Coral Basics Primer

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Ahermatypic, Hermatypic What does it all mean?

A Brief Introduction To coral Anatomy

by Beau Crowley

What are corals? The term coral has several different dictations, but most commonly it refers to the order Scleractinia, all of which have hard limestone skeletons. This order is divided into two main contributors: reef-building and non-reef-building. Most of these two groups are hermatypic and need to acquire sunlight to live. Other organisms do build skeletons similar to those of the same order and these are normally known as non-scleractinian corals.
Other organisms resemble corals but contain no skeleton (Veron, 1993). Most true corals build massive structures that accumulate that cementation of skeleton sand, thus revealing a multitude of coral colonies over thousands of years.

Imposing as they are, the presence of modern coral reefs is the result of a special relationship between the coral polyp and the unseen single celled algae which live symbiotically within the cells of the polyp.

These algae, which are commonly called zooxanthellae, belong to a group of unicellular brown organisms known as dinoflagellates. Most flagellates are part of the phytoplankton of shallow tropical seas where they are a food contributor for zooplankton.

A few members of this group are less benign, occasionally causing lethal red tides and shellfish poisoning. Like land plants, zooxanthellae are able to use the sunlight in photosynthesis by making their own food from CO2.

Symbiotic algae benefit hermatypic corals in two ways. Firstly, 94 to 98 percent of all the organic nutrients are produced by the zooxanthellae, and is used as the major food source by the coral polyp. Secondly, due to photosynthesis of zooxanthellae, hermaphroditic corals are able to deposit their limestone skeletons 2 to 3 times faster in light rather than the dark.

Light enhances the rate of calcification and enables reefs to grow faster than they are broken down by the natural processes of the sea and eroding organisms (Veron 1993).

Even with coral larvae containing zooxanthellae, obtains it directly from their parent polyp during their free swimming stage. The algae multiply as the coral grows and they are responsible for the brown colors in the hermatypic corals. Due to environmental stress, the algae can be expelled by the coral as we will see later in case studies.

Ahermatypic corals that do not contain zooxanthellae are not restricted to high light intensities waters and can exist at almost any depth in these corals all nutrition comes from plankton. Less than one-third of all ahermatypes found in the ocean are colonial (Veron 1993).

These observations raise several questions about evolution of corals and coral reefs. Which evolved first, corals or the algae zooxanthellae?

Were the corals that helped build Paleozoic reefs hermatypic? At this point there is no accurate way of distinguishing hermatypic from ahermatypic fossils. Many corals have evolved in deep water forms and continue to evolve independently.

These may or may not have came from shallow reefs and contained zooxanthellae and their tissues. It seems likely that the symbiotic relationship played some role in ancient reef development, but perhaps a lesser one than it does today. Like a patch work of a miniature forest, the coral reef is made of different communities, each one separate from the other. But somehow they are linked to the next community by a complex web of ecological interactions.

Each reef builds a series of narrow bands or zones that has its own environmental conditions or gradient. The most important factors of reef dynamics are light availability, wave action, salinity, and tidal range.

All of these factors work together to form a complex ecological gradient. These factors are related especially where wave action affects sediments and this affects visibility, which in turn affects water clarity and light availability. Most hermatypic corals require light for photosynthesis of the zooxanthellae that are contained within the tissues.

The light changes as water depth increases, intensity and composition are affected. The changing intensity is not visible to underwater divers, but photographers know that camera flash lights must be used for intensity and color balance. even though the water may be clear. Due to light requirements in different corals, complex communities may evolve.

The different zones will allow for different lighting conditions and water clarity. This may be due to sediment type, tidal zone, and geological contour, of a certain reef type.

The second controlling factor is wave action. Wave action reaches extremes on the reef fronts and the outer flats on a calm day a reef front a benign appearance. The small waves due to tides disturb the peace and yet during a storm the site has a very strong wave action. As a wave comes in building huge force along the fore-reef it crashes down on the outer flat. In this case few species will survive such pounding . Only a few hundred meters away on the lower reef slope there may be little or no water movement at all.

Different types of sediment exist on and around a coral reef. These include coral rubble in different sizes from sand to mud, but it all depends on the exposure to currents and wave actions. Different sizes and different organic components can reduce light penetration and can kill certain organisms as corals, either by choking the polyps or burying them, thus not allowing the zooxanthellae to photosynthesize.

In rare instances salinity of sea water will become high enough to affect coral communities One place is Shark Bay, where a large amount of water is landlocked and combines with a low title range to produce a salinity which may be high enough to kill corals. Reef flat corals are generally tolerant of short periods of low salinity, but when heavy rainfall and very low tides combine, or follow one another, communities may be damaged or destroyed.

Like all other organisms corals require food and inorganic nutrients to live. Hermatypic corals have two major food sources organic nutrients produced and excreted by the symbiotic zooxanthellae into the tissues of the host. The second source comes from their prey in the form of free floating plankton.

Corals growing in shallow clear water communities normally have small polyps. Their main diet consist of sugars that are fed through the photosynthesis. However they can supplement their diet with small zooplankton, mainly at night. Most coral reefs exist in a poor inorganic nutrient environment . Phosphates, nitrates and iron exist in at trace levels, yet these reefs have a productivity that is similar to the rain forest.

Colonies of corals and their zooxanthellae absorb dissolved nutrients from sea water and recycle nutrients from the waste of one another.

Since the reef as itself receives only low nutrient levels from the surrounding ocean it must conserve and recycle. The relationship of zooxanthellae and corals has developed an efficiency that can only be achieved through self regulating processes which, when combined, make up nutrient cycles of most reef inhabitants (Veron 1993).

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Coral Basics Primer Cont.

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).

END.