Scale worms are usually small, and there are many different species that fall under the common name. At least one, Arctonoe vittata, famously enjoys a complex relationship with starfish. The worm lives in the starfish's sucker feet. There's a possibility that this commensal arrangement—neither animal really gets any special benefit from having the other around, but they aren't hurt by it either. On the other hand, a 1979 research paper found that A vittata and its starfish host will seek each other out—mutually—through mazes. The starfish will even choose to move toward the worm over its favorite food. And it's still not really clear why that is.
So these are interesting worms. In fact, they can look damn-near cute—almost like little roly-poly pill bugs. But those are the small ones. This guy is different.
The scale worm species Eulagisca is native to the Antarctic. Scientist and Deep Sea News blogger Miriam Goldstein found him in the Scripps Benthic Invertebrate Collection. Check out his size compared to the coffee cup, and tremble.
Not scared yet? Let's talk about the jaws of the scale worm. Here's something that many scale worm species share—mouths that make them look like extras from Aliens. Miriam has a photo of this specific specimen's jaws on Deep Sea News, but it doesn't quite capture the full horror. Below is a Smithsonian microscope image.
Read about the horrific jaws of scale worms and see more microscope images, courtesy the Real Monstrosities blog
Read about the relationship between scale worms and starfish at The Echinoblog