1. Ecosystem Coral Reef
In every environment – also in coral reefs – you can find 3 main groups of organisms which are directly dependent on each other.
1.1. The Producers
All botanical life forms of the reef are belonging to the group of the producers. Monads, free floating algae (phytoplankton) and solid grown algae as well as algae, which are enclosed in the outer tissues of animals, so called Zooxantellen. These algae assimilate water, carbon dioxide (CO2) and minerals, and transform these by the help of sunlight into sugar. In this process of photosynthesis oxygen (O2) is produced as a side product and all reef inhabitants depend on this production.
1.2. The Consumers
Many organisms live on the products, produced by the photosynthesis. To obtain energy they split, with the help of oxygen (O2), energetic compounds (breathing) and leave carbon dioxide (CO2) as a side product. The group of consumers reaches from monads to big predators and includes therefore all animals, which are not able to perform photosynthesis and have to use different techniques to obtain food.
1.3. The Decomposers
At the end all marine species are going to die and sink down to the ground, where they get decomposed into components (minerals). This work is done, for example, by crabs, snails, worms and sea cucumbers which are shuffling the sand around during their search for organic materials, as well as through bacteria in the ground.
1.4. Cycle
The energetic compounds produced by the plants are absorbed by animals. This passing on of energy is called food chain. Naturally most of the organisms
are at the beginning of the food chain, so that a nutrition – pyramid results: a lot of plants (phytoplankton), many small animals (zooplankton), some larger
animals and animals at the end of the food chain. No wonder that big predators, like sharks, are rare. A cycle develops because the nutrients of the food chain are passed on and the dead links of the chain are decomposed to minerals again. All three groups of organisms, the producers, the users, and the decomposers, are inseparable dependent on each other. The whole cycle is endangered if only one link is missing. In principle reefs are habitats with few nutrients. This poverty forces the reef inhabitants to a great variety of special adaptations and causes the high marine diversity of the different life-forms.
2. Formation of the Reef
2.1. Corals – Builder of the Reefs
Corals are structures built by living organisms. Foundation of this building activity is the possibility of fast growing stone corals (Station 15 and 18, table corals) to absorb calcium from the water and to integrate it into their skeleton structure. Through growing processes these structures become larger. In this way many corals build the spatially strongly divided structure of a reef, which is the life carrying foundation for all other reef inhabitants. It takes a long time, often hundreds of years, until a stable community of species is developed and also able to survive periods of unfavourable conditions. The frequent appearance of raspberry corals (Station 3), which belong to the pioneer settlers, but are without a chance in the competition of a fully developed reef, points to the relatively young age of our house reef.
2.2. Corals are Cnidarians
Corals belong to the group of cnidarians and mostly consist of colonies with single polyps. Each polyp has a vase shaped body that gets narrow at the top. Around their mouth they have tentacles which are supplied with nettle capsules. Some of these nettle capsules contain a poison to anaesthetize and catch their food, e.g. small plankton. On the lower part of their body they continuously store calcium into their skeleton in which the polyps can hide to be save of predators.
2.3. Form and Shape of Corals
Corals can look quite differently because many organisms belong to this group of animals which is systematically divided by classes and families. You will find some of them in our study trail (e.g. Station 3 raspberry coral, Station 10 bubble coral, Station 12 fluorescent peacock coral, Station 13 fan coral, Station 20 anemones and others). To find more information about the systemically division consult our reef library.
2.4. Nutrition Forms of Corals
There are three different ways for corals to obtain food.
2.4.1. Nourishment with the Help of Algae
Most corals carry multiple algae (Zooxantellen) in their outer tissues which practise photosynthesis and therewith produce sugar as a result. This sugar is being placed at the corals disposal and in return the corals provide the algae with minerals and carbon dioxide (CO2). So both partners take an advantage of this biocoenosis. The algae is also responsible for the colouring of the coral, if a coral looks blue, yellow or brown depends on the kind of algae which is settled on it (Station 15 and 18 brown and blue table coral). This kind of nourishment is of great importance for the coral and supplies the corals with about 70% of its complete needs. Without its algae partner a coral could only survive a few weeks!
2.4.2. Nourishment with the Help of Tentacles
Coral polyps have endoblast tentacles to catch plankton, which are mostly visible during night only. During the day the soft polyps would be a fast catch for many reef inhabitants. Therefore most corals are night active plankton catchers. A coral is able to survive even if it has to stop using this nutrition form because it provides only 30 percent of the complete food, the nourishment of its algae partner would be sufficient.
2.4.3. Nourishment through the outer Tissues of the Body
Corals can absorb in water dissolved nutrients like sugar, protein and fat directly through its outer tissues. But unfortunately only 10 percent of the nutrition needs can be covered in this way.
2.5. Reproduction of Corals
Corals have two possibilities of reproduction, which led in its combination to the world wide spread of coral reefs. You can find many corals of our study trail as well in Australia, the South Seas or the Maldives.
2.5.1. Sexual Reproduction
Many corals are separated by gender. Once a year, controlled by the moon’s phase, the corals release at the same time egg- and semen cells into the water. After the fertilization a larva which is able to swim develops and is carried away with the current. It can hover in the water for quite some time and cover long distances. If the larva finds solid ground it settles down, starts to grow and becomes a new polyp.
2.5.2. Asexual Reproduction
The now solid grown polyp splits up and produces by budding other polyps which are still connected to it. A colony of coral develops which can reach huge dimensions. Old, large corals can be some hundred years old as drill examinations prove. The fan coral of our house reef is probably much older than one hundred years (Station 13 gorgonian fan coral).
2.6. Living Conditions of Corals
Reef building corals can only survive in salt water and need certain environmental factors for their growth and development.
