Investigating the seasonal variability in ovary and oocyte development of Antarctic krill (Euphausia superba)

Antarctic krill (Euphausia superba) is a keystone species in the Southern Ocean, however, seasonal variation in ovary development of this species kept unclear, especially within a single sector of the Southern Ocean. In this study, a histological study of krill in the south Scotia Sea region was conducted using a multi-seasonal data, which demonstrated in detail the development of secondary sexual characteristics, ovary development and oocyte development of krill at different stages of sexual maturity. In different years, krill have different sexual development characteristics, indicating the interannual variability in reproductive process and therefore probably the recruitment dynamics. The ovaries of krill are full, the oocytes are rich in yolk, and eggs are about to be laid in summer. After spawning, krill ovaries reorganize and degenerate in autumn and winter. As its reproductive diapause period, it spares no effort to absorb nutrients. Unproduced eggs are reabsorbed by the ovary, the permanent germinal zone is active, and early oocytes begin to develop in preparation for the next egg production. Meanwhile, the ovarian development status is differed in the juvenile, sub-adult and adult stages. An annual or near-annual monitoring program on krill reproductive process is desirable for understanding population dynamics regionally, especially under the context of flux between areas and population connectivity. This study updated the understanding on the seasonal reproductive process of krill in the key spawning and fishing ground.