Same barcode, different biology: differential patterns of infectivity, specificity and pathogenicity in two almost identical parasite strains.

•Two identical cytochrome c oxidase subunit I barcode Gyrodactylus salaris isolates show divergent biology and behaviour.•The salmon-infecting Lierelva strain grew exponentially on salmon.•The charr-infecting Pålsbufjorden strain is non-pathogenic to salmon.•Experiments with isolated worms revealed differences in reproductive rates.•Charr-infecting parasites are more active on salmon than salmon parasites on charr.