marimo; moss ball; lake ball
A spherical green algae (Cladophora aegagropila) found in certain lakes, especially in Hokkaido. Often kept as an ornamental aquatic plant.
The marimo of Lake Akan are designated a special natural monument.
I bought a marimo in a small jar.
Kanji spelling is rare; the word is usually written in kana.
From 毬 (mari, 'ball') + 藻 (mo, 'algae'). The reading まりも is a straightforward compound.