Elongation of fin rays in parental males of Zosterisessor ophiocephalus (Pisces, Gobiidae)

The first description of the elongation of rays of the second dorsal fin in the parental male of the grass goby, Zosterisessor ophiocephalus, is repor ted. The exhibition of fin ray elongation in the male grass goby may be a u seful diagnostic character to identify individuals adopting alternative sty les (i.e. parental males vs. parasitic spawners). The possible role played by this secondary sexual trait in the context of alternative reproductive s tyles adopted by smaller males of this species to gain fertilization is als o discussed.