MESOPHYLLUM SPHAERICUM SP. NOV. (CORALLINALES, RHODOPHYTA): A NEW MAE ¨ RL-FORMING SPECIES FROM THE NORTHEAST ATLANTIC 1 VivianaPen˜a 2 Laboratorio de Algas Marinas, Departamento de Biologı ´a Animal, Biologı ´a Vegetal y Ecologı ´a, Universidad de A Corun ˜a. Campus de A Zapateira, S ⁄ N. A Corun ˜a. E-15071, Spain Walter H. Adey Department of Botany, National Museum of Natural History, Smithsonian Institution, Washington, D.C., 20560, USA Rafael Riosmena-Rodrı´guez Departamento de Biologı ´a Marina, Universidad de Baja California Sur (UABCS). Apartado postal 19-B, km. 5.5 Carretera al sur, La Paz, B.C.S. 23080, Mexico Moon-Yung Jung Division of Life Sciences, Korea Polar Research Institute, KORDI, 7-50 Songdo-dong, Yeonsu-gu, Incheon 406-840, Korea Julio Afonso-Carrillo Departamento de Biologı ´a Vegetal (Bota ´nica), Universidad de La Laguna, La Laguna, Canary Islands, E-38271, Spain Han-Gu Choi Division of Life Sciences, Korea Polar Research Institute, KORDI, 7-50 Songdo-dong, Yeonsu-gu, Incheon 406-840, Korea and Ignacio Ba´rbara 3 Laboratorio de Algas Marinas, Departamento de Biologı ´a Animal, Biologı ´a Vegetal y Ecologı ´a. Universidad de A Corun ˜a. Campus de A Zapateira, S ⁄ N. A Corun ˜a. E-15071, Spain Mesophyllum sphaericum sp. nov. is described based on spherical mae ¨rl individuals (up to 10 cm) col- lected in a shallow subtidal mae ¨rl bed in Galicia (NW Spain). The thalli of these specimens are radially organized, composed of arching tiers of compact medullary filaments. Epithallial cells have flattened to rounded outermost walls, and they occur in a single layer. Subepithallial initials are as long as, or longer than the daughter cells that subtend them. Cell fusions are abundant. Multiporate asexual con- ceptacles are protruding, mound-like with a flattened pore plate, lacking a peripheral raised rim. Fila- ments lining the pore canal and the conceptacle roof are composed of five to six cells with straight elon- gate and narrow cells at their base. Carposporangial conceptacles are uniporate, protruding, and conical. Spermatangial conceptacles were not observed. Molecular results placed M. sphaericum near to M. erubescens, but M. sphaericum is anatomically close to M. canariense. The examination of the holotype and herbarium specimens of M. canariense indicated that both species have pore canal filaments with elongate basal cells, but they differ in number of cells (five to six in M. sphaericum vs. four in M. canariense). Based on the character of pore canal filaments, M. canariense shows similarities with M. erubescens (three to five celled). The outermost walls of epithallial cells of M. canariense are flared compared to the round to flattened ones of M. erubes- cens, the latter being widely accepted for the genus Mesophyllum. The addition of M. sphaericum as new mae ¨rl-forming species suggests that European mae ¨rl beds are more biodiverse than previously under- stood. Key index words: Corallinales; Galicia; Iberian Peninsula; mae ¨rl; Mesophyllum canariense; Meso- phyllum erubescens ; Mesophyllum sphaericum; NE Atlantic; nuclear SSU rDNA; rhodolith Abbreviations: TBR branch, tree-bisection-recon- nection branch; TrN model, Tamura-Nei model The genus Mesophyllum was described by Me. Lemoine (1928) based on the presence of a medulla (hypothallium) disposed in concentric rows (coaxial) separated by thicker walls and the pres- ence of multiporate asexual conceptacles. This genus, originally described as intermediate between 1 Received 27 November 2009. Accepted 2 February 2011. 2 Author for correspondence: e-mail vpena@udc.es. 3 Present address: Facultad de Ciencias, Universidad de A Corun ˜a, Campus de A Zapateira, S ⁄ N, 15071 A Corun ˜a, Spain. J. Phycol. 47, 911–927 (2011) Ó 2011 Phycological Society of America DOI: 10.1111/j.1529-8817.2011.01015.x 911