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 spherical balls that can reach about 30 centimeters in diameter. (Lake Akan's marimo had already been designated a Natural Monument in 1921.)

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. Marimo were discovered at Lake Akan in 1897 by Takiya Kawakami, then a student at Sapporo Agricultural College, who proposed the Japanese name "marimo" ("ball algae") from their shape. Marimo occur in lakes across the northern Northern Hemisphere, but large spherical colonies growing in groups are known only from Lake Akan in eastern Hokkaido.

The Marimo Festival

The "Marimo Festival" (まりも祭り) began in 1950 as part of a local campaign to protect the marimo, with a ceremony to return marimo that had been taken from Lake Akan. At the 6th festival in 1955, some 3,500 marimo were returned to the lake. Held every autumn, the festival continues today as an event of gratitude to nature, featuring Ainu ceremonies. Marimo are listed as Critically Endangered (IA) on Japan's Ministry of the Environment Red List.

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: Marimo are sometimes said to grow extremely slowly, but at Lake Akan a marimo is estimated to grow from 1 cm to 15 cm in diameter in about 20 years. Large ones reach about 30 cm and can become hollow in the center.
• A legend that isn't Ainu: A popular tragic love story says the souls of an Ainu maiden and her lover became marimo, but it is not in fact an Ainu tradition.
• 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 , surrounded by volcanoes and forests and internationally known for its marimo.