Open brain coral

The open brain coral (Trachyphyllia geoffroyi) is a vividly colored, free-living coral belonging to the family Merulinidae. It is the sole member of the genus Trachyphyllia and is found across the Indo-Pacific, from the Red Sea to New Caledonia, at depths of up to 40 meters. These corals are small, rarely exceeding 20 centimeters in diameter, and can be either solitary or colonial. They have a flabello-meandroid growth form, with distinct valleys separated by walls; in colonial forms, a single valley may house multiple polyps.

The valleys range from hourglass-shaped to highly lobed, and the coral typically shows bilateral symmetry. During the day, when the polyp is closed, a fleshy mantle extends beyond the skeleton but can retract if disturbed. Colonies display vibrant colors including blue, green, yellow, and brown. This species also hosts the gall crab Lithoscaptus semperi.

Open brain corals are less common on direct coral reef communities and are more frequently found on sandy reef slopes, near continental islands, and in lagoons, often alongside other free-living corals. Large colonies are rare and usually observed only in marine protected areas.

The IUCN lists the open brain coral as near threatened, primarily due to habitat loss and over-harvesting for the aquarium trade. Indonesia is the largest exporter; in 2005, it exported over 60,000 specimens. Additional threats include disease, ocean acidification, and severe storms.