A species of coral, Trachyphyllia geoffroyi, known for its unusual shape and spectacular flourescent properties under ultraviolet radiation. The open brain coral has a few large polyps and generally lives on sandy surfaces on the ocean floor, often settling on a mollusk while young and detaching as it reaches maturity. It is quite photogenic and popular in aquariums.

The "brain" part of the coral's name should be obvious - it bears a strong resemblence to the well-developed cortex of a higher primate. "Open" is probably due to its diurnal habits - during the day it expands dramatically by filling with water. At the onset of darkness its large mantle withdraws into its calcium carbonate skeleton and small tentacles emerge to feed, resulting in an organism less than half its size a few hours earlier. The coral's color is not constant -it varies depending on the type and intensity of light it's being exposed to, from green to pink to blue to yellow to crimson, and combinations thereof.

Its flourescence under light of shorter wavelengths is due to highly specialized proteins. It has been suggested that their purpose is to negate the harmful effects of this radiation (think sunburn) by absorbing the UV and radiating visible light back.