Diversity of Gracilariaceae (Rhodophyta) in China: An integrative morphological and molecular assessment including a description of Gracilaria tsengii sp. nov.

Gracilariaceae are economically and ecologically important red algae used to produce agar in China. Therefore, understanding the diversity of species in Chinese waters is of great academic and economic value. Although Gracilariaceae diversity in this region has been well studied in the last century, with morphological studies revealing a new genus, 16 new species, and one new variety, molecular phylogenetic studies are rare in China. The aim of this study was to construct a DNA sequence database to study the phylogeny and diversity of Gracilariaceae in this region. We generated 185 rbcL and 170 COI-5P sequences from 211 freshly collected specimens and five herbarium specimens including four types. The rbcL and COI-5P phylograms revealed 19 Gracilaria (G.) and three Gracilariopsis (Gp.) species, including one new diminutive species, Gracilaria tsengii sp. nov., which has specialized spermatangial nemathecia, and one new record for the region, G. perplexa. Based on integrated morphological and molecular analyses, G. chouae is a synonym of G. parvispora and G. firma might be a synonym of G. hainanensis, while G. articulata, G. changii, G. hainanensis, and G. yamamotoi are independent entities. This study also revealed the phylogenetic relationships between Chinese Gracilariaceae species and global taxa, which constitute a gap in several inner Gracilariales lineages compared with previous studies. Most Chinese taxa matched well with the global taxa, except for G. spinulosa, Gp. chorda, and Gp. lemaneiformis, although our collected specimens of these three species matched quite well morphologically with historical specimens. Controversial species will be better placed when molecular data from type specimens become available.