Taxonomy and species diversity of <i>Padina</i> (Dictyotales, Phaeophyceae) from the Indo-Pacific with the description of two new species

Taxonomy, species diversity, and geographical distribution of the marine brown algal genus <i>Padina</i> (Dictyotales, Phaeophyceae) from tropical South-East Asian waters were investigated through a combination of morphological and molecular analyses. The study presents a new schematic diagram showing a suite of morphological characters including thallus hairline structures that are useful for species delineation in the genus. This study also revealed two new species from Myanmar, <i>P. gracilis</i> sp. nov. and <i>P. lata</i> sp. nov. based on molecular and morphological data. <i>P. gracilis</i> is characterised by a 2-layered thallus, thin hairlines on both surfaces of the thallus at equal distances, non-indusiate reproductive sori distally very close to the hairlines on the inferior surface, and the presence of <i>Vaughaniella</i> stage. <i>P. lata</i> is characterized by a 2–4-layered tiny thallus, broad hairlines located only on the inferior thallus surface, non-indusiate reproductive sori abutting the hairlines on the inferior surface and irregularly spreading on the superior surface, and the presence of <i>Vaughaniella</i> stage. This study also revealed new distributional records for four <i>Padina</i> species: <i>P. okinawaensis</i> from Myanmar and <i>P. fasciata, P. ryukyuana</i> and <i>P. terricolor</i> from the Philippines. These findings point to elevated species diversity in central Indo-Pacific waters. This study confirmed that most of the <i>Padina</i> species in the Pacific regions are showing overlapping distribution across the regions, in contrast to species from the Indian Ocean, which tend to be confined to specific localities.