Fluvioralfsia iberica gen. et sp. nov., Fluvioralfsiaceae fam. nov.: The first freshwater member of the Ralfsiales (Phaeophyceae), collected from streams in Spain

A novel brown alga inhabiting freshwater streams in Spain is described herein as Fluvioralfsia iberica gen. et sp. nov. Phylogenetic analyses establish it as a novel lineage within the Ralfsiales, Fluvioralfsiaceae fam. nov. It is an epilithic crust, typically orbicular with a radial growth pattern, formed by dichotomous fan‐like branching of filaments. Crusts show a bilateral symmetry with horizontal medial filaments that bend upward and downward to form circular or semilunar protrusions in concentric bands that overlap at different levels. Older crusts are easily seen with the naked eye, becoming greater than 50 cm in diameter and 1 cm thickness and can cover a high percentage of the stream bottom in optimal habitats. Differentiating characters from other families of Ralfsiales include the absence of phaeophycean hairs and plurangia and, most notably, the unangia originating intercalarily from lower cells of paraphyses. Most populations were located in first‐ to third‐order calcareous streams and rivers, at altitudes of 14–870 m.a.s.l., in the mountainous Cantabrian coast, northern Spain, and one river in southeast Spain. Sites were entirely nonmarine, 2–118 km from the sea, with median conductance of 219 μS · cm −1 (range 112–859) in well‐oxygenated, oligo‐mesotrophic waters and moderate to high current velocities. As freshwater brown algae are a species‐poor group, Fluvioralfsia iberica represents an important addition to the global freshwater algal flora and raises additional questions about the origins of phaeophytes in fresh waters.