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Cilia, nematocyst, gastrodermis, mesoglea and ectodermis all form the coral polyp. A group of simply structured organisms which also include the jelly fish, soft corals and hydroids.
The coral polyp is an basically an anemone-like animal except it secretes a skeleton. Some corals are free swimming and look just like anemones with their tentacles spread open and swollen with inhaled sea water. Others are colonial, and these are the reef building corals.
A coral polyp is a sac with a single mouth at the top with a circle of tentacles protecting the center. The mouth leads into a short straight tube labeled as the pharynx, this opens into the body cavity . The body wall of a polyp is composed of two layers, the ectodermis on the outside and the gastrodermis on the inside.
These layers are separated by mesoglea which is initially non-cellular but will contain a wide range of cell types after initial growth.
The extended polyp with its anemone-like appearance has tentacles composed of the same two layers. The gastrodermis, the inner-cell layer, has specialized cells for digestion. which occurs partly inside the gastrodermis cells themselves (intercellular digestion). Because the body cavity of the surrounding polyps are all interconnected, the polyps of a colony share nutrients.
These polyps do not compete against one another. The gastrodermis is the layer that houses the zooxanthellae, the single celled algae which exist within the gastrodermal cells themselves and are necessary to the growth of most corals. These cells are minute, ranging from 0.008-0.012mm in diameter. Zooxanthellae exist in enormous numbers of the polyp tissues.
Photosynthesis has allowed reflex structures to be built by plants and has allowed reefs to be built by a variety of animals that act as plants due to symbiosis. Zooxanthellae symbiosis is so effective that algae not only meet most of the host requirements, but also allow their host to act as the primary recipient.
The modern reefs have a gross productivity that exceeds most ecosystems. The subject of symbiosis attracts a wide range of theories. The term symbiosis can be defined as the living together of differently named organisms. Endosymbionts (living within a host animal) is the term zooxanthellae is given . Zooxanthellae are a composite of many families, and perhaps classes, of flagellates.
The formal links of this term with algae, taxonomy and the lack of applicability of them to fossil records, exclude it from being readily accepted as a functional descriptor. Vernon (1995) and Schumacher and Zibrowius (1985), have compiled an inventory of uses and misuses of the term zooxanthellae.
Zooxanthellae are now termed "endosymbiotic primary producers". Color variations in corals are most difficult to generalize about, mainly because they involve so many different categories. Also they have so many different casual relationships involving their zooxanthellae symbionts.
Some species have a specific color or colors, and can also range due to geographic locations. The most common variations in color are correlated with the physical environment, especially light.
Colonies that are exposed to intense light are relatively pale. Massive colonies in shallow water are often pale at the top and with dark sides, whereas colonies of the same species in deeper water are one complete color. This is due to the density and color growing in the zooxanthellae (Veron 1995).
Population dynamics of zooxanthellae in corals are being researched as we speak. Population of symbiotic zooxanthellae are characterized by low growth rates relative to populations of cultured zooxanthellae. The low growth rates exhibited by zooxanthellae have been cited as evidence of the host influence over the metabolism of symbiotic algae, either passively through restricted access to space and nutrients, or actively through host specific mitogenic or sytogenic factors (Ove Hoegh-guldberg 1994).
A key experiment in identifying the importance of passive "control" mechanisms is to supply an excess of a particular nutrient, and examine the response of the the growth rate of zooxanthellae. If an increase in the growth rate occurs after the adding of a nutrient then the passive supply of the nutrient is an important factor in explaining the low growth rate of zooxanthellae.
In contrast there is limited information on the biochemical composition of symbiotic algae (Fadlallah 1983).
Ammonium and Phosphate enrichment on the carbohydrate, lipid, and protein content analysis of both zooxanthellae and Stylophoraphistllata coral tissues showed no trends with treatments. However in zooxanthellae the carbohydrate content decreased under ammonium enrichment.
The difference between free-living algae and symbiotic algae is in terms of fate in their metabolites. In phytoplankton most of the excess metabolites are directed toward cell reproduction. In symbiotic zooxanthellae most of the metabolites are translocated and used up by the host (Davis 1984, Muscatine Et al. 1984)
In most shallow waters of reefs hard corals, soft corals and gorgonians can be found and contain zooxanthellae. The zooxanthellae play an important part in nutrition and calcification of their host, and they may also contribute defensive chemicals which assist the host in surviving predation and competition for space on coral reef locations.
Zooxanthellae contribute considerable organic matter to the sediments, some of which may serve as chemical markers. A wide variety if sterols are found in zooxanthellae, and some have been used to trace and identify coral devouring predators. The sterol pattern of zooxanthellae isolated from various host vary, thus indicating the occurrence many different species.
The chemistry of the algae in the host differs from the motile form grown in axenic culture (Ciereszko 1989). Eunicinis is produced by the gorgonian coral E. manosa in large quantities as a defensive chemical that affects a wide variety of organisms (Ciereszko 1989).
The motile forms of zooxanthellae have been traditionally considered to be one species, but accumulating evidence indicates that there may be many (Ciereszko 1989) (Trench Blank 1987). Kokkeetal (1981) examined the sterile composition of zooxanthellae from three Caribbean gorgonians, and found that sterile compositions of the three zooxanthellae cultures were different from one another.
(Withers et al. 1982) found that cultures of zooxanthellae from the sea anemone Aptasia pulchella can synthesize the unique sterol gorgosterol and twenty three-demethylgorgosterol. There are also large differences in zooxanthellae, and that there is no taxonomic affiliation of the host and the sterile patterns of the zooxanthellae.
Studies of the chemistry of corals containing zooxanthellae have led to the discovery of a large variety of natural products that may be relevant to taxonomic chemical ecology and biochemistry. The difference in sterol patterns found in zooxanthellae supports the current view that there are many types of zooxanthellae. The unique presence of sterols such as gorgosterol in zooxanthellae allows the use of bio-markers in sediments, tracers and predatory animals. The active chemical compounds including prostaglandin, and a large variety of terpenoids in animals containing zooxanthellae, serve as defense mechanisms in deterring predation, and in discouraging settling of larvae competing for space on the crowded coral reef (Ciereszko 1989).
Beau Crowley
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