The Central Depression

The peripheral lip encloses a central depression (or inner cup). The diameter of this depression may vary from 2 m on the smaller nearshore reefs to as much as 15 m offshore. Merging and coalescing of cup reefs may expand the diameter of the central depression to 30 m or more. On nearshore reefs, the central depression is often only one or two feet lower than the lips, but may be as much as 2.5m lower. On the nearshore reefs, there is a distinct break between the lip and the inner depression, while on offshore reefs, lips slope towards the center of the cup without any distinct break. A limited number of encrusting organisms inhabit the central depression, although the environment is extremely vigorous and unlikely to become stagnant even at low tide when the pools are slightly above (<30 cm) mean sea level.

In the central depression of the nearshore cup reefs, red coralline algae, foraminifera and bryozoan are common, whereas on the offshore structures a wider variety of organisms is present. Coralline algae, anthozoans, and hydrozoans are commonly observed. Skeletons of coral give the central depression of the offshore reefs a much more irregular topography with higher relief than the more nearshore cup reefs.