Diving Indonesia Periplus Adventure Guid. David Pickell
Coral skeletons are made of aragonite, a very soluble form of calcium carbonate. The material is secreted as a way of disposing of excess ionic calcium.
Grazing and predation of fish and invertebrates causes portions of the coral skeletons to die, and these are immediately encrusted with algae, sponges, soft corals, or any of a myriad forms of small invertebrates. Over time, these too are grazed, silted over by coral sand, or out-competed by other organisms, and their remains become part of another compacted layer.
The lithification of coral rock is not well understood, but a fine-grained carbonate cement seems to form in the pores of the old coral, turning it into dense coral rock. This is thought perhaps to result from bacterial action.
The buildup of limestone on the reef is not a simple process of accumulation. It is a cycle just like the nutrient cycle. Scientists studying a 7-hectare reef in the Caribbean measured an annual production of 206 tons of calcium carbonate; they also measured an annual loss of 123 tons. The greatest part of this erosion was produced by boring sponges, and the rest by grazing fishes and echinoderms.
Not all the limestone produced is created by corals, either. In some areas, particularly where there is very strong wave action, calcareous algaes are the primary producers of carbonate, forming algal ridges at the outer edge of the reef.
Coral Reef Architecture
Coral reefs are generally defined as falling into three main types: fringing reefs, barrier reefs and atolls. In a sense, these types also form a historical progression. As a fringing reef grows outward, a boat channel forms behind. As the reef widens, the boat channel becomes a lagoon, and the fringing reef graduates to a barrier reef. If the fringing reef began around an island, and the island subsequently sinks or the sea level rises, the final result is an atoll, a near circular reef surrounding a central lagoon.
Zooxanthellae and Corals
Reef-building corals have evolved an indispensable, symbiotic relationship with a type of yellow-brown algae called zooxanthellae, which is "farmed" in the tissues of the coral polyp. The relationship is mutually beneficial: the coral receives oxygen and nutrients, and the algae receives carbon dioxide and "fertilizer" in the form of animal waste.
The presence of zooxanthellae is so important to the health of the coral that scientists speculate the symbiotic algae must have been present in the polyp tissue almost throughout modern coral's 50-100 million years of evolution.
The zooxanthellae alga has been dubbed Symbiodinium microadriaticum, part of a "supergenus" of marine dino-flagellate algas, but recent research suggests there are more than one species. These algas are dinoflagellates, which have whip-like processes giving them some limited ability to move. This is probably how the cells initially enter the corals, although once they are there they divide vegetatively, and take on a simpler structural form.
Corals are not the only reef animals to have zooxanthellae. Sea anemones and other cnidarians host the algae, as do some molluscs, most famously the giant clams (Tridacna). Because zooxanthellae is a yellow-brown algae, and the host tissues are generally colorless—to pass the greatest amount of light to the algae—zooxanthellae-containing animals are usually a dull color: beige, brown, olive green. There are exceptions, however, including some of the giant anemones and Tridacna clams, which can be richly hued. As a general rule, however, the most brightly colored invertebrates—such as some of the soft corals—do not harbor zooxanthellae.
Coral nutrition
Corals derive their food energy from three sources: plankton captured by their tentacles, organic nutrients absorbed directly from the water, and organic compounds provided by the zooxanthellae. For the reef-building corals, the latter is by far the most important.
In the presence of sunlight, the zooxanthellae produce oxygen and photosynthetically fix nutrients—glycerol, glucose and amino acids—which are "leaked" to the surrounding tissues of the coral polyp. The raw materials for this process are the waste products of the coral animal: carbon dioxide, ammonia, nitrates and phosphates. It is a very efficient, almost self-sustaining partnership.
— David Pickell
On the steep walls of Indonesian reefs, normally deep-dwelling species can be seen at relatively shallow depths. This is a male square-spot anthias, Pseudanthias pleurotaenia.
These are not the only forms, and scientists studying reef topography offer many more types. For example, bank reefs, reefs that grow up more or less in patches in open water where the depths are relatively shallow, are an important type in the Coral Sea off Australia. These reefs often form at the edge of undersea plates, and appear when geologic activity causes an uplifting of the bottom. If the bottom is pushed up high enough that sunlight can reach it, a bank reef will form.
Fringing reefs. Most of the reefs a diver will be exploring in Indonesia are fringing reefs, coral formations that grow right up to the edge of an island. These reefs can take many forms. The steep coral walls for which Indonesian diving is famous are fringing reefs, with sometimes just a few meters of reef flat, and a reef edge that has an almost vertical slope.
Walls, or drop-offs, fascinate divers because these are where deeper dwelling animals come closest to the surface. Some of the deep-dwelling dwarf angel-fish (Centropyge spp.), damsels, and anthias (e.g., Pseudanthias pleurotaenia) can be found at comfortable depths only along steep drop-offs.
Generally, a fringing reef consists of a reef edge of stout corals, which absorb the brunt of the waves and current; a reef flat, a shallow area exposed at the lowest spring tide; and perhaps a boat channel or back reef, deeper than the reef flat and quite calm.
The reef edge, and the fore-reef area towards the open sea, are the most rewarding areas for the diver. Here the current is strong, bringing plankton and fresh water from the open sea. Here also is where divers will see larger reef fish, and occasional pelagic visitors to the reef. Sometimes the reef edge is indistinct, marked by pinnacles or other formations. And sometimes the area just back of the reef edge will not immediately become part of the reef flat, but instead, protected from the full force of the current, will be rich in more delicate corals and animals.
The reef flat is shallow, and usually light brown with sediment. This is an area of coral sand and detritus, with small boulders of hardy massive corals and clusters of branching Acropora coral, growing in pools. Usually there are fewer than a handful of very hardy coral species on the reef flat.
Most divers will walk or wade across this area (wearing a pair of dive boots, of course) without even looking down. Here there are echinoderms—particularly brittle stars, which sometimes occur in great numbers—small fish, a variety of molluscs and soft algaes. Sometimes there will be meadows of the calcareous alga Halimeda.
The back reef or boat channel is a deeper area, between the reef flat and the shore. Although often deep enough for swimming, the coral growth here is poor because of sediment run-off from the shore. Resistant Pontes, Acropora or Goniastrea grow in the boat channel in patches. Further inland, there may be beds of turtle grass, a rich habitat for juvenile fishes and many crustaceans.
Barrier reefs. The most famous barrier reefs are the Great Barrier Reef off Queensland, Australia, which is 2,000 kilometers long and 150 kilometers wide, and the large barrier reef off the coasts of New Caledonia in Melanesia. A barrier reef is a fringing reef where the back reef or boat channel has become a large lagoon. In the case of the Great Barrier Reef, this "lagoon" is in places 100 kilometers wide.
Trepang drying on a dock in Pagimana, Sulawesi
Trepang Fishing
The lowly sea cucumber, a lumbering, inoffensive detritus feeder, hardly looks like something you would want to touch, much less eat. But this homely