Osedax priapus is a species of bathypelagic annelid polychaete worms that consume the nutrients inside the bones of dead whales or other vertebrates.

The O. priapus males have testis sacs surrounded by the roots and a sperm duct connected to a seminal vesicle that opens just below the palps.

[1] Due to the males of O. priapus still resembling other Osedax species, they use their seminal vesicle on the head to store free sperm for mating with the females.

[1] In other Osedax species, the sexual size dimorphism eliminated competition between male and female worms for limited resources like food or space.

Other species with larger growing females can rapidly colonize a bone, leaving nowhere else to land but other worms' plumes.