Marimo is the Japanese name for algae balls found in Lake Akan. The ones found in Iceland’s Lake Mývatn are called kúlusku. Bologists call them Aegagropila linnaei. These plants have been reported from a very few other locations around the world, but the Japanese and Icelandic colonies seem the largest.For marimo to thrive, a lake must have certain features — temperature is important and not too intense sunlight, but the lake water must also have movement at the depth where the marimo are found. Not only does this rotate them in light but cleans the balls of dirt and debris.
The marimo turn in the water, exposing each green area to sunlight. all parts of the sphere are capable of photosynthesis. Inside the ball, chloroplasts connected to the outside layer lie dormant until triggered by light if the plant breaks open. In Mývatn, the balls may pile up on one another in a layer two or three kúlusku deep. In Lake Akan, marimo may grow to a foot or more in diameter.
In 1950, the Ainu people around Lake Akan came up with a Marimo Festival that is now an annual event. Marimo are taken up from the lake, cleaned and blessed at a local shrine, then returned to the water.
This has become a very large affair with mascots and tourists and has become important to Ainu cultural identity.
Marimo are sold as pets at the festival and other places in Hokkaido, but these are cultured marimo, made from algal filaments. They will still grow, if properly cared for, and can have a place in aquariums or jars that hold nothing else but a single marimo.
Otherwise, this is a protected species in both Japan and Iceland since numbers are said to be decreasing in both Lake Akan and Mývatn. Various theories have been advanced for this, from global warming to hydroelectic dams stopping the necessary water movement. It might be relevant to note that this species was first discovered in an Austrian lake in the 1820s. It isn’t there now.