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.

Small‐subunit ribosomal DNA (SSU) sequences used in combination with anatomical and morphological data have supported elevation of the Plocamiaceae to ordinal status. The Plocamiales currently includes two genera: the free‐living Plocamium Lamouroux; and the adelphoparasite, Plocamiocolax Setchell. Plocamium includes upwards of 40 species, which are widely distributed throughout the world's oceans. Most species are clearly defined anatomically and biogeographically. In this regard the type species, P. cartilagineum (L.) Dixon (type locality: reportedly Northern Europe), is an exception. Plocamium cartilagineum is considered cosmopolitan in distribution, with individuals varying in size, and in the number and habit of ramuli in their alternating series. We have sequenced the large‐subunit ribosomal DNA (LSU) for representatives of P. cartilagineum from France, Ireland, Norway and Scotland as well as isolates from Australia and the North Pacific. These data indicate that many cryptic species, four within Europe itself, are currently included within the concept of P. cartilagineum. We present molecular, biogeographical, ecological, and morphological/anatomical data characterizing these European forms and provide taxonomic revisions that reflect this diversity within P. cartilagineum.