Social flatworms: the minor caste is adapted for attacking competing parasites

Recently, it has been hypothesized that some marine parasitic flatworms (trematoda) within their gastropod hosts form socially organized larval colonies divided into distinct castes adapted for defence and reproduction. However, the evidence in support for advanced division of labour in trematodes is challengeable and generally consistent with the traditional view of ordinary age-structured colonies. Using intramolluscan larval colonies of the trematode Himasthla elongata (Echinostomatidae) as study system, we demonstrate experimentally that contrary to reproducing colony members, non-reproducing larvae are anatomically and behaviourally adapted of defending the colony against competing parasites infecting the same host. We also show that co-infections are significantly less frequent in situ than expected by chance, which emphasizes the efficiency of the antagonistic activity of Himasthla colonies in eliminating competitors. Together, this brings novel and pivotal support to the view of adaptive cast formation in marine trematodes.