The Marimo of Lake Akan: Hokkaido's Living Green Treasures
Deep in the volcanic heart of eastern Hokkaido, in the cool, clear waters of Lake Akan, lives one of Japan's most extraordinary natural wonders. The marimo of Lake Akan are spherical green algae that grow into perfect velvet balls, some reaching more than 30 centimeters across. Although a related algae can be found in lakes scattered across the Northern Hemisphere, only Lake Akan produces these large, beautifully round colonies. Designated a Special Natural Monument of Japan, the marimo are at once a scientific marvel, an ecological symbol, and a sacred presence woven into the culture of the Ainu, the indigenous people of Hokkaido. For visitors from overseas, encountering the marimo is to step into a story that bridges geology, biology, and a century of dedicated conservation.
What Exactly Is a Marimo?
A marimo (毬藻, "ball algae") is not a single organism but a colony. The species, Aegagropila brownii, is a freshwater green alga that grows as fine, branching filaments. In most lakes around the world, these filaments simply attach to rocks or drift in mats along the lakebed. But in Lake Akan, a rare combination of conditions causes the filaments to gather and turn into compact, almost perfectly spherical balls of vivid green that slowly roll along the lake floor.
The form is the result of a delicate dance between sunlight, gentle waves, mineral-rich spring water, and the soft sandy bottom. Wind-driven currents tumble the algae just enough to round their shape; light reaches them at the right angle to allow photosynthesis on every side; and clean, cold water keeps them healthy. Large marimo are estimated to grow only about one centimeter per year, meaning the bigger specimens visitors admire today have been quietly developing for a century or more.
Why the Marimo Was Designated a Special Natural Monument
The marimo of Lake Akan were first studied scientifically in 1897, when Kawakami Takiya, then a student at Sapporo Agricultural College (the predecessor of Hokkaido University), collected specimens at Shurikomabetsu Bay and gave them their Japanese name, "marimo." His observations were published in a botany journal the following year, drawing nationwide attention.
Their scientific rarity, combined with growing concern over their disappearance, led to designation as a Natural Monument on March 3, 1921, under the Historic Sites, Scenic Beauty and Natural Monuments Preservation Act. After the Second World War, lowered water levels caused by hydroelectric demand and persistent illegal collection brought the colonies close to collapse. Recognizing both the urgency of protection and the marimo's unique global value, the Japanese government elevated their status to Special Natural Monument on March 29, 1952, the highest tier of natural heritage protection in the country.
The grounds for designation are striking. Lake Akan is the only place on Earth where freshwater algae spontaneously form such large spherical colonies. The phenomenon depends on the lake's caldera origin, a precise pattern of seasonal winds, a constant flow of underground spring water, and a finely balanced ecosystem that has evolved here over thousands of years. Lose any one of these elements, and the marimo could vanish.
Highlights and Things to See
Marimo Exhibition and Observation Center
The most accessible way to see real wild marimo up close is to visit the Marimo Exhibition and Observation Center, located on tiny Churui Island in the northern reaches of the lake. Reachable only by sightseeing cruise from Akanko Onsen, the center features tanks that recreate the lakebed environment, allowing visitors to view more than 150 specimens of various sizes at eye level. Underwater cameras also stream live footage from natural marimo colonies in nearby Churui Bay. The slow rotation of the green spheres in clear water, their fine surface fibers catching the light, leaves a lasting impression.
The Lake Akan Sightseeing Cruise
The 85- to 90-minute cruise that connects the lakeshore to Churui Island is itself a highlight. Lake Akan is a caldera lake ringed by primeval forest and overlooked by the cones of Mt. Oakan and Mt. Meakan, two of Hokkaido's iconic volcanoes. Passing the four small islands that dot the lake, the boat offers panoramic views that change with the seasons, from emerald summer waters to the fiery reds and yellows of autumn foliage and finally to the ice-locked silence of winter.
The Marimo Festival
Every October, the lakeside town hosts the Marimo Festival, held annually since 1950. Conducted in cooperation with the Ainu community, the three-day festival is built around three solemn rituals: welcoming the marimo from the lake, protecting them through the night, and finally returning them to the water by dugout canoe. A torchlight procession winds through the lakeside streets after dark, accompanied by traditional Ainu song and dance. It is one of the few opportunities to experience an Ainu ceremony of this scale, performed in a deeply spiritual atmosphere of gratitude to nature.
Akan Ainu Kotan
A short walk from the lakeshore lies Akan Ainu Kotan, the largest Ainu settlement in Hokkaido. Visitors can browse craft shops selling wooden carvings and embroidered textiles, sample regional cuisine, and watch performances of Ainu traditional dance, recognized as an Important Intangible Folk Cultural Property. The marimo features in several Ainu legends and traditional names, and learning these stories adds emotional depth to a visit.
