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Comparative studies in some animal taxa have demonstrated that sperm competition reduces inter- and intra-male variation in sperm morphology. However, the influence of fertilization mode on variation in sperm traits remains poorly understood. In this study, we compared the residual variance in sperm traits among and within males of 11 marine fish species with different fertilization modes and sperm competition risks. Both inter- and intra-male variation in sperm length was smaller in internally fertilizing species than in externally fertilizing species. We found no clear evidence that sperm competition risk was associated with inter-male variation in sperm length, whereas intra-male variation decreased with increasing sperm competition risk in externally fertilizing species. Sperm length showed lower variation than other sperm traits when comparing inter- and intra-male variation across sperm traits. Lower inter-male variation is consistent with stabilizing selection on sperm length, whereas lower intra-male variation may reflect tighter quality control during spermatogenesis. Our study provides insights into how fertilization mode and sperm competition risk may be associated with sperm trait variation across hierarchical levels, particularly among males and within ejaculates.<\/jats:p>","DOI":"10.1093\/jeb\/voag076","type":"journal-article","created":{"date-parts":[[2026,8,26]],"date-time":"2026-08-26T14:22:39Z","timestamp":1787754159000},"page":"1200-1214","source":"Crossref","is-referenced-by-count":0,"title":["Inter- and intra-male variation in sperm length is associated with fertilization mode in marine fishes"],"prefix":"10.1093","volume":"39","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-8064-6722","authenticated-orcid":false,"given":"Takeshi","family":"Ito","sequence":"first","affiliation":[{"name":"Sesoko Station, Tropical Biosphere Research Center, University of the Ryukyus , 3422 Sesoko, Motobu, Okinawa 905-0227 ,","place":["Japan"]},{"name":"Department of Biology, Graduate School of Science, Osaka Metropolitan University , 3-3-138 Sugimoto, Sumiyoshi, Osaka 558\u20138585 ,","place":["Japan"]},{"name":"Maizuru Fisheries Research Station, Field Science Education and Research Center, Kyoto University , Nagahama, Maizuru, Kyoto, 625-0086 ,","place":["Japan"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Masaya","family":"Morita","sequence":"additional","affiliation":[{"name":"Sesoko Station, Tropical Biosphere Research Center, University of the Ryukyus , 3422 Sesoko, Motobu, Okinawa 905-0227 ,","place":["Japan"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Seiya","family":"Okuno","sequence":"additional","affiliation":[{"name":"Department of Biology, Graduate School of Science, Osaka Metropolitan University , 3-3-138 Sugimoto, Sumiyoshi, Osaka 558\u20138585 ,","place":["Japan"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Kazuo","family":"Inaba","sequence":"additional","affiliation":[{"name":"Shimoda Marine Research Center, University of Tsukuba , 5-10-1 Shimoda, Shizuoka, 415-0025 ,","place":["Japan"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Kogiku","family":"Shiba","sequence":"additional","affiliation":[{"name":"Shimoda Marine Research Center, University of Tsukuba , 5-10-1 Shimoda, Shizuoka, 415-0025 ,","place":["Japan"]},{"name":"Tateyama Marine Laboratory, Institute of Marine and Coastal Research, Ochanomizu University , 11 Kou-yatsu, Tateyama, Chiba 294\u20130301 ,","place":["Japan"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Yasunori","family":"Koya","sequence":"additional","affiliation":[{"name":"Department of Biology, Faculty of Education, Gifu University , 1-1 Yanagido, Gifu, 501\u20131193 ,","place":["Japan"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Mitsuo","family":"Homma","sequence":"additional","affiliation":[{"name":"Diving Service F. 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