The moss ball is a unique aquatic organism recognized for its dense, velvety, and perfectly spherical shape, often used in home aquariums or decorative jars. Though commonly sold as an aquatic plant, its unusual form and slow growth rate make it a popular, long-lived decoration.
Biological Identity
The popular name “moss ball” is a misnomer, as this organism is not a moss but a rare growth form of a freshwater green algae species, Aegagropila linnaei. In Japan, they are known as Marimo, which translates to “ball algae.” The structure consists of a dense aggregation of fine, filamentous algal strands that grow radially outward from the center, creating the characteristic soft, velvety surface.
Unlike most organisms, the spherical shape of the Marimo is not determined by its internal biological structure; the algae itself forms a loose, spongy mass of filaments. The ball shape is instead maintained by external environmental forces that constantly tumble the organism, ensuring all sides receive light for photosynthesis. These organisms are exceptionally slow-growing, often increasing in size by only a few millimeters each year, but they can live for over a century.
Natural Habitat and Formation
Marimo balls are native to a limited number of shallow, cold freshwater lakes, primarily in Japan, Iceland, and parts of Northern Europe. Large, healthy populations are now rare globally due to habitat degradation and pollution. The key to the spherical structure’s formation and survival is the gentle, continuous movement of water. On the lake bottom, currents or wind-driven waves constantly roll the algal masses, much like a natural tumbler. This tumbling action prevents any single side from remaining fixed to the substrate, which would cause uneven growth and browning.
Basic Care and Maintenance
Caring for a Marimo ball is straightforward due to its adaptation to low-light, nutrient-poor conditions. They thrive best in cool water, ideally below 77°F (25°C), as warmer temperatures can cause browning and decay. Direct, intense sunlight should be avoided, as it can scorch the filaments and encourage the growth of undesirable algae.
The water in the container should be changed every one to two weeks, using dechlorinated tap water or clean aquarium water. If the Marimo is kept in a static container without currents, manually simulate the natural tumbling motion. Gently remove the ball during water changes, squeeze it lightly to remove accumulated debris, and roll it between your palms to help maintain its round shape. Healthy Marimo balls photosynthesize and may float occasionally due to trapped oxygen bubbles, which is a sign of good health.
Common Issues and Pest Concerns
The most common sign of poor health in a Marimo is browning, which usually signals too much light, excessively warm water, or a buildup of debris. If browning occurs, the affected area can sometimes be restored by gently rinsing the ball and moving it to a cooler location with lower light. If a section is severely discolored or disintegrating, it can be carefully trimmed away to prevent spreading.
A far more serious concern is the potential presence of invasive Zebra Mussels, which have been found attached to or inside imported moss balls sold commercially. Zebra Mussels are a highly destructive invasive species that can severely damage native ecosystems and infrastructure. Because of this risk, any moss ball suspected of contamination must be destroyed safely using one of the following methods:
- Freezing it for at least 24 hours.
- Boiling it for a full minute.
- Soaking it in a bleach solution for 10 minutes.
After treatment, the ball and all associated water must be sealed in a plastic bag and disposed of in the household trash, never flushed or dumped outside.