Home | Aquarium Livestock | Marimo – the Tale of Mysterious “Moss Ball” Colonies

Marimo – the Tale of Mysterious “Moss Ball” Colonies

Marimo FormationsA few years ago our plant suppliers introduced new items for sale that were just too fun and interesting to pass up. They were simply called moss balls. If ever there was an aquarium plant with personality, this would be it. They have a certain character, and can almost become like a pet in the aquarium even if they aren’t dazzlingly colored or active like the fish we keep. The vague nomenclature was accurate enough, but the story of where and how these mysterious moss balls come to be is as interesting as they are to look at when you place them in your aquarium.

Marimo, Japanese for “ball seaweed”, were named by a Japanese botanist decades after they were originally described by someone else in Austria around 1820. They originated in shallow areas of a few freshwater lakes in the world including several in Japan, Iceland, Scotland and others. In the trade they are referred to as moss balls or Japanese moss balls, but they also have several other names given by those who encounter them in their native waters. The name seems fitting…each ball has a velvety, thick, fuzzy texture much like a mound of filamented moss. However, moss balls aren’t really moss at all. These unique formations are actually colonies of the filamentous algae Aegagropila. They were previously classified as Cladophora aegagropila, but modern research proved that they actually belong in genus Aegagropila. Don’t let their classification as algae scare you, this is a fun, non-invasive kind!

Moss Balls came to be due to conditions unique in the lakes where they form.  The plants had to adapt to the rather harsh conditions of their habitats to handle low light, strong currents, areas of heavy sediment and other variables. The solution to surviving and even thriving in these conditions for this species was to grow in dense masses where individual filaments radiate outward to form a semi-solid ball.  In most cases the ball is almost perfectly round, Thin Moss ballthough colonies also grow in flattened forms or even attached to rock in dense mounds. Smaller, thinner clusters may grow under the larger clumps. Their round shape is the key to their survival. The balls roll and turn, several layers deep, with the wind-driven currents in the lakes. This keeps their velvety surface relatively free of mud, detritus and sediments. Their entirely green orbicular surface ensures that they always have exposure to the light, allowing them to photosynthesize. Even the dense internal structure is viable. If the ball is damaged or torn in pieces, the newly exposed internal filaments spring to action allowing the new surfaces to feed on the light within hours. Balls may split naturally when they become too large. The moss ball segments should eventually take on a more even form…multiple new balls from a single “parent”.

Marimo are slow growing and rather limited in their maximum size. Growth can be encouraged by providing adequate light and additional nutrients in the aquarium. Depending on their origin, they typically range in size from 2 to 5 inches in the aquarium to some individuals in the wild that reach an average diameter of 8 to 12 inches across (only in specific regions)!  

Moss ballMoss Balls are protected in most of their native waters, an important step taken by conservationists to preserve native colonies. They are still available for aquariums as they are not harvested but “domesticated” or “farm raised” for aquarium use. They are easy to care for, benefiting from regular water changes to the aquarium and placement in an area of adequate circulation. They prefer cooler temperatures and may not fare well in tanks kept above 77 degrees. These critters can be kept in a large jar or bowl if you prefer, just change the water every week or two. Keep in mind that they are adapted to lower light conditions, and they may actually be happier under normal room lighting, but at the least provide them with a place where they will have indirect exposure to bright lights on an aquarium. I’m sure you’ll appreciate your new green “pet” no matter where you make its home.



Moss Ball Formation Types image referenced from wikipedia and originally posted by A. Einarsson

Thin Moss Ball Formation image referenced from wikipedia and originally posted by Zikamoi


  1. avatar

    These moss balls really look great in aquarium.

  2. avatar

    Can someone please tell me, Is it harmful to my cat to drink water from the marimo bowl? It’s th ed only water she drinks!!!

  3. avatar

    Hello Michelle, I wouldn’t expect it to be harmful but it probably isn’t very healthy either. I would recommend contacting your vet with that question…they would be able to give you a better answer than us since we don’t deal with cats much on this aquarium blog. Perhaps a cat water fountain would help as well?

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