Know what it is? It's tiny, about 1/2" across, in the sandbed.
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Looks just like the one I found in my tank too!
http://img.photobucket.com/albums/v505/beakerbob/Inverts/2DSC_4571.jpg
Here is what I found out: http://forum.marinedepot.com/Topic67192-11-1.aspx