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Do they close up when disturbed? Any other features? Looks like a sponge to me, gray might indicate a chicken liver sponge, but hard to tell from that pic.
They don't close up or move at all. They are also almost completly smooth, unlike my other sponges. Just looked up chicken liver sponge (Chondrilla nucula) and it looks pretty close maybe...I'll have to do more pix.