Cryptic, alien and lost species: molecular diversity of <i>Ulva sensu lato</i> along the German coasts of the North and Baltic Seas

DNA barcoding analysis, using <i>tuf</i>A, revealed considerable differences between the expected and observed species inventory of <i>Ulva sensu lato</i> in the Baltic and North Sea areas of the German state of Schleswig-Holstein. Of 20 observed genetic entities, at least four (<i>U. australis, U. californica, U. gigantea</i> and <i>Umbraulva dangeardii</i>) had been introduced recently, whereas three others (one <i>Ulva</i> sp. and two <i>Blidingia</i> spp.) could not be identified at the species level and could also represent recently introduced species. In addition, the observed distributions of <i>Kornmannia leptoderma</i> and <i>U. rigida</i> were much more extensive than indicated by historical records, whereas <i>Blidingia minima</i> and <i>Gayralia oxysperma</i> were absent or much less common than expected. Barcoding analysis also revealed that both <i>U. tenera</i> (type material) and <i>U. pseudocurvata</i> (historical vouchers) from Helgoland, an off-shore island in the North Sea, actually belong to <i>U. lactuca</i>, a species that appears to be restricted to this island. Furthermore, past morphological descriptions of <i>U. intestinalis</i> and <i>U. compressa</i> have apparently been too restrictive and have been responsible for numerous misidentifications. The same is true for <i>U. linza</i>, which, in northern Germany, clusters into two genetically closely related but morphologically indistinguishable entities. One of these entities is present on Helgoland, while the second is present on North Sea and Baltic Sea mainland coasts.