As a first intermediate host, H. pumilio tends to infect and live in the largest (and probably oldest) Melanoides tuberculata snails.

By investing resources into maintaining and protecting its host, H. pumilio reduces the risk of being displaced by competing trematodes.

Soldiers are more aggressive than reproductives, attacking heterospecific trematodes that infect their host in vitro.

They’re found in the highest numbers in the basal visceral mass, where competing trematodes tend to multiply during the early phase of infection.

This strategic positioning allows them to effectively defend against invaders, similar to how soldier distribution patterns are seen in other animals with defensive castes.