How do trematode clones differ by fitness-related traits and interact within a host?

[Display omitted] •Studies of between-clones interactions in parasites mostly focus on infection success.•We studied infectivity and growth of trematode clones in single versus double infections.•Parasite clones were identified by microsatellite analysis and used in fish infections.•The growth rates of clones varied more than their infectivity and both depended on host mass.•When interacting, clones either cooperated during co-infection or competed when growing. Polyclonal infections are widespread and provide evidence of facilitation, competition, and neutral interactions between parasite clones, even within the same host–parasite system. The outcome of coinfections is usually assessed by means of parasite infection intensities, while other important fitness-related traits, e.g., larval growth rates, are often ignored. We experimentally infected fish (Salvelinus malma) with different clones of the common trematode Diplostomum pseudospathaceum and pairs of clones. Clones were identified by microsatellite analysis. Their infectivity and growth rates within the fish were investigated in double-clone infections compared with single-clone ones. In total, 3838 parasite larvae (metacercariae) from 325 fish were measured. The growth rates of the D. pseudospathaceum clones were more variable than their infectivity. Relationships of these parasite traits with host mass were clone-specific. Some clones demonstrated higher infection intensities and growth rates in larger fish. Therefore, specialization toward different size groups of fish hosts may occur in this parasite species. Furthermore, we noticed a positive correlation between population density and parasite growth (Allee effect; rarely reported for parasites) but only in mixed infections. In double-clone infections, evidence of both interclonal facilitation and interclonal competition was found. When clones interacted, they either “cooperated” during infection of the host or competed while growing. There were no clone pairs in which interactions changed in type with time or were present constantly during development of the parasite.