Eusociality represents the highest stage of animal social evolution, defined by three core characteristics: reproductive division of labor (with permanent/obligately sterile worker castes), overlapping generations, and cooperative brood care. Some advanced lineages have further evolved morphologically differentiated castes (e.g., soldier ants, worker ants). This trait has evolved independently more than 20 times across the animal kingdom, spanning multiple phyla from flatworms to mammals. The major groups are categorized as follows:
I. Phylum Platyhelminthes: Class Trematoda
This is the least well-known and most recently widely recognized eusocial group, representing a classic example of parasitic eusociality.
Core taxa: Multiple species of digenetic trematodes (order Digenea), such as Haplorchis pumilio (a fluke infecting freshwater snails) and various marine parasitic flukes.
Social traits: After a single larval fluke invades a host, it produces colonies of hundreds to thousands of individuals via clonal reproduction. The colony differentiates into obligately sterile soldier flukes—smaller in size with highly developed mouthparts and aggressive behavior—that defend clonemate reproductive individuals by attacking invading fluke populations of other strains. Reproductive individuals specialize in producing offspring. This form of eusociality aligns with the "fortress defense" evolutionary model: the host body serves as both a food source and a defensible "nest", and competitive pressure drives the emergence of a defensive caste.
