Seasonal variations in the hemocyte parameters, gonad development, energy storage and utilization of the giant honeycomb oyster Hyotissa hyotis (Linnaeus 1758) in Jeju Island off the south coast of Korea

Abstract

Hyotissa hyotis is a gryphaeid oyster distributed from the Indo-Pacific to Eastern Pacific regions. This oyster has extended its distribution range from tropical and subtropical waters to Jeju Island off the south coast of Korea, possibly due to the recent increase in seawater temperature. This species is considered to be an aquaculture candidate due to its fast growth and high-quality adductor muscle. In the present study, we first investigated seasonal changes of gonad development, energy storage and utilization, and hemocyte functions of H. hyotis on Jeju Island off the south coast of Korea. The taxonomic status of the specimen was confirmed by nucleotide sequencing of the mitochondrial 16S ribosomal genes. Histology indicated that gametogenesis of male and female oysters commenced in May and June, respectively, when the water temperature reached 16.0–18.2 °C. Partially spawned oysters were evident from September to November, when the water temperature was 21–22 °C. H. hyotis had an extraordinarily large adductor muscle, which accounted for 47.6–55.6 % of its total body weight. The ratio of adductor muscle weight to total body weight varied seasonally, and was negatively correlated with maturation of the gonad. Glycogen levels in the adductor muscle were significantly higher than the levels in other tissues and strongly fluctuated during gametogenesis, indicating that the adductor muscle plays a critical role as a major energy storage organ. Finally, post-spawning oysters showed a weakening of cellular defense capacities, characterized by the decreased phagocytic capacity and increased mortality of hemocytes.