Taxonomic investigation of Ralfsia-like (Ralfsiales, Phaeophyceae) taxa in the North Atlantic Ocean based on molecular and morphological data, with descriptions of Pseudoralfsiaceae fam. nov., Pseudoralfsia azorica gen. et sp. nov. and Nuchella vesicularis gen. et sp. nov.

In this study we investigated <i>Ralfsia</i>-like crusts (i.e. excluding <i>Ralfsia</i> sensu stricto, <i>Stragularia</i> spp. and/or Scytosiphonaceae crustose phases) with an emphasis on the North Atlantic Ocean using molecular data (COI-5P, ITS and <i>rbc</i>L-3P) combined with morpho-anatomical comparisons of type material and contemporary specimens. Of the four species of <i>Ralfsia</i> previously reported in Europe, only <i>R. fungiformis</i> is presently recognized as belonging to <i>Ralfsia</i> sensu stricto, Ralfsiaceae, with the remaining species, <i>R. lucida, R. ovata</i> and <i>R. verrucosa</i>, of uncertain taxonomic status. Our study revealed 11 independent genetic lineages of <i>Ralfsia</i>-like taxa, which were not assignable to any of the recognized families of the Ralfsiales. To accommodate this diversity, we propose Pseudoralfsiaceae Parente, Fletcher &amp; G.W.Saunders fam. nov., including two new genera <i>Pseudoralfsia</i> Parente, Fletcher &amp; G.W.Saunders gen. nov. and <i>Nuchella</i> Parente, Fletcher &amp; G.W.Saunders gen. nov. The first genus includes <i>Pseudoralfsia verrucosa</i> (Areschoug) Parente, Fletcher &amp; G.W.Saunders comb. nov. as the generitype (the only species of the three remaining European species of uncertain taxonomic status assigned to <i>Ralfsia</i> that was reassessed here), <i>P. azorica</i> Parente, Fletcher &amp; G.W.Saunders sp. nov. and seven undescribed genetic groups, which require additional vouchers for description. The second genus has as generitype <i>Nuchella vesicularis</i> Parente, Fletcher &amp; G.W.Saunders sp. nov., and also includes <i>N</i>. sp._1MP, which was represented by a single sterile specimen from Swanage, UK and consequently not characterized. Three characteristics differentiate <i>Nuchella</i> from <i>Pseudoralfsia</i> – the former typically with numerous vesicles, non-synchronous development of the plurangia and hair pits arising from both middle and lower cells of the erect filaments. Species of Pseudoralfsiaceae can be distinguished from <i>Ralfsia</i> sensu stricto (Ralfsiaceae) mainly by DNA sequences and by consistently having frequent hair pits, and typically unsymmetrical thalli.