An Unusually Deep Corallimorph Barren on a Remote Coral Reef in the South-Central Red Sea

Corallimorphs are opportunistic cnidarians that can compete with reef-building corals for substrate and thrive in disturbed conditions. Their clonal reproduction often results in a patchy distribution, and they typically occupy a negligible percentage of benthic area on tropical coral reefs. In the Red Sea, the corallimorph Rhodactis rhodostoma has been observed in dense aggregations inhabiting shallow reef flats and nearshore habitats, and is less abundant on reef slopes deeper than 3 m. Here, we describe an unusually deep corallimorph barren on the exposed slope of a remote offshore reef 96 km from the Saudi Arabian coast in the south-central Red Sea. Based on molecular and morphological characteristics, we tentatively identify the species as R. cf. rhodostoma. Mean percent cover of R. cf. rhodostoma ranged from 25% to 35% from depths of 15-30 m, while it was completely absent at 5 m. Although mean percent cover of R. cf. rhodostoma was less than 50% for all depths surveyed, localized patches resulted in areas consisting of ~90% coverage. Contrary to many reports of corallimorph barrens appearing after a physical disturbance, there are no obvious factors promoting corallimorph dominance on this remote reef. Further surveys are needed to document the total extent of the corallimorph barren and to assess if other reefs in the region are susceptible to similar phenomena.