The physical soldier caste of an invasive, human-infecting flatworm is morphologically extreme and obligately sterile

We provide substantial evidence that a globally invasive, human-infectious trematode (parasitic flatworm) in its first intermediate host possesses a physically specialized and obligately sterile soldier caste. Unlike similar-sized immature reproductives, soldier worms both lack reproductive organs and attack and kill competitor trematodes. The soldiers also possess the most extreme morphological specialization for defense yet observed in trematodes, having a giant muscular pharynx analogous to the enlarged mandibles of soldier ants. Partly due to the effectiveness of its specialized soldiers in killing competitors, Haplorchis pumilio substantially structures the ecological community of trematodes infecting its invasive host snail. This work indicates that obligate sterility, akin to the complete division of labor in the social insects, has independently evolved in the flatworms.