Molecular identification of the ubiquitous green algae Ulva reveals high biodiversity, crypticity, and invasive species in the Atlantic-Baltic Sea region

Correct species identification is fundamental for assessment and understanding of biodiversity. Erroneous species identification may impede conservation management and may delay detection of invasive species. The ubiquitous green algal genus Ulva is known for its wide environmental tolerance, plastic morphology, occurrence of cryptic species and ambiguous species concepts that hinder clear identification. We used molecular monitoring to assess species diversity and distribution of Ulva along the full Atlantic-Baltic Sea salinity gradient (> 10,000 km). Ulva specimens were collected from Denmark, Finland, Germany, Norway, and Sweden. DNA barcoding analysis of the tufA gene revealed 20 genetic entities in total, of which 11 could be identified to species level (U. californica, U. flexuosa, U. torta, U. linza, U. prolifera, U. fenestrata, U. australis, U. intestinalis, U. compressa, U. gigantea, U. lacinulata). Nine entities (Ulva sp. 1–9; [Ulva capillata]) yielded novel sequence reads that belonged to either unidentified species, species complexes, or singletons. At least 3 of the discovered species (U. australis, U. californica, U. gigantea) are considered non-native and potentially invasive. Furthermore, considerable differences between the observed and the historically estimated species distributions were found. The highest diversity was recorded in the Atlantic and Skagerrak region whereas only two entities of taxonomically accepted species where found north-east the Blekinge coast. Our study shows that the species diversity of Ulva in the study area is diverging from previous reports, and that molecular methods are imperative for species identification in this morphologically plastic genus. Furthermore, the presence of non-native species indicates a necessity for further fine-scale monitoring in specific areas to e.g. mitigate formation of green tides.