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Akan-Mashū NP, Hokkaido
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A marimo ball isn’t one plant: it’s a colony of fine green-algal threads. Large spherical colonies are unique to Lake Akan, where some marimo grow over 30 cm across.
In Churui Bay, a south wind blowing across more than 2 km of open water can set marimo rolling on the lakebed. That rotation lets sunlight reach every side, helping these algae keep their famous spherical form.
In 1950, local residents and the Akan Ainu Association began the Marimo Festival as a campaign to return collected marimo and protect the lake. Today, Ainu rituals and a ceremony returning marimo keep that conservation purpose visible.
Lake Akan’s himemasu, a landlocked relative of sockeye salmon, helped stock Lake Shikotsu: eggs were moved there in 1894. Around Akan, this local fish is enjoyed salt-grilled or as sashimi.
Strong winds sometimes roll thousands of marimo onto Churui Bay’s shore, in events recorded every 6–9 years. What once looked like a disaster is now understood as part of their life cycle: fragments can regrow.
Researchers used preserved marimo DNA and water-flea remains in Lake Akan’s mud to estimate past abundance. Their analysis suggests the lake had 10–100 times more marimo in the early 1900s than it does today.
MRI scans revealed annual bands inside Lake Akan marimo. Their diameter grows about 0.9–1.26 cm a year, so a 5 cm ball may need roughly 20–28 years to reach 30 cm.
The word “marimo” was published in 1898 by student botanist Takiya Kawakami, after he collected the algae in Shurikomabetsu Bay. That’s another inlet on Lake Akan, not the cruise’s Churui Island stop.
Visitors can’t freely enter the wild marimo habitat. The cruise’s Churui Island stop instead leads to an observation center, where a recreated lakebed tank displays more than 150 marimo, including specimens over 20 cm.
