Marimo (also known as Cladophora ball, moss ball, or lake ball) is a rare growth form of Aegagropila linnaei (a species of filamentous green algae) in which the algae grow into large green balls with a velvety appearance.
The species can be found in a number of lakes and rivers in Japan and Northern Europe. Colonies of marimo balls are known to form in Japan and Iceland, but their population has been declining.
Marimo were first described in the 1820s by Anton E. Sauter, found in Lake Zell, Austria. The genus Aegagropila was established by Friedrich T. Kützing (1843) with A. linnaei as the type species based on its formation of spherical aggregations, but all the Aegagropila species were transferred to subgenus Aegagropila of genus Cladophora later by the same author (Kützing 1849). Subsequently, A. linnaei was placed in the genus Cladophora in the Cladophorales and was renamed Cladophora aegagropila. Rabenhorst and Cl. sauteri (Nees ex Kütz.) Kütz. Extensive DNA research in 2002 returned the name to Aegagropila linnaei. The presence of chitin in the cell walls makes it distinct from the genus Cladophora.
The existence of marimo colonies depends on the adaptation of the species to low light conditions, combined with the dynamic interaction of wind-induced currents, light regime, lake morphology, bottom substrate and sedimentation.
The growth rate of Marimo is about 5 mm per year. In Lake Akan in Japan they grow particularly large, up to 20 – 30 cm. Lake Mývatn, Iceland, had dense colonies of Marimo that grow to about 12 cm in diameter and formed well-defined patches on the lake floor at depths ranging from 2 – 2.5 m.
The round shape of the marimo is maintained by gentle wave action that occasionally turns it. The best environment for that are shallow lakes with sandy bottoms.
The balls are green all the way round which guarantees that they can photosynthesize no matter which side is turned upwards. Inside, the ball is also green and packed with dormant chloroplasts which become active in a matter of hours if the ball breaks apart. The wave action also cleans the balls of dead organic material.
As some colonies have two or even three layers of Marimo balls, wave action is needed to tumble them around so each ball reaches the light. The spherical shape has a low surface-area-to-volume ratio compared to a leaf, which limits photosynthesis and therefore limits the maximum size of the Marimo balls.