For conspecific parasites sharing the same host, kin recognition can be advantageous when the fitness of one individual depends on what another does; yet, evidence of kin recognition among parasites remains limited. Some trematodes, like Coitocaecum parvum, have plastic life cycles including two alternative life-history strategies. The parasite can wait for its intermediate host to be eaten by a fish definitive host, thus completing the classical three-host life cycle, or mature precociously and produce eggs while still inside its intermediate host as a facultative shortcut. Two different amphipod species are used as intermediate hosts by C. parvum, one small and highly mobile and the other larger, sedentary, and burrow dwelling. Amphipods often harbour two or more C. parvum individuals, all capable of using one or the other developmental strategy, thus creating potential conflicts or cooperation opportunities over transmission routes. This model was used to test the kin recognition hypothesis according to which cooperation between two conspecific individuals relies on the individuals' ability to evaluate their degree of genetic similarity. First, data showed that levels of intrahost genetic similarity between co-infecting C. parvum individuals differed between host species. Second, genetic similarity between parasites sharing the same host was strongly linked to their likelihood of adopting identical developmental strategies. Two nonexclusive hypotheses that could explain this pattern are discussed: kin recognition and cooperation between genetically similar parasites and/or matching genotypes involving parasite genotype–host compatibility filters.

P. excavatum Pair-wise Rousset's distance between individual parasitesP. fluviatilis_Pair-wise Rousset's distance between individual parasites