The Annual Gametogenesis of the Mussel, Mytilus unguiculatus (coruscus) (Valenciennes, 1858), Occurring in the Shallow Subtidal of Ulleungdo Island in Korea

The mussel Mytilus unguiculatus (coruscus) (Valenciennes, 1858) occurs at high density on the shallow subtidal hard bottom of Ulleungdo Island off the east coast of Korea, where it plays a crucial role as a primary consumer in the shallow, coastal, benthic ecosystem. In this study, we monitored seasonal variation in the condition index, gonad growth, and subsequent spawning of this species at Ulleungdo to obtain substantial information on its reproductive traits. From May to July 2013, the condition index (CI) dropped dramatically, indicating that M. unguiculatus (coruscus) spawned during this period. The CI also dropped from March to April in 2014 and then increased in May. Histology revealed that the onset of gonial mitosis in males and females occurred in September and October, and individuals in the spawning stage were observed in April 2014 and May 2013, when the surface seawater temperature (SST) ranged from 14 to 15 °C. The monthly mean oocyte diameter measured from the digitized images of the gonad also indicated that most females spawned completely in July. In summary, the annual reproductive cycle of M. unguiculatus (coruscus) at Ulleungdo could be categorized into (1) onset of gonial mitosis in late summer to early fall, (2) active growth of gonad cells ready for spawning during winter and spring, and (3) spawning and reabsorption of the residual gametes during late spring and summer. Seasonal changes in the gonads are closely linked to temporal changes in the SST.