Madracis ridges Below 90 feet on the high diversity reef, you will also see, nestled among the larger corals, ridges or knolls, high concentrations of the small branching finger coral Madracis mirabilis. These unusual thickets also feature finger sponges, encrusting sponges and algae. The Madracis ridges are also found scattered around the low diversity reef habitat. To examine the low diversity reef habitat, you must venture below 120 feet into water as deep as 170 feet, not advisable for the average recreational diver. At these depths, corals grow in a flattened manner to maximize their exposure to light, a critical element of life to the symbiotic algae living in the corals' tissues.
Habitat relief is much lower than on the shallow reefs.
Fewer corals live in this zone, primarily because most species need more light. At about 150 feet, you will begin seeing the algal-sponge habitat, which extends to around 270 to 290 feet deep. Dominated by coralline algae, this habitat covers several square miles, a much larger area than is inhabited by corals. Algal nodules, up to fist size, cover 50 to 80 percent of the bottom in places. Because of the area covered and the amount of carbonate deposits produced, the algae may be more important to reef formation, overall, than corals are on the banks.