Systematic revision of the non-geniculate coralline genus Phymatolithon (Hapalidiales, Rhodophyta) from Taiwan, including three new species

Phymatolithon is a dominant genus in the algal reefs in Northwest Taiwan. In this study, we describe three new species of Phymatolithon, P. fragile sp. nov., P. taiwanense sp. nov. and P. variabile sp. nov., based on morpho-anatomical characters examined with both light and electron microscopy, and DNA sequence analyses. Phymatolithon fragile is only found in northern Taiwan (a subtropical area) and can be separated from the other species from Taiwan based on its thin thallus (<130 µm thick when mature vs >200 µm in other species) and the possession of bisporangia only. Phymatolithon taiwanense and P. variabile are widely distributed in both northern (subtropical) and southern (tropical) Taiwan and can only be distinguished by a combination of characters relating to the sizes of tetrasporangial compartments and the number of cell layers in cortex and medulla. In addition, P. taiwanense and P. variabile are the first two species of Phymatolithon shown to occur in tropical waters. This study suggests that our knowledge of the species diversity of Phymatolithon in tropical regions, particularly in the Indo-Pacific, will increase when more collections are made and analysed by a combination of morphological and molecular approaches.