CORAL REEFS
Coral reefs are the most diverse and beautiful of all marine habitats. Large wave resistant structures have accumulated from the slow growth of corals. The development of these structures is aided by algae that are symbiotic with reef-building corals, known as zooxanthellae. Coralline algae, sponges, and other organisms, combined with a number of cementation processes also contribute to reef growth.
The dominant organisms are known as framework builders, because they provide the matrix for the growing reef. Corals and coralline algae precipitate calcium carbonate, whereas the framework- building sponges may also precipitate silica. Most of these organisms are colonial, and the slow process of precipitation moves the living surface layer of the reef upward and seaward.
The reef is topographically complex. Much like a rain forest, it has many strata and areas of strong shade, cast by the overtowering coral colonies. Because of the complexity, thousands of species of fish and invertebrates live in association with reefs, which are by far our richest marine habitats. In Caribbean reefs, for example, several hundred species of colonial invertebrates can be found living on the undersides of platy corals. It is not unusual for a reef to have several hundred species of snails, sixty species of corals, and several hundred species of fish. Of all ocean habitats, reefs seem to have the greatest development of complex symbiotic associations.
Branching and Mound-Shaped Corals
Photo by Phil DustanReef-building corals may occur in a variety of growth forms, and there often is strong variation in coral shape even within a species. In this photograph, we can see the branching elkhorn coral Acropora palmata (upper left), with arms showing strong orientation into an oscillatory current between left and right. In the right foreground is the mound-shaped coral Montastrea annularis. It is of great interest that such differently shaped corals can occur side by side. What might that indicate to you?
Intraspecific Variation in Growth Form
Here is an intraspecific example of variation in growth. We see two colonies of Montastrea annularis, but one (center) is platy in appearance, whereas the other (right) is more mound-shaped.
Deeper Water form of "Monastrea annularis"
Photo by Phil DustanIn deeper waters (ca. 30 m) in the reefs of the north coast of Jamaica, one finds a greatly flattened species of Montastrea, which is a very close relative of the mound-building shallow water Montastrea annularis. The flattened mushroom-like shape may be an adaptation to capture light efficiently.
Plate-like Growth of Agaricia sp.
Photo by Phil DustanBy contrast, species of Agaricia tend to be plate-like in form and do not display nearly as much variation as some other dominant corals on the reef. A large fauna of sponges, bryozoans, articulate brachiopods, and other sessile epibenthic invertebrates live on the undersides of these platy corals.
Oriented Branches of the Elkhorn Coral
Photo by Phil DustanHere we see the occurrence of a group of colonies of the elkhorn coral Acropora palmata, with arms conspicuously oriented left to right. This probably reflects a predictably strong bidirectional current. Because of the extensive growth plasticity of some corals, it is often difficult to distinguish between distinct biological species and plasticity of form within a species.
Platy Form of the Elkhorn Coral
Dancing Lady Relief, Jamaica
Photo by Phil DustanThis shot of a reef on the north coast of Jamaica vividly captures the large amount of shade cast by the dominant elkhorn coral Acropora palmata. Because reef-building corals depend upon zooxanthellae for colony growth, the shaded areas are not good areas for vigorous coral growth, which depends upon light.
Hexacoral Polyps, Closeup
Photo by Robert RichmondHexacoral polyps (e.g., stony corals) have tentacles in groups of six. In hermatypic - or reef-building - corals symbiotic zooxanthellae are found within the endoderm and are usually concentrated in the tentacles, which increases exposure to light. The tentacles are also the site of enormous numbers of nematocysts, which can ensnare or puncture prey.
Polyps of Montastrea cavernosa
Photo by James W. PorterThis is a closeup of the massive scleractinian coral Montastrea cavernosa. The tentacles capture smaller zooplankton, by means of nematocysts.
Octocoral Polyps
Photo by Robert RichmondOctocoral polyps (e.g., sea whips, sea fans) have 8 tentacles, instead of 6. Particles such as smaller zooplankton are probably captured by direct interception on the tentacles. In some cases, such as in sea fans, water streams around the colony and polyps capture particles in the quiet eddies that are formed on the downstream side of the colony.
Crown-of-Thorns Starfish, Acanthaster planci
Photo by Robert RichmondAcanthaster planci is a carnivorous asteroid starfish whose principal prey are corals. It has sharp dorsal spines that penetrate human skin and cause sickness. Normally it occurs at low density and is nocturnal. In the 1960s, population explosions were found all over the Indo-Pacific and the starfish were moving in large numbers during the day. Instead of being of minor impact, they were now major predators on the reef. In Guam, for example, over 90% of the corals were killed. The explanation of the origin of the outbreaks is still very controversial. Charles Birkland suggested that they were related to storms that could wash out nutrients from atolls, which stimulated phytoplankton grown, which, in turn, increased the survival of starfish planktotrophic larvae. Others have argued that larval starvation is not a limiting factor in larval survival. It was also argued that typhoons and other storms killed corals, causing starvation among the starfish and a switch to roving in aggregations in search of food.
Pacific shallow reefs are often dominated by the the erect coral Pocillopora damicornis. When the crown-of-thorns starfish tries to mount and consume the coral, several species of shrimps and crabs that live among the coral branches move to attack the starfish, thus defending the coral.
