antenea

I have been watching these forums for a good while, but never have posted so this is my first post. I’ve recently had a very small worm hitchhike into my tank. Initially I noticed in in the sand and it was reaching out and hitting an amphipod. It didn’t appear as if it was trying to catch it...

Can anyone id this please. The grey part on the left side is sucking onto the rock. The top is a shell. The right side has 2 antennae. It doesn’t swim or walk. It just sucks on to the rock to move.