Marimo Day

What Is Marimo Day?

Every March 29th is "Marimo Day" (マリモの日, Marimo no Hi) in Japan. On March 29, 1952, the marimo (まりも, Aegagropila linnaei) — spherical algae colonies that grow in Lake Akan (阿寒湖) in Hokkaido — were designated as a "Special Natural Monument" (特別天然記念物) of Japan, recognizing their unique biological character and the importance of protecting them. Lake Akan's marimo are among the world's most remarkable examples of this algae species, growing into perfectly spherical balls that can reach 30 centimeters in diameter over centuries of slow growth.

What Are Marimo?

Marimo (まりも, literally "ball seaweed") are colonies of Aegagropila linnaei algae — tiny individual filaments that form into compact, spherical shapes through the gentle rolling action of lake currents and waves. The result is a velvety green ball that grows slowly — approximately 5-10 millimeters per year — producing the large specimens that take decades or centuries to achieve their full size. Lake Akan in eastern Hokkaido contains one of the world's largest and best-developed populations of large marimo, making it a unique natural phenomenon.

The Marimo Festival

Lake Akan's Ainu indigenous community has developed a traditional "Marimo Festival" (まりも祭り) held every October, in which marimo are ceremonially returned to the lake after being displayed on shore — a ritual expression of gratitude to the natural world and of the Ainu community's relationship with the lake and its inhabitants. The festival has become a major cultural tourism event that combines Ainu cultural performance with environmental awareness.

Marimo in Japanese Popular Culture

Marimo have become an unlikely cultural phenomenon in Japan — sold as souvenirs in glass jars of water, featured in manga and anime, and kept as unusual pets by people who appreciate their slow, meditative existence. The marimo's spherical perfection, its slow growth, and its association with the clean, cold waters of Hokkaido give it a quality of natural perfection that has made it one of Japan's most beloved natural curiosities.

Fun Facts

• Growth speed: Large marimo in Lake Akan — specimens measuring 20-30 centimeters in diameter — are estimated to be 150-200 years old, having grown at the rate of approximately one to two millimeters per year. Their patience and persistence as living things give them an almost philosophical dimension in Japanese cultural interpretation.
• Lake Akan: Lake Akan (阿寒湖) is a caldera lake in Akan Mashu National Park (阿寒摩周国立公園) in eastern Hokkaido — a UNESCO Global Geopark with extraordinary volcanic and natural heritage, including active volcanoes, ancient forests, and the rare marimo that make it internationally known.