LESSON 5:
DOMINICA'S CORAL REEFS

What are coral reefs?

Coral reefs are formed by coral animals (each called a coral polyp), which can range in size from 1/10 of an inch to several inches in diameter. The polyp is tubular and saclike in shape, with a central mouth ringed with tentacles. Hexacorals, whose tentacles number in mutiples of six, are the primary reef-building corals of today. Each coral polyp of reef-building corals sits in a hard, calcium carbonate skeletal cup, called a corallite, which becomes cemented together with others to form colonies . Some hard corals form small colonies, others form colonies that are several feet high, all in a variety of shapes. Together, the different types of reef-building hard corals combine to create the foundation of a coral reef, that provide homes and places of attachment for many other kinds of reef organisms.

Where and why are reef-building corals found in Dominica?

Reef-building corals are found in Dominica primarily on the leeward west and southwest coast attached to volcanic walls and pinnacles. Dominica's volcanic rock provides a solid place of attachment that corals require to build reefs. (Substrates such as mud and sand are too unstable to support coral reef building.)

Dominica lies within the latitudinal band on earth (between 30°N and 30°S) where reef-building corals, which require warm ocean temperatures of 70°F to 85°F (20° to 29°C) to grow. Dominica also provides the clear water and light that reef-building corals need. This is because reef-building corals have a symbiotic relationship with photosynthesizing, microscopic algae (called zooxanthellae) that live in the surface tissues of the coral polyps. Both the coral and the zooxanthellae mutually benefit from this relationship and so this type of symbiotic relationship is called mutualism. The algae are provided a protected environment, and nutrients from the coral's wastes (in the form of CO2, nitrogen and phosphorus). The coral polyp, in turn, utilizes some of the algae's photosynthetic products (oxygen and carbohydrates) for respiration and nutrition. The algae also increases the ability of coral polyps to form large amounts of calcium carbonate, which helps build coral colonies, and reefs, faster.

This is important because corals are often broken back by wave action from storms, and grazed upon, etched or bored into by some reef animals, and must be able to continually rebuild to maintain the reef. Reef formation reflects the balance between the corals' ability to build (accretion) and the forces which break corals down (erosion). Windward slopes on islands tend to have stronger wave action, and so greater erosive forces on coral reefs. Leeward slopes on islands are more shielded from the wind, and with less wave-associated erosion, there is greater accretion and coral reef formation.

Why do reef-building coral colonies form different shapes and sizes?

Reef-building corals obtain their nutrients from both their symbiosis with zooxanthellae algae (autotrophic element) and by the capture of plankton using their tentacles (heterotrophic element). The degree to which a particular coral type gains its nutrients from either the autotrophic or heterotrophic elements varies among species. Some corals obtain over 95% of their energy from the photosynthesis of their algae (and so are more autotrophic), others as little as 50% (and so are more heterotrophic).

The more autotrophic corals have tree-like growth forms and relatively small polyps. These corals tend to live in more shallow, bright light areas where light is scattered in many directions, so a mutilayered, branching growth form, projecting out in many directions, maximizes the collection of light for algal photosynthesis. Thin tissue-layers and small polyps exposes the maximum area of the zooxanthellae algae to the light. These corals also have a large amount of algae in their tissues, with the amount of algal protein present nearly that of the polyps themselves.

The more heterotrophic corals have less algal material in their tissues, thicker tissue layers, and larger polyps. Their colonies tend to have a single-layer, spheroidal form, which allows them to better remove plankton from the waters flowing over the reefs. A branching colony would be inefficient, since inner layers of polyps would be deprived of food.

Some corals of the same species will change their growth form as light levels decrease with depth. The boulder coral (Montastrea spp.) will change from a rounded, volumetric growth form in shallow, well-lit water, to a flattened form in poorly-lit areas in deeper water. Lower light levels reduce the photosynthetic rate of the coral's algae, which limits the coral's production of calcium carbonate. A flattened, plate-like growth form, minimizes the amount of skeletal material needed for each polyp, since a flattened form is more two-dimensional than the more three-dimensional, rounded form. ANd a flattened form is more efficient in capturing the lower light intensities in deeper water.

What type of coral reef is found in Dominica?

