Why does internal fertilization in phallostethoid fishes not necessarily result in live birth?

In phallostethoid fishes, internal fertilization does not necessarily result in live birth because the species are not viviparous. While these fishes have evolved a specialized, asymmetrical intromittent organ known as a priapium—formed from modified pelvic fins—to facilitate internal fertilization, this anatomical adaptation does not equate to the development of the embryos within the female.1

In the context of fish reproduction, internal fertilization is a distinct process from the subsequent development of offspring. While some fish species utilize internal fertilization to support live-bearing (viviparity or ovoviviparity), others, including phallostethoids, remain oviparous. In these cases, the female sheds the fertilized eggs, and the embryos complete their development outside of the mother's body. Thus, the presence of an intromittent organ for internal fertilization is not a definitive indicator of live-bearing reproductive strategies.231