These fascinating green spheres are an kind of algae that grow on the bottom of a select few lakes in the norther hemisphere. Due to the negative implications of the term ‘algae, ‘these balls are usually referred to as moss even though they are formed by strings of algae rolled together to form a ball. The common name Marimo was coined by a Japanese botanist in 1898.
These mysterious algae live in cold waters. They float and sink according the available light for photosynthesis. In domestic settings people refer to this sinking and floating behavior as "playing" and will also gravitate towards other Marimo in the same body of water, which is also often characterized as a social attribute. They can be kept in a variety of water temperatures but tend to do better in cooler water. Any fresh water will do. They can be housed in decorative vases, fish tanks or even just a cup of water. Although Marimo are not particularly picky about light, they do need some so keep them near a window.
They require little by way of maintenance, with the water needing to be changed just once a week. If green algae forms on the outside of the ball (will be slimy) to clean, just remove them from the tank and gently squeeze - like you would wring out a sponge. They grow very slowly averaging about 2cm in diameter a year. The oldest recorded Marimo Moss ball is over 100 years old!
Marimo are a great addition to any fish tank as well as they can actually help suppress the growth of pesky brown and green algea that tends to plague fresh water fish tanks.