Diversity and Ecology of Species Associated with Coral Reef Systems in the Western Gulf of Thailand, including the Description of Two New Species.

The brown macroalgal genus plays important ecological roles in many marine ecosystems. This group has received much attention over the past decade, and a considerable number of new species have been identified globally. However, our knowledge of the genus diversity and ecology along south-east Asian coasts are still limited. Given the growing body of research that uses a combination of molecular and morphological data to identify cryptic species, this study investigates the diversity of in the western Gulf of Thailand using morphological and molecular data, as well as their interactions with scleractinian corals. A total of 36 specimens were collected from 15 sites in the western Gulf of Thailand and used for molecular and morphological analyses. One mitochondrial (3) and two chloroplast (A and L) genes were amplified and sequenced for molecular phylogenetic analyses. Based primarily on phylogenetic evidence, two new species were formally described, sp. nov. and sp. nov. Additionally, was newly recorded from Thailand. Two new lineages of were identified, 12 and 13. Among the species identified, three were found in interaction with corals, the most notable of which was the massive coral . sp. nov. only interacted with by growing on bare coral skeleton between colonies. Furthermore, 13 was observed under the branching coral . Our findings revealed that presented both effects and absence of effects on coral. A thorough understanding of diversity and ecology is essential for ongoing and future research on coral-macroalgal ecological relationships.