
"What's a Marimo?"
Well, really it's just an algae that grows in a spherical shape but they're kept as "pets" in fish tanks. Click the image for a larger view and notice the two green balls. I took the screen capture from this website where you can learn more about Marimo. I guess they're the pet rocks of the plant lover world and I want one-actually I'd like a whole colony of them growing in a fish tank like in these images from Google.
You can learn more about the classification, nomenclature and growth habits of Marimo on Wikipedia.
Update:
I just purchased one. You can see the new post at Marimo, Japanese Moss Ball.