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Aegagropila linnaei, known as Marimo (毬藻, literally “ball seaweed”) in Japanese and 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.
You might be thinking that marimo aren’t actually pets, but in fact they are. They don’t usually require food (though you might have suppliers that will sell you a marimo with marimo food), but you need to clean them every once in awhile just by rolling it between your hands. They are very soft, kind of squishy and are really fun to hold! They also come in lots of shapes and sizes so you can own a whole aquarium of marimo.
Marimo come in all sizes, but only one shape: round. You can find marimo the size of your pupil and marimo the size of your hand. Usually if you buy them online, the S sizes are about the size of your pinky nail, the M sizes are the size of your thumbnail if not a bit bigger and the L sizes are about the size of your palm. This may differ, though, so make sure to check.
Marimo CAN get sick if something goes wrong with their environment.
Marimo turning white / lighter means that it is probably receiving too much light. If the Marimo also seems a bit slimy or if its texture seems otherwise unusual, you may be dealing with a hostile type of algae growing around it. These algae choke the slow-growing Marimo, so it’s best to carefully wash them off or remove them with tweezers.
Marimo turning brown may be a sign that it’s time to gently clean it. If this doesn’t fix it, try carefully picking off the brown (dead) parts and adding a bit of salt to the water to stimulate the Marimo’s growth. If the bottom of the Marimo has turned brown because it didn’t receive light for longer periods of time, be sure to start rolling the Marimo around a bit more often to prevent parts of it from dying off again.
Marimo turning black and/or falling apart Marimo are unfortunately known to start decaying from the inside out sometimes, especially when it has been covered by hostile algae for a while or when it is simply too big for clean water to reach the inside. In order for it to become healthy again, the black parts should be removed and the Marimo should be gently re-rolled. It’ll be smaller than it was before, but it now has a good chance of surviving and growing back just fine!
However, marimo are usually fine if you don’t completely forget about them and remember to clean them and change their water!
Depending on how big your marimo is, you can put it in any kind of glass or plastic container, or even a fishbowl or a jar. As long as you can see your marimo from the outside and they have some room to roll around, then it will be fine.