They can rise when they’re hungry. They can sink when they’re thirsty. And they prefer things with a pinch of salt.

In the shallower parts of the Akanko, or Lake Akan, there live many round green balls. They range in size from tiny marbles to huge creatures the size of footballs. Usually resting on the sandy lake-bed, they sometimes float up on their own, to get closer to the surface and soak in more sunlight, or take a ride closer to the shore.

These are the Marimo.

The creatures get their name from two Japanese words: mari, meaning the soft bouncy ball that people play with, and mo, the name for any plant that lives in the water. At Lake Akan, the marimo also have local names in the language of the Ainu people who live there. They are called tokarip — “lake roller” — or torasampe, which means “goblin of the lake”.

As the lake waters lap the shore, they also turn the marimo around, giving all sides a chance to see the sunlight. This gentle rolling is what gives marimo their round shape.

Marimo are actually not single creatures, but large families of a certain type of green algae, living all packet together into a ball. The algae, Aegagnopila linnaei, can live in other ways too. They can cling onto the surface of underwater rocks, usually on the shaded side. They can simply float around as free filaments, sometimes forming a carpet at the bottom of the lake. Or, they can cuddle together to make a round marimo.

While the algae themselves are commonly found around the world, it’s very rare for them to become marimo. That’s because marimo need perfect conditions to grow. They like cool, freshwater lakes that are not too warm for them to live in, and they prefer it if the lakes are a bit saltier than usual. They must have a constant supply of wind, to move the waves and keep them turning. Many of the lakes are near ‘old growth’ forests and volcanoes, from where nutrients are washed in by run-off water. And, finally, marimo also need enough access to sunlight, all the year round.

Such perfect places are hard to find, but the places they can be are quite spread out. The three major marimo lakes are in quite different places: Japan, Iceland, and the Ukraine.

Still, the marimo are very delicate. Nowadays, with humans changing the landscape more and more, the perfect balance of the lakes are getting disturbed, and the marimo are slowly beginning to disappear.

Lake Mývatn, in Iceland, was one such place. It was home to some of the largest marimo, about ten to fifteen centimetres across. The only other places with such large marimo are Lake Svityaz, in the Ukraine, and Lake Akan, which has the largest marimo in the world.

The local Icelandic name for marimo is kúluskítur, which means “round shit”. That’s what fishermen used to yell at the algae balls when they got caught in their nets, in the days when both fish and balls were still plentiful.

Those days are now gone. It was in the 1960s that humans began mining the area around the lake for minerals. This mining released extra nitrogen and phosphorus into the lake — both of which act as fertilisers. That may sound like a good thing, but unfortunately they didn’t fertilise the marimo. They only fertilised the bacteria that lived at the top of the lake, letting them burst into dense blooms that covered the whole surface.

The new bacteria formed a barrier: it didn’t let much sunlight come through.

As you know, the kúluskítur need sunlight all the year round. They can’t do without it. So as the bacteria spread, the kúluskítr slowly began to wither and die.

The kúluskítr of Lake Mývatn didn’t live alone. They lived along with their algae-mat form, which made a carpet at the bottom of the lake — a carpet that held down all the mud and sediments underneath. With the sunlight supply down, those algae began to die too, leaving great holes in the mat. And, through those holes, the kúluskítr began to sink.

The round shape of the kúluskítr helps them stay clean. If any mud or dirt falls onto them, that side becomes heavier, so they roll over and let the mud fall down. But they’re not used to having dirt underneath, without an algae-mat to go in between. They’re too heavy to stay up on their own.

So, as they algae balls were struggling for sunlight, the lake bed was beginning to pull them in like quicksand.

By 2013, there were no kúluskítr left in the lake.

What happened at Lake Mývatn often happens to other species in other places as well. In a process called ‘eutrophication’, it’s usually fertilisers from fields which run off into rivers and lakes, and fertilise the bacteria and algae on top. Those bacteria or algae then grow so fast that they take over the top spaces, and the creatures at the bottom begin to suffocate.

