Morphological and molecular characterization of Gracilaria tortifolia sp. nov . (Gracilariales, Rhodophyta): A new foliose species from Hainan Island, China

Flattened Gracilaria species with spiny blade margins represent one of the most taxonomically challenging groups in the northwestern Pacific Ocean, with broadly applied names such as G. spinulosa often masking unrecognized species diversity. During fieldwork at Yinggehai, Hainan Island, China, we collected flattened Gracilaria specimens that morphologically resembled G. spinulosa. We examined these specimens using an integrative approach combining detailed morphological observations with rbcL phylogenetic analyses. Our specimens formed a strongly supported clade together with material previously reported as G. ‘spinulosa’ from Hainan and Guangdong but were clearly separated from authentic G. spinulosa from Tainan, Taiwan (the type locality) and were recovered as sister to G. huangii. Morphologically, the Hainan specimens were distinguished from both G. spinulosa and G. huangii by their conspicuously twisted upright blades, distinctly thickened cystocarp floor, and well‑developed basal nutritive filaments. Based on this combined evidence, we describe these specimens as a new species, Gracilaria tortifolia sp. nov. The recognition of this species highlights the long-standing problem of how the name G. spinulosa has been applied across the north-western Pacific and underscores the need for integrative re-examination of G. spinulosa records elsewhere in the north-western Pacific to clarify its true geographical range.