Not a clue...

[ThunderGoose]

Sorry this in under blues. This shell with the circle on top has moved around my tank. Not very far and I haven't watched it move but it's in a different location every day. This morning is the first morning I've been able to photograph it.

It would fit on my pinky nail and came in on live rock/sand from TB Saltwater (Gulf of Mexico). Any ideas?

 

[ThunderGoose]

Solved my own mystery. While trying to remove the shell to put it in better light and under my microscope I realized - there's several things all clumped together, and they're firmly attached...

Lightbulb moment

The shell is an urchin hat! I knew I had a tiny urchin in this tank but I hadn't seen it for a few days. Now I know why.

PS. The Husband has claimed Urchin Hat as the name of his new band.

 

[saltyfilmfolks]

Lol.