in the Iwagaki oyster and July-September in the Pacific oyster. Glycogen contents of cultured Iwagaki oysters were lower in July and September than those of wild ones. This was probably due to differences in the timing of the decrease in glycogen contents. Some unknown environmental factors might affect glycogen contents.