Morphological and molecular characterization of two species of Gelidium (Gelidiales, Rhodophyta) from Mexico: G. nayaritense sp. nov. and the new record of G. sanyaense Author Quiroz-González, Nataly Posgrado en Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México; Av. Ciudad Universitaria 3000, C. P. 04510, Coyoacán, Ciudad de México, México. Author Ponce-Márquez, Ma. Edith Facultad de Ciencias, UNAM Av. Universidad 3000, Circuito Exterior S / N Delegación Coyoacán, C. P. 04510 Ciudad Universitaria, D. F. México. Author López-Gómez, Norma Escuela Nacional de Estudios Superiores Unidad Morelia, Antigua Carretera a Pátzcuaro No. 8701, Col. Ex Hacienda de San José de la Huerta, C. P. 58190 Morelia, Michoacán, México. Author Rodríguez, Dení Facultad de Ciencias, UNAM Av. Universidad 3000, Circuito Exterior S / N Delegación Coyoacán, C. P. 04510 Ciudad Universitaria, D. F. México. text Phytotaxa 2021 2021-12-03 527 3 161 176 journal article 3132 10.11646/phytotaxa.527.3.1 09a3523d-5fa7-4643-8610-3108b16d077e 1179-3163 5751325 Gelidium nayaritense Rodríguez D., Quiroz-González N. & Ponce-Márquez M. sp. nov. ( Figs 3–4 ). Type: — MEXICO . Nayarit : Sayulita , intertidal, 20°52’29’’N ; 105°26’72 W , 17 June 2017 , D. Rodríguez & N. Quiroz-González , holotype FCME 10875 ( Fig. 3a ) in FCME (Faculty of Science, Mexico) , isotypes : FCME 10869 , 10876 , 10880 , 10881 , 10884 in FCME (Faculty of Science, Mexico). FIGURE 4. Reproductive features of Gelidium nayaritense sp. nov. A. Tetrasporangial branch with tetrasporangial sorus (TS). Scale = 400 µm. B. Tetrasporangial sorus with steril margin (EM) Scale = 100µm. C. Tetrasporangial sorus with tetrasporangia (TE). Scale = 100 µm. D. Cruciately divided tetrasporangia (TE). Scale = 40 µm. Description: —Plants a turf of purple-red thalli, with cylindrical prostrate stolon attached to substratum with brushlike haptera and erect linear to clavate axes, with irregular to smooth margins, cylindrical at base and flattened above, mostly without ramification. When branches occasionally present, irregularly arranged with one order of ramification, 0.4–1.0 cm high, 0.2–1 mm width, 0.1–0.3 mm thick ( Fig. 3a–d ). Axes in transverse section ovoid, elliptical, elliptical acute or elliptic in half moon, decreasing in diameter towards apex. Shape of vegetative apices acute ( Fig. 3e–f ). Cortex of 2–3 layers of rounded cells, 6–9 µm in diameter ( Fig. 3g –h ), outer cortical cells irregularly arranged in surface view. Medulla of 3–6 rows of circular or subcircular cells in transverse section, 12–15 μm in diameter, and with very large intercellular spaces. Internal rhizoidal filaments in small groups within the medulla and concentrated in the subcortex ( Fig. 3g –h ). Tetrasporangial sori located in proximal parts of ordinary branches, ovoid, 180–390 µm wide and 285 µm long, with small sterile margins. Tetrasporangia irregularly arranged in sori, ovoid, spherical, 20–28 µm in diameter, 21–25 µm long, cruciately divided. Gametophytes not observed. Etymology: —The species epithet referred to the state Nayarit , Mexico , where the type specimen was collected. Distribution and Habitat :—Sayulita in Nayarit to Zihuatanejo in Guerrero . Intertidal rocks, with low incidence of light and high waves. Barcode sequences of holotype specimen: —rbc L GenBank accession number MT 798865 and COI-5P GenBank accession number MT 799807 . Other examined material :— MEXICO . Guerrero : Troncones , intertidal, 17°46’4’’N ; 101°43’31’’W , 15 January 2017 , D. Rodríguez & N . Quiroz-González , 10888, 10889 (FCME).