Coral in Tokyo Bay

© Tomohiko Higuchi

The coasts of Japan are offering new perspectives for Tara scientists. It’s in Japan that the most northerly surface corals of the planet are found. Situated very far north compared to the Pacific reefs, these corals are of particular interest to scientists: their specificity will complement the studies we’ve done on corals found in Japan’s tropical zones.

The geographical position of Japan makes it a special region for study. The archipelago stretches 3,300 kilometers from north to south, passing from the latitude of Quebec to that of Cuba. These climatic variations are not without consequence on the temperature of the water, nor even on the ecosystems. In Tokyo, where the climate is temperate, Laminaria — long, ribbon-like macro-algae — form the majority of underwater landscapes. Further south towards Okinawa, coral reefs find ideal conditions for their development. It’s easy to conclude that coral is an animal that prefers warm waters (like the Tara divers). But not at all !




Few people know that Tokyo Bay is home to what scientists call “marginal coral communities”. Sylvain Agostini, scientific coordinator of Tara’s research in Japan explains: “A coral reef is a geomorphological structure, like a mountain made of corals and limestone algae that form an entire ecosystem. But the presence of coral species is not always limited to the reef. Corals can sometimes be found in these so-called “marginal areas” like Tokyo Bay where the water is colder. Those that manage to live in more hostile, colder environments are often few in number and fail to develop quickly enough to form a true coral reef.”


Two conditions are essential for the presence of a coral reef: a minimum temperature of 18°C and the precise chemical parameters* necessary for the formation of the animal’s limestone skeleton. In other words, coral needs a favorable environment to “build” a reef.




The presence of these marginal corals in the waters near the Tokyo megalopolis raises another question: how were they able to travel over such great distances to higher latitudes? Responsible for these movements is the “Kuroshio”, a marine current that transports warm waters from south to north. Not as well-known as the Gulf Stream, the Kuroshio current operates discreetly, along the Pacific coast of Japan. It carries masses of warm water from the Philippines and the China Sea to the center of the Pacific Ocean. Acting like a gigantic underwater conveyor belt, the Kuroshio connects tropical regions to regions in more northerly latitudes.


In 2 weeks Tara will begin the next phase of scientific research. For now the boat is continuing her mission of educational outreach: 1,500 people are expected to visit the schooner at the next stopovers. In the communications headquarters (the “PC Com”) Captain Samuel Audrain refines logistical details. First mate Nicolas Bin goes over maps that will allow us to navigate from one sampling site to another. The entire team is getting ready to begin the second half of the Tara Pacific expedition. Objective: to study corals in all latitudes of the Japanese archipelago.


Noëlie Pansiot


*Chemical parameters: development of the coral skeleton depends on a sufficiently high saturation of aragonite. Aragonite is the mineral which constitutes the skeleton.



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