There are three basic types of coral reefs. Fringing reefs grow out from shore, generally parallel the coastline, and are considered the youngest reef-type geologically. Barrier reefs also generally grow parallel to the coastline, but far offshore (in some cases more than 25 miles), and are separated from the mainland by a relatively shallow area of water called a lagoon. Atolls are reefs (primarily found in the tropical Pacific Ocean) that form in the open sea, usually around older islands that have sunk back into the ocean. The islands submerge due to geological forces, leaving behind a ring of coral around a large lagoon where the island once sat above sea level.

Dominica's coral areas are geological young, having been occasionally disrupted by the island's volcanic past, and form small fringing reefs on steep volcanic walls, primarily along the island's leeward coast.

What are the general types of corals?

Corals are animals placed in the Phylum Cnidaria, named for the stinging structures called cnidae, or nematocysts, which are found on the coral's tentacles and used to capture prey, and for protection. Jellyfish, anemones and hydroids are also cnidarians. Hexacorals, as previously mentioned, have tentacles in mutiples of six , and can be divided into six orders: hard or stony corals which are the main reef builders; black corals; anemones; tube-dwelling anemones; and zoanthids. Octocorals, have eight tentacles, and colonies that usually lack hard, rigid, permanent skeletons. A group of octocorals called gorgonians include animal colonies known as sea fans, sea rods, sea feather plumes, flat sea whips, and orange sea whips. Hydrocorals are in the class Hydrozoa, a separate taxonomic class from the hexacorals and octocorals (who are both in the class Anthozoa). Although hydrocorals may look like stony, hard corals, they are hydroid colonies that secrete hard, calcareous skeletons, and some, the fire corals, can inflict a painful, but not deadly, burning senstation on the skin from the sting of their tentacles' nematocysts.

What are some important characteristics of a coral reef food web?

Coral reefs are usually found in warm tropical waters that are generally quite nutrient poor. Yet, coral reefs have one of the highest rates of primary production (the total rate at which biomass is made by photosynthetic organisms) of any marine ecosystem. Reefs also support one of the highest levels of biodiversity (kinds of living things) on the planet. How can such a great amount of primary productivity and number of life forms be supported over time in such nutrient-poor oceanic waters?

The high rate of primary productivity results from the large surface area of photyosynthetic tissues found on the reef in the form of the single-celled algae living in and efficiently recycling nutrients with the coral polyps, and other reef animals also symbiotically associated with algae. The photosynthetic rate is possible because of the favorable levels of light and temperatures in the tropics. And, the levels of nutrients, although low, are sufficient over a period of time, due to the constant flow and circulation of seawater through the reefs.

Coral reefs also have a very stable environmental climate in which to live, with light, temperature, and salinity levels, remaining relatively constant day to day, year to year. This allows organisms a better chance to diversify and develop intricate symbiotic relationships over time, rather than expending energy responding to a constantly changing, harsher environment. It has been coral's important symbiotic relationship with algae, that has helped build reefs in some tropical areas for thousands of years. And with an expanding reef, comes a large variety of microhabitats (cracks and crevices) along the coral reef's terrain. Many available micohabitats helps accomodate the high number organisms found associated with coral reefs. The more types of places to live, the better chance more kinds of organisms can find a suitable home.

With so many organisms living on the coral reef, the high rate of primary productivity can only support a high diversity of reef organisms by having the nutrients produced, recycled, very efficiently, within the reef community. The degree of nutrient recycling within coral reef communities is extreme, and has led to a very complex food web with up to six trophic (feeding) levels, probably the most of any marine ecosystem. With so many trophic levels, nearly all the original energy from primary production is used up by the time it reaches the top of the food web. This results in low numbers of top carnivores being produced on coral reefs (in relation to the original amount of energy made by primary production).

Some of Dominica's coral reef animals close-up :

QUESTIONS TO LESSON 5

After reviewing the information in Lesson 5, write a brief, but exact explanation for the following questions. See how your ideas match the answers that will be provided Friday Jan. 21.

Question 1: Based on the environmental requirements of reef-building corals, what are some human-related actions that would be detrimental to the health of coral reefs?

Question 2: With six trophic levels estimated for a coral reef food web, why are large coral reef fishes susceptible to overfishing?
All Dominica lessons and photography © 1999 Lance Leonhardt