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Three common growth forms of Aegagropila linnaei, the lake ball proper, "free-floating filaments" and epilithic
Scientific classification
Kingdom: Plantae
Division: Chlorophyta
Class: Ulvophyceae
Order: Cladophorales
Family: Cladophoraceae
Genus: Aegagropila
Species: A. linnaei
Binomial name
Aegagropila linnaei

Marimo (Japanese: 毬藻 marimo), also known as Cladophora ball, Lake ball, or Moss Balls in English, is a species of filamentous green algae (Chlorophyta) found in a number of lakes in the northern hemisphere. A marimo is a rare growth form of the species where the algae grow into large green balls with a velvety appearance. Colonies of such balls are known only from Iceland, Japan and Estonia.


Classification and nomenclature

They were first discovered in the 1820s by Dr. Anton E. Sauter in Lake Zeller (de:Irrsee), Austria. The genus Aegagropila was established by 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 has been accommodated in the genus Cladophora in the Cladophorales and has been renamed Cladophora aegagropila (L.) 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 plant was named "marimo" by a Japanese botanist Tatsuhiko Kawakami 川上龍彦 (Kawakami Tatsuhiko) in 1898 (Meiji 31). Mari is a bouncy play ball. Mo is a generic term for plants that grow in water. The native names in Ainu are tōrasanpe (lake ghost) and tōkarippu (thing that rolls in mud)[1]. They are sometimes sold in aquaria under the name "Japanese moss balls" although they are unrelated to moss. In Iceland the lake balls are called kúluskítur by the local fishermen at Mývatn (kúla = ball. skítur = muck) where the "muck" is any weeds that get entangled in their fishing nets.

Growth forms

There are three growth forms. One is epilithic (growing on rocks) and is usually found on the shaded side of the rocks. Another growth form lives as free-floating filaments, as small tufts of unattached filaments that frequently form a carpet on the muddy lake bottom. The third growth form is the lake ball proper, where the algae grow into sizeable balls of densely packed algal filaments that radiate from the centre. The balls do not have a kernel of any sort.  


Marimo colonies in Lake Akan Hokkaidō and Lake Mývatn Iceland rank among the strangest plant communities on Earth. Their existence depends on the adaptation of the species for 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 they grow particularly large, up to 20-30 cm (8-12 inches). Mývatn, Iceland, has dense colonies of marimo that grow to about 12 cm in diameter and form well defined patches on the lake floor at depths ranging from 2-2.5 m. The colonies were discovered in 1978 but have shrunk considerably in size since then. The round shape of the marimo is maintained by gentle wave action that occasionally turns it. 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 matter of hours if the ball breaks apart. The wave action also cleans the balls of detritus. 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 to volume ratio and is therefore not optimal for a plant. This limits the maximum size of the marimo balls.

Conservation and cultural aspects

  The rapidly declining population of lake balls in Mývatn is of special concern, but for unknown reason some of the main patches have all but vanished in recent years. At Lake Akan a great effort is spent on the conservation of the lake balls. This includes an annual three day marimo festival in which the Ainu people, the indigenous people of Hokkaidō, play an important part. Because of their appealing appearance the lake balls also serve as a medium for environmental education. They bear a certain likeness to the Earth in being green and round and in their need to rotate in order to receive light on all sides. In Japan the marimo has been a protected species since as early as 1920, defined as a natural treasure. Small balls sold as souvenirs are hand rolled from free-floating filaments. The marimo was given a status of protected species in Iceland in 2006. It is said that taking good care of the plant will make one's wishes come true. Both Lake Mývatn and Lake Akan are protected, the former as a nature reserve, the latter as a National Park.

Care for Domestic Marimo

Tap water is enough for a domestic marimo bought as souvenirs to grow healthily. The water should be changed every one or two weeks, more frequently in summer and less often in winter. The marimo can be placed in the refrigerator if the weather becomes too hot (>25° Celsius), but do not place the marimo in the freezer. The marimo is able to carry out photosynthesis with normal household lightings, otherwise, indirect sunlight is also acceptable, but the marimo should never be placed in direct sunlight since the marimo is adapted for low light conditions only. If the marimo green colour turns brownish, try to leave it alone for a while and see if it recovers, if not, a very minimal amount of salt can be added to the marimo to try to remedy the problem.

Appearances in manga

  • In the anime & manga One Piece, the Straw Hat Pirates's cook, Sanji, often calls swordsman Roronoa Zoro "marimo" (in a derogatory manner) because of his fuzzy, green hair. Also in One Piece, there is a henchman to the villain Wapol named Kuromarimo, who uses static afros to attack his opponents. He is named that way because of his afros, which resemble marimo algae.
  • Also, in the anime Nerima Daikon Brothers, the character Ichiro loves these small balls of Algae.
  • In the Anime Daa Daa Daa it is revealed during episode 58 that the Character Nanami Tenchi Keeps a pair of Marimo named Aega and Gropila as pets. She presumes them to be female and sisters.
  • An early Sailor Moon episode features a monster that holds innocent people hostage by entrapping them in giant marimo. Sailor Moon declares she will defeat the monster for his crime of "exploiting the protected species of Lake Akan". Appropriately enough, the name of the aforementioned marimo-throwing monster is "Akan".


  1. ^ Note: the Ainu names were transliterated into kana, then transliterated into the Latin alphabet. Therefore the spelling may not be a close representation of the original pronunciation.


  • Einarsson, A., Stefánsdóttir, G., Jóhannesson, H., Ólafsson, J.S., Gíslason, G.M. Wakana, I., Gudbergsson, G. and Gardarsson, A. 2004. The ecology of Lake Myvatn and the River Laxá: variation in space and time. Aquatic Ecology 38: 317-348.
  • Hanyuda, T., Wakana, I., Arai, S., Miyaji, K., Watano, Y. and Ueda, K. 2002. Phylogenetic relationships within Cladophorales (Ulvophyceae, Chlorophyta) inferred from 18S rRNA gene sequences, with special reference to Aegagropila linnaei. J. Phycol. 38: 564–71.
  • Jonsson, G.S. 1992. Photosynthesis and production of epilithic algal communities in Thingvallavatn. Oikos 64: 222-240.
  • Nagasawa, S., Wakana, I. and Nagao, M. 1994. Mathematical characterization of photosynthetic and respiratory property regarding the size of Marimo’s aggregation. Marimo Research 3. 16-25.
  • Yokohama, Y., Nagao, M,, Wakana. I. and Yoshida, T. 1994. Photosynthetic and respiratory activity in the inner part of spherical aggregation of “Marimo”. Marimo Research 3: 7-11.
  • Yoshida, T., Nagao, M., Wakana, I. and Yokohama, Y. 1994. Photosynthetic and respiratory property in the large size spherical aggregations of “Marimo”. Marimo Research 3: 1-6.
  • Yoshida, T., Horiguchi, T., Nagao, M., Wakana, I. and Yokohama, Y. 1998. Ultrastructural study of chloroplasts of inner layer cells of a spherical aggregation of “Marimo” (Chlorophyta) and structural changes seen in organelles after exposing to light. Marimo Research 7: 1-13.
  • Wakana, I. 1992. A bibliography relating to “Marimo” and their habitats. Marimo Research 1: 1-12.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Marimo". A list of authors is available in Wikipedia.
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