Cladophora rhodolithicolasp. nov. (Cladophorales, Chlorophyta), a diminutive species from European maerl beds

Cladophora rhodolithicola sp. nov., a small green macroalgal epiphyte on rhodoliths, is described from the Atlantic coasts of the British Isles, France and Spain, based on morphological and molecular evidence. Molecular phylogenetic analyses reveal that the rhodolith epiphyte is closely related to C. pygmaea, a dwarf species that also grows on maerl and small stones and whose systematic position was previously uncertain. The fact that both species, along with two other distinct Cladophora species (C. echinus and C. battersii), are resolved among species traditionally assigned to the Cladophora section Longi-articulatae, has implications for our understanding of the evolution of the genus. The section Longi-articulatae is one of the most distinctive groups in Cladophora, being characterized by coarse thalli with conspicuous basal cells, strict acropetal growth and the lack of intercalary rhizoids. Here we show that deviant character states such as reduced and irregular growth, and intercalary rhizoids have evolved independently several times within this clade.