Photo copyright Tim Nicholson.
Taken at night on Middle Reef, in the Egyptian Red Sea.
Cup coral, Tubastraea aurea
Tubastraea lack symbiotic algae and do not contribute to reef building. They tend to grow on underhangs, drop-offs and reef slopes in strong currents. The name combmes from the Latin for tube (tubus) and the Greek for star (sastron). At night the coral skeleton (corallum) is hidden by a ring of bright yellow tentacles, as shown in the photo above. During the day the tentacles are retracted into deep circular calices.
The skeletal tubes measure 5-15 mm across and protrude by 10 mm or more from the coral surface. Tubastraea species occur from the surface to depths of nearly 1500 m. There are four or five species in shallow Indo-Pacific waters (Red Sea, Thailand, Indonesia, Hawaii, etc.).
Cup coral is also known as sun coral.
Corals of the World, by Dr Elizabeth Wood, T.F.H. Publications Inc. ISBN: 0-87666-809-0.
The Underwater Photographer: Digital and Traditional Techniques, by by Martin Edge, Paperback, 536 pages (2009)