Feature Article: Tropical West Atlantic Corals, Part 1: General Information and Atlantic SPS Corals
Through its personal, day-to-day interactions with coral, the reef aquarium community as a whole has learned a lot about reef-building corals. However, since West Atlantic stony corals have been banned from collecting for the ornamental trade, most of the coral which is distributed through the aquarium trade originates from the Indo-Pacific region. Due to this collection disparity, aquarists in general aquarists know much less about the stony corals of the West Atlantic, including many of the unique endemic species. Furthermore, many marine aquarium books omit information about corals which are unavailable in the trade resulting in an even greater gap in our knowledge of West Atlantic corals. This is particularly disappointing for American reef keepers who know more about corals from seas and islands they will never visit than the corals which grow in U.S. territorial waters. Furthermore, the literature which does cover West Atlantic corals is usually written for divers and naturalists. These texts often discuss and present corals from a point of view which has little relevance for aquarists. By illustrating West Atlantic corals through close-up photography and relating the species to well-known Pacific species, this article introduces West Atlantic corals in a perspective which is familiar and informative for aquarists as well as divers.
Geography of the Caribbean Sea
The diversity of stony corals and the types of reef development in the Caribbean Sea can loosely be categorized into major geographic zones. These zones are designated by their climate, oceanography and their geography. The west coast of Florida and the Gulf of Mexico support reasonably good coral diversity. Since these reefs frequently experience strong storms, the coral cover is only moderate compared to other Caribbean reefs. Southeast of Florida, the Bahamas Archipelago contains an estimated 3000 islands which provide for heterogeneity of reef types, even on different sides of the same island. Northern shores are subjected to the force of the Atlantic Ocean whereas southern shores facing the Caribbean Sea are relatively sheltered from the swell of the open ocean. Below the Bahamas, the Antilles are an island chain stretching from Cuba in the West to Trinidad and Tobago in the East. The first few islands in the chain are known as the Greater Antilles and they include Cuba, Haiti and the Dominican Republic and Puerto Rico. The remaining smaller islands are known as the Lesser Antilles. As with the Bahamas, the northern shores of the Antilles are steeper and more exposed than the shallow southern shores which support better reef formation. The Greater Antilles support better coral growth than the Lesser Antilles with the exception of Aruba, Bonaire and Curacao which occur below the Hurricane Belt. Reef development is hindered on South American coastlines due to a large influx of freshwater but on Central American Coastlines, coral reefs thrive. The reefs of Panama are well developed and the Gulf of Honduras contains the Belize Barrier Reef System which is the longest barrier reef in the Caribbean Sea.
The Hurricane Belt
One of the major forces shaping the way reefs form in the Caribbean is known as the hurricane belt. The occurrence of hurricanes has a strong impact on the formation of reefs in terms of shallow water coral cover and coral diversity (Rogers 1993). As tropical storms move westward through the Caribbean Sea, they are barred from moving southward by a ridge of high atmospheric pressure which is generally agreed to occur near 12.5 degrees North. Reef areas in the path of the hurricane belt experience more frequent and more severe tropical storms. The increased damage to reef-building corals in this area leads to less coral cover but it fosters a greater diversity of shallow water corals. Reef areas outside of the hurricane belt experience fewer and less severe tropical storms. The reduced damage allows corals to grow larger and to cover more of the reef but it also enables more competitive corals to dominate, resulting in a decrease in the diversity of shallow water corals.
Diversity and Taxonomy
The Caribbean Sea is home to over 70 species of corals which are distributed in 20 genera and 14 families. All of the coral species and nearly half of the genera are endemic to the Caribbean Sea. Until very recently, it was believed that all Atlantic stony coral species belonged to families which also occurred in the Pacific Ocean. Classic taxonomic tools have relied primarily upon making distinctions between a coral's form and function or its morphology to determine its relation to other coral species. More modern taxonomic tools employ a variety of techniques including a growing emphasis being placed on genetic markers to differentiate between corals at all levels of taxonomy. Recent research using genetic markers has shown that certain Caribbean coral species which were once believed to belong to families also common to the Pacific are actually more closely related to each other in the first coral family to be endemic to the Caribbean Sea (Fukami et al. 2004).
Before we dive in to the eye candy portion of this article, it is our responsibility to understand some of the forces which are threatening the ecology of Caribbean reefs and the diversity and abundance of Caribbean corals. The causes for the decline in coral cover in the Caribbean are many and research is just beginning to understand some of these causes, many of which have links to human activities on land and in the sea. One of the most alarming causes of coral mortality is disease which in some cases has been shown to be aggravated and sometimes directly attributable to human sewage (Patterson et. Al 2002). Coastal development in many regions has led to increased erosion and with it, an influx of nutrient and sediment in coastal waters. The increased nutrients fuel algae growth and the sediments can irritate and kill coral (Rogers 1990). Furthermore, the depletion of predatory fish species by commercial and recreational fishermen has allowed damselfish population to grow unchecked. Since damselfish scrape away coral tissue to grow algae farms, their increased numbers lead to an increase in the loss of coral cover (Potts 1997). In many respects, Western society is often more concerned about the future of exotic ecosystems when our own natural resources are greatly imperiled. In North America, this is especially true of aquatic resources where over a third of 300 endemic freshwater mussel species are threatened or in need of conservation status (Neves 1993). Although it is noble and worthy to protect ecosystems with a high biodiversity of species, there is ample room for an increased awareness of the threats to our own local ecosystems.
