Cladophora aegagropila

Marimo (毬藻?, literally "ball seaweed"), 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 only known to form in Iceland, Scotland, Japan and Estonia.

Marimo colonies in Lake Akan, Hokkaidō in Japan and also in Lake Mývatn in 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 a 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-area-to-volume ratio and is therefore not optimal for a plant.[clarification needed] This limits the maximum size of the marimo balls.

 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 sizable balls of densely packed algal filaments that radiate from the center. The balls do not have a kernel of any sort.

Reference

http://en.wikipedia.org/wiki/Marimo 

http://www.ilec.or.jp/database/asi/asi-22.html