Molecular divergence and morphological diversity among four cryptic species of Plocamium (Plocamiales, Florideophyceae) in northern Europe

The Plocamiaceae currently includes two genera: the free-living Plocamium and the adelphoparasite, Plocamiocolax. Plocamium includes ca. 40 species that are widely distributed throughout the world's oceans. Most species are clearly defined anatomically and biogeographically; however, the type species Plocamium cartilagineum (type locality: considered northern Europe) is an exception. Although reportedly cosmopolitan in distribution, our investigations unequivocally indicate that there is substantial genetic diversity among isolates from different geographic locations and that there are at least eight divergent cryptic species currently included in P. cartilagineum. In this investigation we focused on populations of P. cartilagineum from northern Europe. We used a combination of molecular and morphological characters in order to establish boundaries between four cryptic species in this geographical area. Our molecular results not only resolve these four entities as distinct, but also indicate that they are among the most divergent of all Plocamium species worldwide. As a result, we now recognize P. cartilagineum, Plocamium subtile, Plocamium nanum sp. nov. and Plocamium maggsiae sp. nov. in northern Europe. A preliminary assessment of 'P. cartilagineum' in other geographical areas is provided.