There are things in our environment that we can’t see but are ‘invisibly important.’ For example, sunlight: we can see through its presence but we do not see the ‘energy’ that it is composed of. There are organisms that are dependent on its existence, and without it would surely perish. Same goes for the air we breathe, we need it but we can’t see it. Let’s explore how this affects our precious coral reefs, which are most dependent on sunlight and temperature.
Zooxanthellae is part of the genus Symbiodinium that encompasses the largest and most prevalent group of endosymbiotic dinoflagellates known to science. Did I lose you already? OK… so let’s put it more simply. It is a photosynthetic algae that lives in the tissues of most reef-building corals and it is vital for their health and nourishment.
New studies have revealed that zooxanthellae are a highly diverse group of organisms, which might include several hundreds of species. They may be very coral-specific or two or three species may even share a single coral host.
These singled-celled algae commonly live in the tissues of tropical cnidarians such as corals, sea anemones, and jellyfish where they process and move products of photosynthesis (energy from the sun) to the host and in turn receive inorganic nutrients. They are also harbored by various species of sponges, flatworms, mollusks (e.g. Giant clams), etc. Generally, these enter the host cell where they are processed into another form of energy. WOW, this is a lot to ponder. If you’re having trouble, Wikipedia can help: http://en.wikipedia.org/wiki/Symbiodinium
One of the great limiting factors for the distribution of life in our oceans is the energy from the sun. Its light energy is used for photosynthesis and can only penetrate to a depth of around 200m beneath the sea surface (give or take, depending on water turbidity). Its heat determines the range of all marine organisms on the planet. The basis of ocean productivity is its plant life – phytoplankton, sea grass and kelp each support entire ecosystems. Like the Zooxanthellae, all need to use photosynthesis to grow and reproduce.
Photosynthesis is the process whereby plants capture the sun’s energy to convert nutrients, carbon dioxide and water into complex sugars, oxygen and energy. Marine plants, particularly phytoplankton, play a significant role in regulating the amount of oxygen within our atmosphere. At the same time, they support the vast majority of marine life from the equator to the poles, their concentrations so great that at times, blooms can be seen from space.
The coral and the zooxanthellae algae maintain a mutualistic relationship. The coral provides the algae a protected environment, safe from the saturation of light and predation and allows the zooxanthellae to utilize the coral’s metabolic waste products to fulfill their dietary needs. In return, the algae produce oxygen and help the coral eliminate its waste. Most importantly, zooxanthellae supply the coral with glucose, glycerol and amino acids, which are the products of photosynthesis. The coral uses these products to make proteins, fats and carbohydrates and to produce its calcium carbonate, which is vital for making its skeleton. This perfect symbiotic relationship between the algae and coral polyp facilitates a recycling of nutrients even in nutrient-poor tropical waters. In fact as much as 90% of the organic material photo-synthetically produced by the zooxanthellae is transferred directly to the host coral tissue.
In addition to providing corals with essential nutrients, zooxanthellae are responsible for the beautiful and unique colors of many corals.
In addition to providing corals with essential nutrients, zooxanthellae are responsible for the beautiful and unique colors of many corals. Sometimes, when corals become physically stressed, such as at times when the water gets too warm, or pollution too high, the coral polyps expel their algal cells and the colony takes on a stark white appearance. This is commonly described as “Coral Bleaching.” Although corals can survive without zooxanthellae by feeding on plankton captured by their tentacles, they cannot thrive, and if the polyps go for too long without their partner zooxanthellae, the coral bleaching may result in coral death.
Because of their intimate relationship with zooxanthellae, reef-building corals respond to the environment like plants. Because their algal cells need light for photosynthesis, reef corals require clear water. For this reason corals are generally found only in waters with small amounts of suspended material, in water of low turbidity. This leads to an interesting paradox: Coral reefs require clear, nutrient-rich water, and they are among the most productive and diverse marine environments. The crucial interdependence between the coral and its symbiotic algae zooxanthellae is the driving force behind the growth and productivity of entire coral reefs.
The soft corals are part of a group known as Octocorals. They are identified by the sets of eight tentacles that surround each polyp (as opposed to the six in the hexacorallia). This species are mostly slow-growing and long-lived, despite over half of all such species having the same symbiotic relationship with zooxanthellae as the hard corals. The soft corals are filter feeders, with their stinging cells less developed than the hard corals, relying on the currents to carry the planktonic particles towards their polyps. Some of these coral species are able to live in the deepest part of our oceans and have few predators, as many produce toxic substances.
Corals are not the only ones to use zooxanthellae algae. Indeed, it has been found in others living in the sea. The Indo-Pacific is where we can find the greatest distribution of nudibranchs. These creatures look like the creation of talented and exuberant artists.