There are hundreds of Acropora species in the Indian and Pacific Oceans but the genus is represented by only two species in the Caribbean Sea. A. cervicornis is called staghorn coral and A. palmata is called elkhorn coral. For a time it was believed that A. prolifera was a third species but recent research has shown that this coral is actually a hybrid of the two true Acropora species (Oppen et Al 2000). Acropora are important reef building corals for Caribbean reefs but their sharp decline in abundance and susceptibility to disease has warranted their placement on the endangered species list.
Acropora cervicornisis an important habitat for coral reef fish. The colonies grow into a staghorn shape with golden brown branches and sharp white tips. The branches are pretty robust but they are prone to breaking which facilitates asexual propagation. Due to the effects of the hurricane belt, extensive stands of A. cervicornis are most common in southern Carribean locations such as Roatan, Curacao and the Grand Cayman Islands.
Acropora palmata is commonly known as Elkhorn coral because its colonies are composed of thick, broad branches. This coral can grow to produce some of the largest colonies of all corals. Elkhorn coral is critical in building reefs because it grows quickly and it produces a sturdy reef structure. Although entire reef zones formerly consisted almost entirely of A. palmata, in recent years this species has declined by 80-95% of its former population abundance throughout much of its range (Gladfelter 1991). A. palmata occurs almost exclusively in very shallow, high energy environments.
To the seasoned aquarist, Agaricia will resemble commonly kept Pavona species as well as other Pacific genera belonging to the family Agariciidae. Agaricia is endemic to West Atlantic reefs and the seven recognized species occur on reefs at all depths. The colonies are usually encrusting, submassive or foliaceous. The corallites are small and closely set with shared walls and visible septa which radiate from the coral calices. The species are distinguished partially by the degree of ridge development between the corallites. Light brown and grey are the most common colors of Agaricia but the genus also occur in yellow, green, orange and shades of purple and bluish grey.
Agaricia agaricites is a highly variable species which grows into several recognized forms. Depending on the environment, colonies can grow encrusting or sub-massive (agaricites), plating or foliaceous (purpurea), thick bifacial plates (carinata) or upright bifacial lobes (danai). The surface has well developed intersecting ridges which give this coral a reticulated appearance. A. agaricites is usually colored a light golden or dark brown, sometimes with a pale growing margin. This species superficially resembles Gardinoseris from the Pacific.
Agaricia humilis is a somewhat cryptic species which rarely grows very large. Colonies are encrusting with closely spaced small corallites. The well developed ridges intersect frequently around the corallites making them appear deep and dark. The color is usually dark brown but it can also be shades of green and orange. A. humilis usually grows in protected crevices and underneath ledges of relatively shallow reefs.
Agaricia fragilis usually grows into small disc shaped colonies. The surface is very smooth with weak development of ridges. The outer growing edge frequently has a pale margin which fluoresces a brilliant green or orange. It most commonly occurs in sheltered reef areas at intermediate to great depths. It somewhat resembles certain Pacific Leptoseris species.
Agaricia lamarcki is a foliaceous species which can grow very large in the right environment. The colonies are usually found at intermediate depths and they frequently will develop into sheets and whorls. The surface of A. lamarcki colonies has a moderate development of ridges. The most notable feature of this species is the characteristic star-like pattern around each corallite which can be golden, white or blue.
Agaricia tenuifolia is a fast growing coral species which is an important contributor to the reef-building process. A. tenuifolia can grow very large and it frequently becomes the dominant species in certain high energy reef zones. The colonies are composed of thin upright bifacial plates with strongly developed grooves running parallel to each other and the growing edge. The color is a uniform light brown or grey with thin pale lines between the corallites. Agaricia tenuifolia is most common in shallow reef zones with strong regular wave action.
Porites is a genera which is common in the Pacific Ocean as well as modern reef aquaria. The five currently recognized Atlantic Porites species grow colonies which are massive, plating, encrusting or branching. The branching species can be very abundant in shallow protected waters whereas massive Porites is more common in shallow exposed reef areas. At just a millimeter across, the corallites are very small and they are closely spaced giving the skeleton a porous appearance. Colors are frequently yellow, brown or grey but some species occur in vivid red, pink and blue. Porites species appear very smooth when the polyps are retracted or they can appear fuzzy if the polyps are extended.
Porites astreoides is a common and highly visible coral on Caribbean reefs. It is most often vivid yellow or yellowish-green but it can also occur in grey and brown. In shallow water with strong water motion P. astreoides has a mostly encrusting growth form. At intermediate depth this species takes on a more typical dome or rounded ball shape with large lumps covering the surface. On deeper reefs the colonies can become plating but they usually retain the lumpy appearance on the colony surface.
Porites divaricata is one of the less abundant Porites species in the Atlantic. Although it can be found in a range of habitats, it occurs at relatively low densities. It is the thinnest branching species of the Atlantic Porites and its delicate branches are prone to breaking. In calmer back reef areas it can develop into small patches but it is most often encountered as a loose aggregation of unconnected branches. P. divaricata occurs in grey, brown and greenish but it will sometimes be a vivid red, blue or purple, usually with bright white or yellow branch tips. This species superficially resembles Montipora digitata.
Porites furcata occurs almost exclusively in very shallow water. It commonly grows up to the water surface in moderate wave action, in lagoons and backreefs. Of the branching Porites species, P furcata has medium sized branches which bifurcate frequently. The colonies of P. furcata provide very high quality habitat for a variety of small fish and invertebrates.
Porites porites has the thickest branches of all the Atlantic Porites species. This coral's branches have a somewhat enlarged appearance at the tips. The polyps can extend to a great degree making the branches appear fuzzy. P. porites occurs at all depths but it is the most common species in intermediate to deep reefs. The color is uniformly grey.
The next article will continue the coverage of West Atlantic SPS corals and it will also cover the diverse coral species in the Faviid family.
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