This is a marine worm, Dinophilus, and it’s parasitized by unicellular organisms called Grellia. You can see the parasites as blue, banana-shaped structures in this polarized view. . Dinophilus are a few millimeters in length and inhabit all marine environments. They eat algae and mind their own business without bothering anyone. However, the large population I had in one of my fishless tanks were all infected by a species of unicellular organism. . When I checked an odd-looking Dinophilus under