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Got this on a mushroom rock today. Didn't notice it until the shroom retracted. It's about 1 inch and it opens in the miidle of the pointed end.
When open, it raises slightly from the rock.
Yes, it opens where the lines come together. It moves like a clam: slowly opening then snapping shut when startled. I can't see a definitive hinge, though. The mushroom has since covered it, at least for today. Working on a better pic.
It looks like a clam that has attached its self to a horse shoe crab. I am really stumped at this point. Does the back end have a tail? Can you pick it up and take a pic of it;s underside?
I found this pic online. It's called a heart cockle. It looks similar, but from other pics it seems cockles (though not specifically the heart cockle) are shaped like a regular bivalve clam. So that would mean that the creature in my tank is buried lip-down in the rock. And what I'm looking at is the hinge. Don't know if all cockles are shaped this way