Marimo moss balls – extraordinary plants in Hokkaido’s clear lakes

Akanko Marimo Observation Center

They look like toys. Some of them have the size of a fuzzy football, but most are smaller, and all of them are green and live in the water. Marimo are sometimes called moss balls in English, but in fact they are not moss, but colonies of algae. These tiny string algae cling together to form a globe. Then, they grow slowly outward so that they eventually become these fluffy green balls.

Green balls in the lake

But to our disappointment, no green balls are to be seen from the small excursion boat. We are on a boat trip on Akan-ko, a large lake in the east of Hokkaido / Japan. From the shores near the village and from the landing stage we could see that the lake’s water is remarkably clear. But still, once the boat is in motion all we see is the bright autumn colours around, the beautiful islands dotting the lake, the sky reflected in the water, and the smoke rising from the crater of Meakan-dake, an active volcano right next to the lake.

Lake Akanko in Hokkaido
Mt. Oakan-dake in Hokkaido

Akan-ko, too, is a crater lake, and this is one of the reasons for the large number of sizeable marimo found there. Crater lakes often have a high visibility, and the algae, which lie in the shallow water on the lake’s ground, need light to grow. A lot of plankton, particles and sediment in the water will hamper their development. Even so, in normal conditions they grow only about 5 mm per year, scientists estimate.

Marimo in Austria

The first to discover these strange plants in 1820 was an Austrian botanist named Anton Sauter. And yes, he found them in a lake in Austria. He called them Kugelalgen, or spherical algae. When they were found in Japan they acquired the name of marimo by which they are now known in English. 毬藻 means “felt ball algae”. Meanwhile, only a few lakes in the world, e.g. in Iceland, Estonia and Ukraine, have clear enough water for marimo to survive.

Akanko Marimo Observation Center

The boat ride on Lake Akan-ko leads to a small island surrounded by reeds in which ducks are quacking. From the landing a short path leads to the Marimo Observation Center. It is just a single room with several aquaria, exhibition panels and an underwater live camera. The screen is showing several layers of green balls floating gently on the lake’s ground. As the current turns them around a little, each side of the algae ball has a chance to get towards the light. Thus they can perform photosynthesis and grow. The oxygen generated by this process may form air bubbles. That’s why they can hover upwards a bit at the end of a sunny day. This is demonstrated in an aquarium nearby.

Back in the village, the souvenir shops display single marimo in water tanks for the visitors to touch. They also sell tiny balls of algae. But these are mostly hand-rolled from normal string algae. Of course, the real marimo are protected. Nobody has found a method yet to grow them naturally except from pieces of broken-up marimo.

How to see the Marimo moss balls in Lake Akanko

Akanko is a small onsen village on the shores of Lake Akan-ko in the Eastern part of Hokkaido, Japan. Boats leave regularly for 1.5-hour cruises of the lake, including a stop at the Marimo Exhibition Center (1900 ¥).


  1. which place in Hokkaido is able to see the moss ball on the ground not under the sea or lake?

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