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What is “Umibouzu”?

“Umibouzu” is a reef-building stony coral. Corals create skeletons of calcium carbonate in a variety of shapes. These hard skeletons protect the soft coral polyps. The polyps have tentacles with which they can catch food. In addition, zooxanthellae (unicellular algae) live inside the coral polyps. These zooxanthellae provide the corals with energy by performing photosynthesis.

With the energy of the food caught by the tentacles and the energy provided by the zooxanthellae, the corals can grow their skeletons. After a few millenniums the corals will have built a big network of skeletons, also called a ‘coral reef’.

Coral reefs stop high waves coming into the bays. Thus creating calm lagoons, where a variety of creatures live and thrive. Many beautiful creatures, such as fish, shrimps, crabs and many others, are living inside the colourful coral reefs.

At the night of full moon, corals release eggs and sperm. Shortly after, small babies, called a planura, are born. The planura swim with the ocean current, until they find a nice place to settle down on the ocean floor. The planura then turn into polyps and will start to build a skeleton.

“Umibouzu” is a massive Porites coral. It can grow up to a few meters in height. The Umibouzu corals are trying to grow up to the ocean surface to get more sunlight. This leads to problems with fishing boats hitting the corals with the bottom of their boats. Fishermen are very worried about their fishing boats. However, the damage to the hundreds of years old Umibouzu corals is a lot more serious.

The Umibouzu corals stay in the same place for hundreds of years. Japan is the most northern country in the world, where corals exist. The very large and warm current, Kuroshio, bring in planura from tropical oceans to Japan. Every Umibouzu coral records the history of its own area in their skeletons. The mission of the Umibouzu scientists is to read the hidden stories in the coral skeletons.

How to research Umibouzu

The skeletons of Umibouzu have annual bands like tree rings. We observe the structure of the annual bands and analyze the chemical composition to map the environmental changes in the ocean and on land. Coral scientists attempt to reconstruct a variety of past changes or events, such as global warming, ocean acidification, marine pollution, earthquakes and typhoons. In addition, we study the response of the ocean ecosystem to the environmental changes and events by investigating the coral skeletons. Coral fossils also tell us about the past environmental conditions.

We begin by locating the areas where Umibouzu corals live. Coral reefs can be found using photographs taken from airplanes and satellites. The more precise location of the Umibouzu corals is then retrieved from historical documents, local fishermen and divers.

After specifying the area, we find the Umibouzu corals by snorkeling in the area. After we find a large Umibouzu coral we take our air-pressured drill and go down with our scuba-diving gear. We drill coral cores with a 3-5 cm diameter. The small holes are then filled up with a concrete plug to ensure the regrowth of coral tissue.

The coral cores are brought back to the laboratory and cut into thin slices with a rock cutter. When taking x-radiographs, we can see the annual bands on the coral slices. We can then count the age of the Umibouzu coral. The annual bands of the Umibouzu coral act like a calendar. We analyze the various chemical compositions along the calendar to read a detailed history of seawater temperature, precipitation, solar radiation, nutrient concentration, and so on. We also measure the skeletal density and calcification rate to know the growth history of the Umibouzu coral.

Reef-building corals will lose the symbiotic zooxanthellae when the seawater temperature increases. This is called “coral bleaching”. If corals live without zooxanthellae for longer than a few weeks, they will die. Will corals be able to survive in these times of global warming? Another problem for corals is the increasing carbon dioxide in the atmospheres. The carbon dioxide dissolves into the oceans, leading to ocean acidification. Ocean acidification makes it very difficult for corals to build their calcium carbonate skeletons. Will corals be able to survive this?

Will our children still be able to enjoy the beautiful coral reefs in their future?

From our research we know that ancestors of Umibouzu survived much warmer conditions in the ages of the dinosaurs. Maybe this means that Umibouzu will be able to adapt to the current conditions and survive after all.  By studying the stories of many Umibouzu corals we can learn about the past of Umibouzu corals. This can help us to predict what the future holds for all the Umibouzu corals!