Phymatolithon atlanticum sp. nov. (Hapalidiales, Rhodophyta) from the northeast Atlantic Ocean

Phymatolithon atlanticum is newly described from France and Northern Ireland (the United Kingdom) using both molecular (rbcL sequence data, concatenated psbA and COI-5P datasets) and morpho-anatomical analyses. The species shows sequence divergences from other Phymatolithon species by 10.7–15.3% in rbcL, 5.1–9.2% in psbA and 9.6–16.8% in COI-5P. Morpho-anatomically, P. atlanticum is encrusting to lumpy, has slightly sunken female conceptacles with raised rims, lacks buried conceptacles, has five to six rosette cells surrounding tetra/bisporangial conceptacle pores, and has tetra/bisporangial conceptacle roofs that are pitted with depressions as a result of the disintegration of rosette cells. This latter feature is what currently separates the species morpho-anatomically from all other species of Phymatolithon. The tetra/bisporangial conceptacle of P. atlanticum closely resembles those of Lithothamnion crispatum and Leptophytum bornetii and may well represent additional examples of convergent evolution in the non-geniculate coralline algae.