Polymorphic parasitic larvae cooperate to build swimming pseudo-colonies luring hosts
Multicellular parasites have evolved a variety of astonishing strategies to enter and survive within their host organisms. Amongst these, the Digeneans, a group of parasitic flatworms known as flukes, stand out for their diversity and ingenuity in infecting their host organisms, which include invertebrates, fish, mammals, and birds. They are particularly renowned for their complex life cycles, which often involve multiple host species. Many digeneans rely on a free-living intermediate stage known as a cercaria, which infects the next host. Through the use of live observations, light and electron microscopy, 3D reconstructions and histological analysis, we discovered that in a most peculiar instance, cercaria larvae of two distinct morphological types cooperate to build a locomoting pseudo-colony that swims in the ocean and acts as a lure. The functional specialization of the two cercaria types comprises the pseudo-colony with the larger morphotype enabling motility and the smaller morphotype presumably facilitating invasion. Through the comparative analysis of a genome, we established that both cercaria types possess the identical genome and belong to digeneans from the family Acanthocolpidae, likely genus Pleorchis. This discovery provides a unique example and a novel insight into how morphologically and functionally heterogeneous individuals of the same species cooperate to build pseudo-colonial organisms for the purpose of infection. This strategy bears resemblance to the cooperating castes of the same species found among insects, or inducible defenses and morphotypes found in representatives of zooplankton.