Abstract
We investigated the reproductive strategies and embryonic development of Acheilognathus rhombeus (a bitterling species that spawns in autumn) within its freshwater mussel host in the Bongseo Stream, South Korea. By focusing on survival mechanisms during critical stages of embryonic development, the selective use of mussel gill demibranchs by the bitterlings and associated adaptive traits were observed over 1 year. A significant diapause phase occurs at developmental stage D, which lasts for approximately 7 months, allowing embryos to survive winter. Development resumes when the temperature exceeds 10 °C. Minute tubercles on the embryos (crucial for anchoring within the host gill demibranchs and preventing premature ejection) exhibit the largest height during diapause, and the height decreases when developmental stage E is reached, when growth resumes. Acheilognathus rhombeus embryos were observed in 30.5% of the mussels, mostly within the inner gills, thereby maximizing spatial use and oxygen access to enhance survival. These results highlight the intricate relationship between A. rhombeus and its mussel hosts, demonstrating the evolutionary adaptations that enhance reproductive success and survival. This study provides valuable insights into the ecological dynamics and conservation requirements of such symbiotic relationships.