Open brain coral

[1] They are free-living and exhibit a flabello-meandroid growth form, meaning they have distinct valley regions separated by walls.

[4] During the day when the polyp is closed, the coral is covered by a mantle that extends beyond the skeleton, but can retract when disturbed.

[1][7] Large colonies of open brain corals are uncommon, and are typically only observed in marine protected areas.

[1] The IUCN lists open brain corals as "near threatened" due to habitat loss and over-harvesting for the aquarium trade.

[1] Other threats to open brain corals include disease, acidification, and severe storms.

A red variety of Trachyphyllia in a reef aquarium