Among the eight species of Yonagunia described in the Indo-Pacific region, five species have been reported from Taiwan. In this study, we re-examined additional herbarium specimens which were morphologically similar to the other Taiwanese species of Yonagunia. Two new species of Yonagunia (Y. robusta sp. nov. and Y. stipitata sp. nov.) were identified based on rbcL sequence analyses, and their vegetative and reproductive structures were documented in detail. Yonagunia robusta is the largest species (up to 13 cm high) found in Taiwan and superficially resembles a previously described species, Y. palmata from northern Taiwan, but the thallus size is much larger and the branches are coarser in Y. robusta. Young plants of Y. stipitata may be confused with Y. maillardii, a species widely distributed in the Indo-Pacific, but the thallus branches are much thinner and numerous reproductive branchlets are formed in clusters in fertile Y. stipitata. Yonagunia robusta is widely distributed from northern to south-eastern Taiwan, whereas Y. stipitata is restricted to Orchid Island, a remote small island off the south-east of Taiwan. The species diversity (seven out of 10 described species) of Yonagunia is high in Taiwan and it is speculated that some more cryptic species will be found from the Indo-Pacific by using a combination of DNA sequence data and morphological observations. HIGHLIGHTS Ten species of Yonagunia have been described from the Indo-Pacific. Seven out of 10 species of Yonagunia are found in Taiwan. Cryptic species of Yonagunia will be found in the Indo-Pacific.
