Knobby brain coral

The knobby brain coral is a massive coral that either forms hemispherical domes or, particularly in areas of high wave action, forms plates and encrusts the seabed. It can grow to a diameter of about 1.3 metres (4 ft 3 in). The surface of the dome usually has a number of bulges or knobs but this species is not easy to distinguish from the symmetrical brain coral which tends to have a smoother outline. The surface consists of sharply delineated, convoluted ridges with valleys in between. There is no trough-like groove in the top of the ridge as is the case in the rather similar grooved brain coral (Diploria labyrinthiformis). The coral polyps are strung along the valley bottoms, each sitting in a little stony cup or corallite. The sides of these have minute walls called septa which come in four different sized cycles. They extend outside the corallites as costae that join one corallite to another but are discontinuous in this species, another distinguishing factor. The colour of the coral is usually some shade of yellowish or greenish brown and is caused by the presence of symbiotic dinoflagellates called zooxanthellae in the coral's tissues.