A huge Phyllodesmium longicirrum has been seen on Conchita, a dive site on the southeast side of Wakatobi just a short boat ride from the resort. In this picture of this amazing nudibranch you can see the golden zooxanthellae packets. They are ingested from the soft corals it feeds upon and passed live through the nudibranch’s digestive system. Stored just beneath the skin, the algae capture light energy, producing nutrients that can sustain the nudibranch for months. The same chemicals that feed the animal are also exuded from its skin as a defensive shield.
Although not plants and therefore incapable of photosynthesis themselves, many sea anemones form an important facultative symbiotic relationship with the green algaes’ zooxanthellae, zoochlorellae, or both. The algae reside in the animal’s gastro dermal cells. The sea anemone benefits from the products of the algaes’ photosynthesis, namely oxygen and food. The algae in return are assured a reliable exposure to sunlight and protection from micro feeders, which the sea anemone actively maintains. The algae also benefit from protection thanks to the presence of the anemone’s tentacles and stinging cells or nematocysts, thus further reducing the likelihood of being eaten by herbivores.
The Giant clam is the largest living bivalve mollusk. They can weight more than 200kg, measure as much as 120cm across and have a life span in the wild of 100 years or more! Its range covers the Indo-Pacific; they are prolific in Wakatobi, but sadly, global populations are diminishing quickly. The Giant clam is considered a delicacy in Japan, France and Southeast Asia, while the Chinese believe it has aphrodisiac powers.
The clam cultivates zooxanthellae in a special circulatory system, which enables it to maintain a substantially high number of these symbionts. The clams release sperm and eggs into the water column and their fertilized eggs float in the sea for about 12 hours until eventually the larvae hatch. At roughly one week of age, a young clam settles on the substrate (although it changes location frequently within the first few weeks). The larvae does not yet have the symbiotic algae, so it depends completely on plankton, and like this, free floating zooxanthellae are captured while filtering food, thus completing the cycle.
Fish are naturally the most dynamic and colorful features of a healthy coral reef environment. It is here that fish reach their maximum diversity, ranging in size from the smallest of fish, like a goby, which weigh 0.01 grams as adults, to the huge whale shark, which is more than one billion times heavier.
While fish are an obvious component of reef ecosystems, the basis of biological production on the reef is less obvious. We have seen the importance of zooxanthellae for coral growth. However, an even greater amount of plant tissue is present on the reef substrates in the form of “algal turfs.” Algae grows over coral and shades corals beneath it, which can lead to the death of corals due to light not reaching their zooxanthellae – bringing us to the importance of fish, and in particular, grazing.
Grazing fish reduce the amount of algae on coral reefs by intensive feeding to such an extent that the herbivores, such as parrotfish, damselfish or blennies, are thought to be keystone in shaping coral reef communities. Their work is a constant scraping of the coral rock, mowing down the turf and scraping off the algae, which always quickly replenishes. Grazing is vital to coral reef survival because it stops the coral having to compete for its compulsory light with canopy-forming algae. In contrast, predatory fish are also good for reefs. Because coral reefs are very productive systems that occur in low nutrient waters, supplies of extra nutrients to the reef system are important. This is where the role of plankton–feeding fish comes in; the fish feed on zooplankton drifting in the current. Some of the nutrients go into their own growth, while others are lost in the feces, which are themselves a source of food and nutrients to reef systems.
The importance of the survival of the coral reef goes without saying. It is our natural heritage and the most diverse and valuable ecosystem on earth. With 4000 species of fish, 800 species of hard corals and hundreds of other species we cannot find such diversity anywhere else on Earth. This biodiversity is key to finding new medicines for the 21st century. Many drugs continue to be developed from coral reef animals and plants as possible cures for cancer, arthritis, human bacterial infections, viruses and other diseases. Storehouses of immense biological wealth, reefs also provide economic and environmental opportunity to millions of people. Healthy reefs attracts tourism, creating millions of jobs. And thousands of people survive directly from local fishing of reef fish. Coral reefs also buffer shorelines from wave action and prevent erosion, property damage and loss of life. The lives of millions of people depend directly on the survival and well-being of the coral reef. Our further understanding of this delicate ecosystem can only help to improve the ways to protect it.
Luckily, at some places on the planet, coral reefs are still as pristine as we expect them to be. With water temperatures varying from 26 to 30 degrees and minimal human impact, Wakatobi’s reefs are extremely healthy. Wakatobi is located in a protected marine preserve and is recognized as one of the only places on Earth where the health of the coral reef has remained pristine. Abundant marine life and diversity reflect the efforts to protect this enchanted kingdom and provide hope to future generations who may be lucky enough to see such pristine corals.
Without a doubt, Wakatobi Dive Resort sits on the edge of one of the world’s most diverse and intriguing marine environments, a place where nature reveals itself in myriad details large and small. Those who come to immerse themselves in this remarkable ecosystem often seek a deeper understanding of their natural surroundings, and with the help of our educated, professional staff find a whole new world.
Many thanks to our guests who contribute their incredible images and share our passion for the mystery and wonders of nature Wakatobi has to offer.