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Aiming to address the current lack of knowledge with regard to the male reproductive system, this study employed histological techniques to provide the first systematic description of the microstructure of the male gonad and its developmental dynamics in krill. Using high-resolution microscopy, we characterized the typical morphological features of germ cells at each developmental stage. The Spermatogenic Index was applied to quantify stage-specific differences in spermatogenic functional maturity within the testis, revealing a significant enhancement in spermatogenic synchrony during the adult stage 3BM. Furthermore, by systematically measuring germ cell cross-sectional diameters and estimating cell volume based on a morphometric model, we quantified the morphological changes of germ cells during spermiogenesis. The spatial position of the testis and the microstructure of spermatophores were also determined. This study provides key morphological evidence for understanding the male reproductive traits of krill, establishing a theoretical and data foundation for deciphering reproductive strategies and optimizing resource management models.<\/jats:p>","DOI":"10.1093\/mam\/ozag019","type":"journal-article","created":{"date-parts":[[2026,3,19]],"date-time":"2026-03-19T17:19:49Z","timestamp":1773940789000},"source":"Crossref","is-referenced-by-count":0,"title":["The First Systematic Histological Observation of the Male Antarctic Krill (\n                    <i>Euphausia Superba<\/i>\n                    ) Reproductive System"],"prefix":"10.1093","volume":"32","author":[{"ORCID":"https:\/\/orcid.org\/0009-0001-4074-929X","authenticated-orcid":false,"given":"Yuanfang","family":"Chang","sequence":"first","affiliation":[{"name":"College of Marine Living Resource Sciences and Management, Shanghai Ocean University , Shanghai 201306 ,","place":["China"]},{"name":"Center for Polar Research, Shanghai Ocean University , Shanghai 201306 ,","place":["China"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6081-7811","authenticated-orcid":false,"given":"Guoping","family":"Zhu","sequence":"additional","affiliation":[{"name":"College of Marine Living Resource Sciences and Management, Shanghai Ocean University , Shanghai 201306 ,","place":["China"]},{"name":"Center for Polar Research, Shanghai Ocean University , Shanghai 201306 ,","place":["China"]},{"name":"Polar Marine Ecosystem Group, The Key Laboratory of Sustainable Exploitation of Oceanic Fisheries Resources, Ministry of Education , Shanghai 201306 ,","place":["China"]},{"name":"National Engineering Research Center for Oceanic Fisheries , Shanghai 201306 ,","place":["China"]},{"name":"International Research Center for Marine Biosciences, Ministry of Science and Technology, Shanghai Ocean University , Shanghai 201306 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