Coitocaecum parvum is a digeneic trematode or flatworm (Platyhelminthes) that is parasitic to the intestine of the common bully (Gobiomorphus cotidianus) or upland bully (G. breviceps). The common and upland bully are freshwater fish of New Zealand that C. parvum uses as its definitive host. C. parvum is a hermaphroditic freshwater trematode that can omit its definitive host and produce eggs by selfing or progenesis inside its amphipod second intermediate host (Lagrue et al., 2001)