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Akan-Mashū NP, Hokkaido
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Marimo algae grow in many places, but large spherical colonies are known only from Lake Akan. Churui Bay, just beyond the island, is one of the lake’s special habitats for these remarkable living balls.
At Churui Bay, waves are marimo’s sculptors: winds of about 5 m/s blowing across more than 2 km of open water can roll the balls along the lakebed, polishing them into spheres. Too little—or too much—wave action can spoil the balance.
Giant marimo aren’t solid green balls. Above about 10 cm across, they develop a hollow, decomposing core; the active algal layer is only about 4–5 cm thick, a hidden structure revealed by scientific imaging.
In 1961, local cruise operators voluntarily stopped entering Churui’s marimo beds, and a viewing pond opened on the island instead. Sunlight proved harmful, so in 1978 it was replaced by an indoor center with tanks and a research lab.
Marimo washing ashore can be part of their life cycle, as balls break apart and fragments return to the colony. But in December 2021, strong winds stranded them beyond the waves’ reach along 150 m of Churui Bay, prompting a rescue back to the water.
An Ainu-language dictionary records the name to-karip for marimo: to means lake or swamp, kari means to turn, and p means thing. It’s a vivid description of the round algae balls found beside the island.
Churui comes from the Ainu ciw-ruy. Local Ainu-language materials gloss it as “where waves are fierce”; another government translation gives “fast-flowing current.” Either way, the name evokes moving water.
The Lake Akan Marimo Festival began in 1950 as a campaign to protect and return marimo to the lake. Ainu participants continue to make it a living tradition, with prayers and ceremonies connected to the algae’s protection.
Churui Island’s marimo beds depend on more than the lake itself: the Churui River flows into their habitat, so preserving forest in the river’s upstream watershed is part of protecting the algae.
Surveys found that the amount of spherical marimo in Churui Bay fell by about 40% between 1973 and 1985. Later surveys found little change through 2019—but no recovery to the earlier level.
