4. The resulting structure—a reef

Reef profiles are remarkably consistent in general design because they are essentially biological constructions responding to environmental factors. All have reef flats, a horizontal expanse at low tide level that develops when the reef can no longer build upwards and extends horizontally. This is commonly the largest expanse and is the only part that early naturalists could examine until the advent of scuba diving relatively recently. However, it is always relatively depauperate because of the extremes of heat, salinity, and light that affect it. To seaward is the reef crest, commonly constructed by calcareous red algae that are the only form of life that can withstand the severe waves that pound the reef. To seaward of the crest, the reef slopes downwards, and is where environmental conditions are most benign and where the greatest biodiversity is found. The rugosity or 3-D structure of a reef is a key factor in reef biodiversity because it provides a vast number of niches for the huge diversity of life. Reefs are constantly subjected to erosion from burrowing and tunnelling forms, whose activity creates sand and sediment. This is then forced into crevices or pushed up onto the island behind it, so this erosion is also the starting point for further reef and island construction. Expanses of sand are associated with all reefs, and are home to another rich assemblage of species.