Abstract Artificial fertilization experiments were performed and male and female gonads observed histologically to confirm the oldest known description (published 1934) of the reproductive mode for internal gamete association (IGA) in Vellitor centropomus (Cottidae). Stripped eggs from mature females were unfertilized such commencing after their immersion in seawater, indicating that V. centropomus is an IGA species. Many spermatozoa were stored in the immature ovary with non-vitellogenic oocytes, evidence that males of the species mature before females and that copulation occurs when females are still immature. The male reproductive organ consisted of paired testes, a specialized seminal vesicle and a functional urogenital papilla. A sperm duct in the testicular hilus was absent. Spermatogenesis occurred sequentially, posterior most first, suggesting that it occurs over an extended period of time. A ventral luminal ampulla, central expanded main sperm duct and dorsal saccular ampulla were observed in the seminal vesicle, all parts being involved in sperm storage. The saccular ampulla had an exocrine function, secreting a periodic acid–Schiff (PAS)-positive substance. Sperm heads stored in the posterior part of the testis or lumen of the seminal vesicle were also PAS-positive. Sperm heads were slender, a typical morphology in IGA species.