Marimo, unlike fertilised bacteria, don’t grow fast. Quite the opposite, in fact — their diameter usually increases by only five millimetres a year. Starting from scratch, it could take hundreds of years for them to reach their maximum size.

Of course, they don’t always have to reach their maximum size. Marimo sometimes get bumped and naturally break apart. In that case, both halves get rolled around, to form two smaller marimo. Even so, those two small marimo will still take a long time to get bigger.

Scientists have found that it wasn’t always like this. It’s just that the conditions today aren’t good enough for them to grow faster.

What helps marimo grow faster, it seems, is salt.

Around the Akanko are many natural springs of dense, salty water that flow into the lake. It is near these springs that epilithic Aegagnopila — the version of marimo algae that lives not in balls but on rocks — grows thickest.

Apart from Akanko, there are many other lakes, in and around Japan, where marimo algae live in one form or another. Six thousand years ago, it turns out, many of these were not lake at all. They were shallow areas of the sea!

Those lakes still hold traces of the sea they once were. The Japanese name for them is kaisekiko, which literally means “traces-of-the-ocean lake”

The marimo, it seems, also hold memories of the sea. They grow in the lakes, which have a bit of salt, but they grow much more slowly than their ancestors in the ocean.

That was one story of the first marimo. But it’s not the only one. Another story begins with a tribe of humans, living on the banks of the Akan Lake.

The chief of the tribe had a daughter, who fell in love with a commoner. Her parents were not happy: they wanted her to marry into a chief, or at least some other important family. The couple tried to run away, but on the way,they fell into the lake and sank to the bottom. It was there that they turned into marimo, and could finally be together.

That is why, since that day, marimo are a symbol of love, affection, and good luck. When given to you as a gift, they help to accomplish your heart’s desires. No wonder, then, that marimo are a very popular gift in Japan. In fact, at one point, they became a bit too popular.

It all started in 1921, when the government of Japan made the marimo into a protected species, and declared it a National Treasure. That was meant to help the marimo. Unfortunately, it had the opposite effect.

After the announcement, people from all over the country — and out of it — began flocking to Akanko and other lakes to get some marimo of their own. Tourists would take them home as souvenirs to keep in their aquariums. Others would steal them, and sell them in towns and cities for high prices.

Meanwhile, a new hydroelectric plant had been built, a year earlier, on the Akan River than flowed out of the lake. The water level went down. Many of the marimo that were left got overexposed to sunlight, and began to wither away.

Soon, it seemed, there were many marimo all over the country, but hardly any left in Lake Akan itself.

By the 1950, the marimo were severely endangered. That was when the local people, the Ainu, decided to do something about it. They launched a campaign, requesting people to return the marimo to the lakes from which they had been taken. And, lots of people actually came forward to do it.

It was in honour of those people, and to thank them for releasing the marimo, that the first Marimo Festival was held. Forty-eight marimo were returned by people, “blessed” by a senior Ainu, and returned to their homes in the lake.

Today, the Marimo, apart from its previous meanings, has also become a symbol of the community and natural environment of Akan.

The creatures are still in danger, and have even disappeared from many areas of the Akanko itself. The Akan Lake Marimo Preservation Society is working to help them come back. Researchers are hopeful that marimo can be reintroduced to Lake Mývatn as well, once the conditions are made better.

The Marimo Exhibition and Observation Centre, reopened in 1996, helps to make the public aware of the how the marimo work. It conducts research, so scientist can know more about the marimo to be able to help them better.

And, every October, there is the three-day Marimo Matsuri, the largest event in Hokkaido. Running from the 8th to 10th of October, it commemorates the time the first forty-eight marimo were returned to the lake.

The celebration includes traditional Ainu songs and dances, and ends with a senior Ainu ‘returning’ marimo back to the lake. It’s the time when people gather to thank the god of the lake for the blessing of Mother Nature.

It’s a time to celebrate the marimo, and be thankful that they are still around.

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Sources: Some information in this article is taken from MossBall.com, Tofugu, National Geographic, and Boedeker et al. (2010).