Nearby Attractions
Akanko Onsen, the hot spring town on the southern shore of the lake, is an ideal base for exploration. Within easy reach are the bubbling mud pools of the Bokke Nature Trail, the Akan International Crane Center for those interested in the rare red-crowned crane, and the night walk experience Kamuy Lumina, an immersive light-and-sound show inspired by Ainu legends. Travelers with more time can extend the journey east to Lake Mashu and Lake Kussharo, the other jewels of Akan-Mashu National Park.
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Q&A
- Can visitors swim with or touch the marimo?
- No. As Special Natural Monuments, the wild marimo are strictly protected, and entering their habitat is prohibited. Visitors can observe them at the Marimo Exhibition and Observation Center on Churui Island, where they can be seen at very close range in carefully maintained tanks.
- Are the small marimo sold in souvenir shops the real Lake Akan marimo?
- No. Wild marimo from Lake Akan cannot legally be collected or sold. The small souvenir marimo are made by hand-rolling filamentous algae from other lakes (or, more recently, imported sources) into spheres. They are a charming keepsake but are not the protected wild specimens themselves.
- When is the best time of year to visit?
- Lake Akan is rewarding in every season, but autumn (late September to mid-October) is particularly memorable. The forests around the lake turn brilliant colors, and the Marimo Festival takes place from October 8 to 10. Sightseeing cruises generally operate from spring through late autumn, while winter brings frozen-lake activities such as ice fishing and snowmobile tours.
- How do I reach Lake Akan from outside Hokkaido?
- The most convenient gateway is Kushiro Airport, served by daily flights from Tokyo (Haneda) and Sapporo (New Chitose). From the airport, Akanko Onsen is about a 60-minute drive or roughly 70 minutes by direct airport bus. Buses also connect from JR Kushiro Station in around two hours.
- Why is Lake Akan the only place where giant marimo form?
- It is a fortunate coincidence of geology and climate. The lake's volcanic caldera shape produces gently curved bays where wind-driven waves rotate the algae evenly. Underwater springs deliver mineral-rich water, the lakebed is sandy enough to allow rolling, and the sunlight reaching the shallows is just right. No other lake in the world combines all these conditions in the same way.
Basic Information
| Name | Marimo of Lake Akan (Akanko no Marimo) |
|---|---|
| Designation | Special Natural Monument of Japan (highest tier of natural heritage) |
| Designation Dates | Natural Monument: March 3, 1921 / Special Natural Monument: March 29, 1952 |
| Scientific Name | Aegagropila brownii (family Pithophoraceae, green algae) |
| Location | Lake Akan, Akan-cho, Kushiro City, Hokkaido |
| Habitat | Mainly Churui Bay and Kinetanpe Bay, along the northern shore of Lake Akan |
| Maximum Size | Recorded specimens exceed 30 cm in diameter; growth rate roughly 1 cm per year |
| Conservation Status | Critically Endangered (Category IA) on Japan's national Red List |
| Where to Observe | Marimo Exhibition and Observation Center, Churui Island (accessible by sightseeing cruise from Akanko Onsen) |
| Access | About 60 minutes by car from Kushiro Airport; about 2 hours by bus from JR Kushiro Station |
| National Park | Within Akan-Mashu National Park |
References
- Cultural Heritage Online — Marimo of Lake Akan
- https://bunka.nii.ac.jp/heritages/detail/204523
- Kushiro City Official Site — Marimo of Lake Akan
- https://www.city.kushiro.lg.jp/museum/bunkazai/1005708/kinenbutsu/1005712.html
- Kushiro City Official Site — World Significance of the Marimo of Lake Akan
- https://www.city.kushiro.lg.jp/machi/kankyou/1004295/1008323/1005713.html
- Marimo Official Web Site (Kushiro City Board of Education Marimo Research Laboratory)
- http://marimo-web.org/
- HOKKAIDO LOVE! — Marimo Exhibition and Observation Center
- https://www.visit-hokkaido.jp/en/spot/detail_10124.html
- National Parks of Japan — Lake Akan Boat Cruise and Nature Trail
- https://www.japan.travel/national-parks/parks/akan-mashu/see-and-do/lake-akan-sightseeing-boat-and-lakeside-walking-trail/
- Japan Tourism Agency — The Marimo Exhibition and Observation Center
- https://www.mlit.go.jp/tagengo-db/en/R1-01898.html
Last verified: 2026.05.05