2.6.1. Temperature
Water temperatures between 27°C and 29°C offer perfect growing conditions. Temperatures above 30 °C or below 20°C for a longer period will limit the survival of corals.
2.6.2. Light
Corals need light for their algae partners and therefore grow best in shallow water. This can be observed well in our reef study trail!
2.6.3. Current
Corals prefer areas with current since it provides plenty of plankton.
2.6.4. Water Quality
Corals need clean water, poor nutrient conditions and water without floating sediments.
3. Coexistence of Different Marine Species
In coral reefs exist as in all other biotopes, different relations between species, some of them are introduced here:
3.1. Symbiosis – Cooperation
The cooperation of different species with mutual advantages for all partners is called symbiosis. Remember, even corals have symbiotic algae in their tissues and therefore represent a kind of symbiosis. The following symbiosis can be found in our reef study trail:
3.1.1. Goby – Crab
If you dive slowly and carefully you can find small holes in the ground on the sandy areas of our reef study trail where a small fish - named goby - is guarding the entrance. These holes are often secured by small pieces of broken corals and are dug out by a small crab with poor eyesight. The goby is always keeping contact with the crab and warns it if an enemy is coming close. In this situation both of them will disappear back into the hole. Like this the goby is provided with a well secured place to hide and the crab is having a private guard (sandy area between Station 4 and 5).
3.1.2. Sea Anemone – Clownfish
As all other corals sea anemones have cnidoblasts with poison. To prevent that single tentacles nettle each other they are clad in mucus. Clownfish use this safety cover for themselves by rubbing their body carefully against the mucus-clad tentacles. The mucus impregnates the skin and slowly the fish becomes “invisible” to the stinging cells´ sensors. Therefore clownfish are quite safe against predators. Adult fish defend their anemone, in which their juveniles live, vigorously even against much larger creatures (Station 20 magnificent anemone – clownfish).
3.2. Competition – Quest for Competitive Advantages
A continuous fight for survival is taking place even though the reef community looks harmonious. The dense, narrow growth of corals in the shallow areas of the reef is the expression of this fight for light and space. Every centimetre of free space is used immediately and different species often try to overgrow each other (Station 9 red sponge overgrowing slab coral. Notice the change between 2002 and 2007, the sponge is now covering the coral completely!).
3.3. Commensalisms – Food Sharing
If you are lucky you can see goatfish or blue spotted stingrays searching the sand for food. Often they are followed by other species, e.g. wrasse/hogfish which catch pieces of food from the dispersed sediment.
3.4. Episetism – Predators
Episetism is also part of a reef biotope, maybe you are lucky to see a cornet fish hunting. Many species try to protect themselves against predators by poison and show this clearly. Snails, like the beautifully dyed pyjama slug, present their poison through their colourful appearance.
Also lionfish and scorpion fish protect themselves with venomous spines.
4. Threats to Coral Reefs
Like almost all other environments, coral reefs are under a high environmental pressure. Around 60 percent of the world’s coral reefs are seriously endangered. Corals push off their algae partners if they are under stress. In this way they loose their colour, the so called “coral bleaching” starts. Now they can only survive a few more weeks, if the ecological damage continues. The polyps die and the dead lime skeleton is left over. The following threats endanger the world’s coral reefs today:
4.1. Global Warming – Greenhouse Effect
The rising temperature of the atmosphere increases the water temperature. Corals will die within a couple of weeks if the water temperature is above 30 °C as they can only survive in water below 30 °C.
4.2. Building Activity
On many islands the lime of corals is often the only accessible building material. To build new houses, streets or even airports corals are broken off the reef. Unfortunately you will find a good example of this between Station 14 and 15.
4.3. Sedimentation
The clearing of vegetation (mangroves) close to the shore and the chopping of large rainforest areas in tropical regions causes a higher amount of sediments in rivers. These floating particles, basically sand and clay are carried by the rivers to the reefs close to the shore and smother corals, which are cut off from the sunlight and die. Building activities close to the shore also influence the reef in the mentioned way.
4.4. Excessive fishing
All over the world more fish than naturally can reproduce are caught. The fishing methods of the fish industry, dynamite and cyanide fishing contribute to a worsening of this situation.
4.5. Tourism
The direct contact of swimmers, snorkelers, divers or anchoring boats damage corals. Extra nutrients caused by feeding or excrements are resulting in a competition advantage for fast growing algae, which are completely covering up the corals and cause their death. Harpooning, sports fishing and the collection of marine souvenirs, like shells or stuffed fish, cause a selective impoverishment of the underwater world.
5. How can I Help to Protect the Reefs?
There are many possibilities to help the intact reefs to survive in their diversity. With your correct behaviour you can be an example for others and achieve a significant contribution in protecting the reefs!
- Inform yourself about local conditions, safety areas and environmental projects.
- Try not to touch the ground during diving as well as any corals.
- Do not feed any marine animals and avoid any pollution of the water.
- Do not buy or collect marine souvenirs like corals, shells and others.
- Choose diving centres which practise environmentally safe diving: no anchoring at the reef, no waste dumping into the water, no fish feeding.
- Support local environmental projects.
- Education and support of the local population help to develop a sensible environmental behaviour.
- Many forms of energy consumption produce emissions and contribute to the global warming. Avoid waste of energy also at home.
Diving Map
The following pages show the diving map of our house reef and photos of the reef study trail to introduce the different stations and guide you through your way underwater. During only one year a lot of changes occurred and document that reefs are in constant change. If you would like to have any further information’s you are more than welcome to consult the books of our reef library. Also our instructors are pleased to give you more detailed information, especially about the great variety of the reef community. We wish you lots of great discoveries during your dive in the fascinating underwater world of the Red Sea.
Blue Water Team
