Phallostethidae

Phallostethidae, also known as priapium fish, is a family of atheriniform fish native to freshwater and brackish habitats in southeast Asia. They are small, no more than 3.5 cm (1.4 in) in length, with partially translucent bodies. They are found in fresh and brackish water from Thailand to the Philippines and Sulawesi. They are named for a muscular organ found under the chin of males. This organ, which may possess small testicles, is used together with the pelvic fins to grasp the female during mating. Unlike most other fish, priapium fishes exhibit internal fertilisation, although they are oviparous.