Cozumel ball with tentacles

[dfx]

These are usually seen hiding in holes, but this one was out. What is it?

[video=youtube;QTuKJ1-pAuU]https://www.youtube.com/watch?v=QTuKJ1-pAuU[/video]

 

[TSandM]

Looks like some type of swimming scallop, to me. We have them in the PNW, and they always make me laugh.

 

[LowVizWiz]

It looks like a rough file clam to me