Is it rigid or wiggly?
The last few weeks, I’ve noticed this thing stretching off the rock covered in my King Kong Yumas, into the sandbed. I only see it in the mornings, before the lights come on. The first few weeks it would retract the second I would shine light onto it. Now, it just chills and keeps doing what it’s doing. I’ve only ever seen the single arm/body, and only from this particular rock. Even after moving sides of the tank the rock is on. Any idea what it could be?
Wiggly. If it was larger, I’d imagine it was something like a tiger’s tail cucumber or something. But it’s tiny. Only a few inches long fully stretched out, and the thickness of lead in a #2 pencil roughly.
Interesting. If you've only seen one and it has been living in the same place for more than a day, it's probably not a brittle star arm.
Your suggestion of a baby peanut worm may be it. Quick google search results in similar images.
Yeah, that's a peanut worm (Sipuncula); spoon worms (Echiura) are fairly similar, but their stretchy parts (the proboscis) are flattened rather than tubular or branch into two parts the ends.
