Anatomical and functional specialization of the soldier caste in a heterophyid trematode, Cercaria batillariae

The presence of a soldier caste is a hallmark of the reproductive division of labour. Trematodes reproduce asexually and form a colony in snails, the first intermediate host. Several trematode species produce morphologically and behaviourally specialized soldiers for defending colony members. These soldiers often possess disproportionately large pharynxes, which serve as a weapon for sucking and attacking their enemies. Here, we demonstrate further anatomical and functional specialization in trematode soldiers. We found that Cercaria batillariae HL7 exhibits the reproductive division of labour with reproductive and soldier castes. Transmission electron microscopy (TEM) revealed that the pharynxes of the soldiers were not just larger than those of the reproductives but were covered by a tegument with serrated projections, coupled with thick bundles of radial muscle fibres, consistent with enhancing the role of soldiers in attaching to and sucking on enemies with their mouths and pharynxes. These sucker structures are analogous to those seen in octopuses. The TEM images further revealed that at least some soldiers of C. batillariae lack a germinal mass, suggesting they are at least temporarily sterile. Our study facilitates the understanding of the trematode social system by providing fine-scale structural differences between the reproductive and soldier castes.