Studies of inshore and offshore coral reef systems usually conclude that there are fundamental differences between them assuming that terrigenous influences diminish with distance from the mainland. However, with few exceptions reefs in Thailand are fringing reefs around continental islands, whether inshore or offshore. We describe patterns in coral community composition along a 70 km transect from the mainland to offshore islands in the southern Andaman Sea. We found that coral assemblages are comprised of two basic types. Differences in coral assemblages were associated with obviously different environmental conditions, including underwater visibility and level of suspended matter, which were not always aligned with distance from the mainland. While reef communities were quite variable between sites, the inshore coral assemblages tended to be characterized by higher numbers of Montipora, Pectinia, Podabacia, and Acropora compared to reefs around offshore islands. Offshore reefs were characterized by a relatively high number of Lobophyllia and Plerogyra colonies. The expected relationship between water quality and sensitive indicator groups like Acropora did not materialize with Acropora and Montipora being more common inshore. While there were strong inshore-offshore reef habitat differences in environmental conditions and reef communities in southern Thailand, they were not consistent with expectations from the literature.
