Marimo, also known as lake moss or ball algae, scientifically named Aegagropila linnaei, is a type of algae commonly found in the northern hemisphere.
Marimo represents a rare growth form of Aegagropila linnaei, where this aquatic plant develops into large green spherical masses resembling soft velvet balls. This type of algae is considered a natural treasure of Japan and has become a popular pet.
Aegagropila linnaei has long been regarded as a mystery in biology, particularly due to its attractive spherical growth form. This algae can only be found in a few water environments in four countries: Iceland, Scotland, Estonia, and Japan. They exist as free-floating filaments, growing on flat stones. These “green balls” can reach diameters of up to 40 cm. Unsurprisingly, this biological entity has captured the attention of scientists and algae enthusiasts for centuries.
Among the four mentioned countries, Marimo (literally meaning “water ball”) is particularly popular in Japan. Large and impressive algae specimens are primarily found in Lake Akan, located in eastern Hokkaido.
When given sufficient time to develop in nature, Marimo algae in Lake Akan can grow to significant sizes, with diameters reaching up to 40 cm, much larger than moss balls found anywhere else.
Marimo is considered a natural treasure of Japan.
Algae enthusiasts in Japan have been captivated by the beauty of Marimo for centuries, and this type of algae was designated a national treasure in 1921. However, this recognition has ironically worsened the situation for the algae, as increased attention has turned Marimo into a commodity, leading to a gradual decline in their natural population. At times, the price for a Marimo ball in Tokyo could reach 1,000 yen (over 6,500 USD today). Currently, the situation seems even more dire with a water plant built near Lake Akan, causing a sudden drop in water levels and resulting in the death of hundreds of Marimo specimens.
Aegagropila linnaei has long been regarded as a mystery in biology.
Meanwhile, conservation efforts in Japan began in 1950 when photographs of piles of dead Marimo were published in national newspapers, shocking the country. In response, many people across Japan purchased Marimo balls taken from Akan and returned them to their natural homes. To honor the goodwill of these individuals, the first Marimo festival was held on October 7, 1950, which continues to be celebrated annually.
Marimo is sometimes sold for display in aquariums, often sourced from lakes in Ukraine like Lake Shatsk. Spherical Marimo sold in Japanese pet shops may originate from Europe or be cultivated by local residents around Lake Akan, using gentle wave-making devices, while extraction from Lake Akan is prohibited.
Currently, you can still purchase Marimo as souvenirs and “pets,” but these are artificially rolled from free-floating fibers rather than the naturally formed Marimo shaped by currents at the bottom of the lake. However, they remain soft and smooth to the touch like their natural counterparts and can literally last a lifetime if properly cared for.
Additionally, knockoff Marimo—plastic balls with a thin layer of algae—are also very popular and serve as a perfect choice for those wanting to keep Marimo.
Due to their attractive appearance, spherical Marimo also serves as a medium for environmental education. Small balls sold as souvenirs are hand-rolled from tiny free-floating fibers. A widely sold plush toy named Marimokkori incorporates the shape of the Marimo algae into its design.
One thing to note about Marimo is that they grow very slowly. They grow at an average rate of 5 mm per year, so it would take decades to grow a giant algae ball like those at the bottom of Lake Akan. However, if you have the time to wait and care for them properly, they can indeed live very long, possibly even outlasting you.
Although Marimo thrives in nature, they cannot reproduce on their own. However, if you have a large “child” Marimo and want to get a second “child,” all you need to do is split it in half and roll them into smaller balls.
Marimo commercially popular typically grows in the form of large spheres created by dense algae filaments radiating from a central point, without a nucleus. On average, spherical Marimo grows about 5 mm per year. The largest spherical Marimo ever found can live up to 200 years, with a diameter of 95 cm. Spherical Marimo is a rare and fascinating entity.
Regarding why Aegagropila linnaei grows into a spherical shape, scientists have proposed several hypotheses. Some believe it is a natural protective mechanism, as in a free-floating filament form, they would be more susceptible to being eaten by fish. Others suggest that in a spherical form, they can more easily return to the surface if accidentally washed ashore. However, these are merely hypotheses, and to date, there has been no definitive answer to explain this mysterious formation.
