<p>Antarctic krill (<i>Euphausia superba</i>) is a keystone species in the Southern Ocean; however, seasonal variations in reproductive development for krill are complex and remains unknown. A histological investigation with observations of external secondary sex features of krill in the south Scotia Sea region was carried out using a multi-seasonal dataset for detail reproductive descriptions for this commercially important species. The monthly development of secondary sexual characteristics, the thelycum, as well as of oocytes over a period of two years were described. It was observed that krill have diverse reproductive development characteristics within the ovary, and that this function differently between the juvenile, sub-adult, and adult stages. During the summer, adult krill ovaries are large and ripe with oocytes rich in yolk ready for release in late summer. Post spawning, the ovaries resorb, fragment, and regress throughout the autumn and winter. During reproductive diapause period, krill focus on absorbing nutrients. Un-released eggs are reabsorbed by the ovary, the permanent germinal zone is active, and early oocytes begin to develop in preparation for the egg production phase. Krill that are about to spawn have an ovary that fills the space between the digestive gland and muscle. The ovarian development of krill is divided into 10 sexual developmental stages. As a part of this study, data on the carapace thickness with similar development patterns in krill size and carapace width, was investigated for the first time to help understand krill growth and development.</p>