The tale of the founder flounder: assessing the origin of a recently established fish in Icelandic waters

The geographic origin of a newly established species is an important aspect of a species' colonisation history used for classification and management plans but can additionally deliver information on introduction pathways that require the attention of managers and scientists. The invasion history of European flounder Platichthys flesus, first documented in Icelandic waters in 1999, has not been fully elucidated. In previously published Icelandic literature different hypotheses on the European flounder's origin and introduction pathway have been raised, suggesting either an arrival from the Faroe Islands or an introduction via ballast water from the coasts of northwestern Europe. To clarify the geographic origin of European flounder found in Iceland, we used microsatellite analysis of purposefully collected samples from 14 sites around Iceland and reference data stemming from previously published research of flounder population structure in the Faroese and other European populations. Our results indicate that the Faroese population is the most likely source of the European flounder found in Iceland. There was weak isolation by distance between Icelandic samples and the Faroese population. Additionally, we identified nine hybrids between European flounder and native European plaice Pleuronectes platessa and further documented signatures of introgression between the species. European flounder x European plaice hybrids are commonly found throughout the overlapping native range of both species but has previously not been documented in Iceland.