The semen of Hemilepidotus gilberti, a non-copulatory marine cottid fish, includes two sperm types; eusperm for fertilization and parasperm, which cannot be employed in fertilization. A previous study having revealed the function of parasperm as antidispersive (reducing the lateral dispersion of semen) during semen transportation, the number of eusperm arriving at an egg mass was estimated so as to confirm the significance of parasperm function. Two simple models, one including and one excluding the size effect of the egg mass at ejaculation, were used. A considerable antidispersive effect was apparent when the egg mass was small, resulting in the latter being contained within the dispersion area of semen. When male H. gilberti emit semen, eggs that hang down from the genital opening do not extend beyond the semen dispersion area. Therefore, it was concluded that parasperm in this species ensures semen transportation and promotes fertilization success in males via an antidispersive effect related to egg mass size.
