The Netherlands Antilles consist of two groups of islands:
• Curaçao, Aruba and Bonaire belong to the Leeward Group of the Lesser Antilles, off the coast of Venezuela, between latitudes 12º00'N - 12º40'N and longitudes 68º12'W - 70º04'W
• St. Martin (the northern part of which is French), St. Eustatius and Saba belong to the Windward Group of the Lesser Antilles, east of Puerto Rico, between latitudes 17º27'N - 18º08'N and longitudes 62º56'W - 63º16'W.
Well-developed coral reefs are only found in Curaçao and Bonaire and in parts of the southwest coast of Aruba. In the Windward Islands, there are no extensive coral reefs. These islands are surrounded by flat sandy bottoms, which slope gently to great depths. Coral species only occur isolated or in patches.
The coral reefs of the southwest coasts of Curaçao and Bonaire are generally of the same type. There's a submarine terrace stretching across a distance of 50 to 100 m from the coast to a depth of 8 to 12 m, where there is a drop-off and a steep slope to a depth of 50 to 60 m.
On the terrace and slope, the following reef zones may be recognized (Bak, 1976):
1. A shore zone, either in front of a shingle beach or in front of a cliff. In the first case, the bottom is mostly sandy and the most conspicuous organisms are algae, echinoderms and sponges (eg, Desmapsamma anchorata), while there are only a few encrusting corals present. In the shore zone in front of a cliff, the bottom is rocky and the environment is typically one of high energy. The rock surface is covered with algae, the White encrusting zoanthid Palythoa caribaeorum and encrusting Diploria clivosa.
2. The Acropora palmata zone. Between 1 and 3 m, the Elkhorn coral Acropora palmata was by far the most abundant coral, but after the hurricane Lenny in 1999, this species has strongly diminished in number. As it is a fast growing species, 6 to 10 cm per year, it is expected that this zone will recover. Other species dominant in this zone are encrusting calcareous red algae (Porolithon pachydermum) and the Blade fire coral Millepora complanata.
3. The barren zone, between 3 and 4.5 m, is devoid of great quantities of reef builders. The bottom consists of sand and coral fragments, most often branches of the Elkhorn coral (Acropora palmata). The Orange lumpy encrusting sponge (Scopalina ruetzleri) can be found on the coral fragments.
4. In the lower terrace zone, between the barren zone and the drop-off, the number of coral colonies increases, reaching a great density and diversity in the area before the drop-off. The Sun anemone (Stichodactyla helianthus) may occupy large areas between the coral heads.
5. In the drop-off zone, between 8 and 12 m, the corals Montastrea annularis, Agaricia agaricites and Madracis mirabilis are most dominant (drop-off boulders). Between the corals, the Vase sponge (Ircinia campana) and the Lavender rope sponge (Niphates erecta) are abundant.
6. In the upper-slope zone, between 15 and 30 m, a great density and diversity in species continues. Boulders of the star corals Montastrea annularis and Montastrea cavernosa and plates of the Tan lettuce-leaf coral (Agaricia agaricites) alternate with sponges. Especially dominant are the Lavender rope sponge (Niphates erecta), the Orange elephant ear sponge (Agelas clathrodes), the Brown tube sponge (Agelas conifera) and Agelas nsp.
7. In the lower-slope zone, below 30 m, coral cover starts to decrease. Flattened growth forms of, among others, Lamarck's sheet coral (Agaricia lamarcki) start to become dominant. In this zone, the black corals Antipathes gracilis, Antipathes pennacea and the coiled Wire coral Cirrhipathes leutkeni can be found. This zone ends in a sand-covered bottom with scattered sponges. At some localities, small specimen of the Giant barrel sponge (Xestospongia muta) are dominant (Xestospongia muta-deep water), while at other localities Niphates recondita and Haliclona vansoesti are abundant (Haliclona vansoesti-deep water).
At some localities (for example, Holiday beach and Porto Marie, Curaçao and the area around Alice in Wonderland, Bonaire), the upper-slope zone is interrupted by a horizontal sand ridge that can reach a length of more than 25 m.
The northeast coasts of Curaçao and Bonaire have steep shore cliffs. The sea here is very rough because of the trade winds (northeast coast). In front of the steep cliffs, extensive fields of Sargassum platycarpum and Sargassum polyceratium can be found. These algae fields, located on substrates with many big and deep holes, may cover tens of meters in the direction of the drop-off. The Sargassum fields are the habitat of several fish species, including some groupers and juvenile snappers. Few corals are found here because of the strong water movement.
The coral species composition of the drop-off zone and deeper zones is about the same as that of the leeward reefs, but the growth form of the corals is more flattened and more gorgonians are present.