Fertilization modes drive the evolution of sperm traits in Baikal sculpins

Abstract

AbstractBaikal sculpins are a unique group of fishes, including both externally and internally fertilizing species. Sperm traits, such as morphology and motility, are predicted to undergo adaptive changes due to species fertilization modes. However, studies exploring such traits in Baikal sculpins are lacking. Here, we conducted a field study and investigated sperm traits of three sculpins with external fertilization (Cottocomephorus grewingkii, Leocottus kesslerii and Paracottus knerii) and one sculpin with internal fertilization (Comephorus dybowskii). Sperm head morphology was spherical in both types of fertilizing species; however, the head cross‐sectional area of the sperm of the internal fertilizer was smaller than that of external fertilizers. This result implies that sperm head morphology might be adapted to swim in a viscous ovarian fluid. The internal fertilizer, C. dybowskii, had longer and faster‐swimming sperms with longer motility duration than the external fertilizers, perhaps due to different sperm competition levels. Sperms of the internal fertilizer were only motile in isotonic solutions, whereas sperms of the external fertilizers were motile in lake waters, suggesting that sperm motilities are specialized according to their external/internal environments. To our knowledge, this is the first study to demonstrate sperm traits of Baikal sculpins, which may have evolved adaptively along with changing fertilization modes.