Eusociality in parasitic flatworms: Trematodes form soldier and reproductive castes

When considering flatworms, one does not generally imagine colonies, much less behavioral specialization, polymorphism, and caste formation. However, a division of labor has now been documented for the trematode parasitic flatworms and it involves a caste of soldiers that defend colonies against invading trematodes. Soldiers do not reproduce, and, compared to reproductives, are smaller, have larger mouth parts, are more active, are common in areas where invasions start, and more readily attach hetero- and conspecific trematodes. I will share the story of the discovery of trematode sociality and present recent published and unpublished evidence for a soldier caste in numerous other trematode species around the world. I will then show parallels with other animal societies that also fit the fortress-defense mode of sociality. Several attributes of the Trematoda make it clear they should provide fruitful systems to study the general principles underlying the evolution and ecology of sociality.