Marimo (Aegagropila linnaei): grower data and the zebra mussel find
Sold as a 'moss ball', marimo is a green alga that Tropica rates easy with low light and CO2 demand and slow growth [1]. In March 2021, invasive zebra mussels were found attached to imported marimo balls in some pet stores in 29 US states [5].
Grower data
As published by Tropica Aquarium Plants[1]
- Light demand
- LowTropica: medium light demand is 0.5 W/L.
- CO2 demand
- LowTropica: medium need 6–14 mg/L; high approx. 15–25 mg/L.
- Growth rate
- Slow
- Difficulty
- Easy
| Type | Moss |
|---|---|
| Origin (grower) | Cosmopolitan |
| Height after two months | 3 - 10+ cm |
| Placement | foreground, midground[2] |
| Growth (second grower) | Very slow[2] |
| Size (second grower) | 1–10 cm[2] |
| Water hardness (maximum) | Hard[2] |
Name and range
A green alga, so it is outside the World Checklist of Vascular Plants. The GBIF Backbone Taxonomy accepts Aegagropila linnaei Kützing in the family Cladophoraceae (Chlorophyta) and lists Cladophora aegagropila (L.) Rabenh. as a synonym [3, 4].
What the grower data says
Tropica rates marimo easy, slow-growing and low in light and CO2 demand, and gives a size of 3–10 cm across [1]. Aquasabi agrees on easy, low light and no need for added CO2, and calls growth “very slow” [2]. Both file it with the mosses: Tropica’s type field says “Moss” [1] and Aquasabi lists it in its moss category.
It is not a moss, and Tropica’s own description says so plainly: “not really a plant, but a ball of algae”, and a decorative exception to the rule of avoiding algae at all costs [1].
Keeping the shape
In nature, Tropica explains, marimo is normally found in shallow lakes, where the movement of the waves rolls it into a sphere [1]. An aquarium has no waves, so Tropica’s advice is to turn the ball regularly to keep its shape [1]. It can also be divided into smaller pieces, which either become round with time or, if attached to roots and stones, form a carpet [1]. Aquasabi lists it for the foreground, middle ground and nano tanks [2]. Tropica adds that it is protected in parts of Japan [1].
The 2021 zebra mussel discovery
Marimo became a biosecurity issue in 2021. According to the US Geological Survey’s Nonindigenous Aquatic Species database, in March 2021 zebra mussels were discovered attached to imported marimo balls in some pet stores in 29 states, from Alaska to Florida [5]. The same USGS profile notes that the U.S. Fish and Wildlife Service lists the zebra mussel as injurious wildlife, and that it is established in all the Great Lakes and in the large navigable rivers of the eastern United States, and notes that it can survive several days out of water in cool, humid conditions [5].
For anyone keeping marimo, the practical point is containment: water, packaging and any unwanted balls from a batch of uncertain origin should never go into drains, ponds or waterways. Check your state’s natural resources agency for current guidance, and see invasive aquarium plants and safe disposal.
Names and taxonomy
Marimo is a green alga in the order Cladophorales. The GBIF Backbone Taxonomy accepts Aegagropila linnaei Kützing in the family Cladophoraceae, phylum Chlorophyta [3], and treats Cladophora aegagropila (L.) Rabenh., the name Tropica’s description still uses, as a synonym [4, 1]. As an alga it is outside the World Checklist of Vascular Plants. Tropica’s origin field reads “Cosmopolitan” [1].
Questions
Is a marimo moss ball a moss?
No. Tropica says it is not really a plant but a ball of algae [1], and the GBIF Backbone Taxonomy places Aegagropila linnaei in the green algae, family Cladophoraceae [3].
How do I keep a marimo round?
Tropica says it must be turned regularly in an aquarium to keep its shape [1].
Similar plants
- Anubias barteri var. nana: grower data, planting and namesrhizome, epiphyte / foreground / midgroundLight: LowCO2: LowEasy
- Riccia fluitans: a floating liverwort, by the grower datamoss, floatingLight: MediumCO2: MediumMedium
- Java moss / Bogor moss (Taxiphyllum barbieri): grower data and namesmoss, epiphyteLight: LowCO2: LowEasy
Related guides
- Algae & problemsHair, thread and fuzz algae: telling them apart and what helpsGreen filamentous algae in planted tanks: how thread, hair, fuzz and Cladophora differ, the light, nitrate and CO2 triggers growers name, and the grower remedies.
- Algae & problemsInvasive aquarium plants and how to dispose of trimmings safelySeveral common aquarium plants are established in US waters and two are federal noxious weeds. What the official records say, and how to dispose of trimmings safely.
Compare it with the other profiled plants in the plant parameter table.
Sources
- Aegagropila linnaei (plant details, 000CST). Tropica Aquarium Plants. Type, origin, growth, height, light, CO2 and difficulty; description: a ball of algae 3–10 cm wide, formed by waves in shallow lakes, turn it regularly, can be divided, protected in parts of Japan. Accessed October 12, 2026.
- Aegagropila linnaei (Aquarium Plant Profile). Aquasabi. Placement: nano aquarium, middle ground, foreground; easy; very slow; low light; hardness up to hard; 1–10 cm; CO2 not required. Accessed October 12, 2026.
- Aegagropila linnaei Kützing (GBIF Backbone Taxonomy record 5272847). GBIF Secretariat. Accepted; Plantae, Chlorophyta, Ulvophyceae, Cladophorales, Cladophoraceae. Accessed October 12, 2026.
- Cladophora aegagropila (L.) Rabenh. (GBIF Backbone Taxonomy record 5272828). GBIF Secretariat. Synonym of Aegagropila linnaei. Accessed October 12, 2026.
- Zebra mussel (Dreissena polymorpha): species profile. U.S. Geological Survey, Nonindigenous Aquatic Species database. In March 2021, zebra mussels were discovered attached to imported Marimo balls (an alga in the aquarium trade) in pet stores of 29 states from Alaska to Florida. Accessed October 12, 2026.