A marimo’s rounded appearance is the visible result of algae interacting with a moving lake. Understanding that relationship helps explain how these unusual balls retain their shape, receive light, and grow.

Research on wild marimo, particularly those in Japan’s Lake Akan, reveals several processes working together. It also shows why familiar claims about their growth deserve a closer look.

How waves help marimo stay round

In Lake Akan, wind-generated waves cause marimo to rock and rotate. A 2021 study combined field observations with magnetic resonance imaging (MRI) to investigate how this movement affects their structure.

The researchers found that rotation helps polish the outer surface, producing a more densely packed layer of algal filaments. Their findings connect the ball’s appearance with the physical conditions around it: wind, waves, and seasonal ice cover all influence its movement. [1]

Japan’s Ministry of Land, Infrastructure, Transport and Tourism describes the importance of Lake Akan’s setting. The surrounding topography helps produce waves that tumble the balls, while stronger conditions can wash them onto the shore. [2]

Conceptual diagram showing wave-driven marimo rotation, light exposure, and removal of surface deposits in Lake Akan.
Figure 1. Wave-driven movement helps shape wild marimo, exposes different surfaces to light, and shakes off deposits. Original ModMoss conceptual illustration, not to scale, based on sources [1] and [2].

Why turning matters for sunlight

A ball resting on a lake bottom cannot expose every side to sunlight simultaneously. Tumbling changes which parts face the light.

The ministry’s explanation connects this movement with photosynthesis across the marimo’s surface and the development of its rounded shape. Movement therefore affects both the ball’s form and the light available to its algae. [2]

How movement clears the surface

Fine sediment and organic particles can accumulate on marimo. Rocking and rolling help shake off these deposits, supporting continued photosynthetic growth.

The 2021 Lake Akan study also found that wave-driven movement promotes water exchange between the ball’s interior and its surroundings. The researchers linked that exchange with nutrient recycling, showing that movement influences conditions inside the ball as well as its exterior. [1]

What happens when inner cells receive light

Cells inside a marimo experience different light conditions from cells at its surface.

A study published in Phycological Research examined the chloroplasts, the cellular structures involved in photosynthesis, of inner and outer cells. Inner cells kept in darkness had chloroplasts with a different structure from those at the surface.

After exposure to light, the inner-cell chloroplasts changed. Within 48 hours, researchers observed substantial starch grains and other structural changes consistent with previously reported recovery of photosynthetic activity. This provides a more specific account than simply describing the interior as dormant. [3]

How fast do marimo grow?

A single annual growth figure can conceal differences in size, habitat, and measurement method.

A 2023 Lake Akan study used MRI analysis to estimate growth from internal ring patterns. Estimated annual diameter growth increased with ball size in the sample. The researchers’ model estimated a maximum annual diameter increase of approximately 11.5 millimeters, with a reported standard deviation of 1.8 millimeters. [4]

2023 Lake Akan study: model-estimated maximum annual diameter growth of 11.5 millimeters, plus or minus 1.8 millimeters standard deviation; not an aquarium forecast.
Figure 2. A model-estimated maximum annual diameter growth rate for the studied Lake Akan marimo. The dot marks 11.5 mm/year; the whisker shows ±1.8 mm/year, one reported standard deviation, rather than an observed minimum-to-maximum range. Graphic by ModMoss using source [4].

That estimate applies to the studied wild marimo and the researchers’ method. It should not become a promise about the growth of every aquarium moss ball. A useful growth figure needs context: what was measured, where the algae grew, and whether the measurement describes filament length, radius, or diameter.

Sources and further reading

  1. Nakayama and colleagues (2021). The structure and formation of giant Marimo (Aegagropila linnaei) in Lake Akan, Japan. Scientific Reports 11, 22017.
  2. Ministry of Land, Infrastructure, Transport and Tourism, Japan. Marimo: Formation and Shapes. Explanatory PDF.
  3. Horiguchi, Yoshida, Nagao, Wakana, and Yokohama (1998). Ultrastructure of chloroplasts in ‘Marimo’ (Cladophora aegagropila, Chlorophyta), and changes after exposure to light. Phycological Research 46, 253–261.
  4. Nakayama and colleagues (2023). Ideal water temperature environment for giant Marimo (Aegagropila linnaei) in Lake Akan, Japan. Scientific Reports 13, 16834.

Research summary by ModMoss, based on the publications above. The illustrations explain published findings; they do not present new experimental data.