We don't know what this is. And neither did these three graybeards, who between them have nearly a century and a half of experience in the Hood Canal oyster industry: 
Nathan is the only one who wasn't shocked at the bright red bivalve. He said that they grow way out deep and called them "blood oysters." We love that delightfully dramatic name, and will continue to use it, but the consensus is that it's definitely not an oyster. Are there any scuba divers out there who know what this is? Below is a series of photos showing Jim shucking the red... thing. First he pries it off of a Pacific oyster shell: 
...and it comes all the way off. If it had been a real oyster, he would have only been able to pry off the top shell.
The thing used a muscle to attach itself to the oyster shell. Here's the hole through which the muscle passes: 
And here it is fully open: 
We've called every expert we could think of to try to get an ID on this creature, but for now we'll have to end this post with a cliff hanger. Stay tuned!
[…] other day two of the graybeards were driving to work in the woods above the oyster farm, at around 7 in the morning, when they […]
[…] adding it to stirfrys or pilafs. We’ve even successfully fed it to a vegetable-averse graybeard who, upon seeing something green in his rice dish, nervously wondered if it was spinach. He seemed […]
Pododesmus macrochisma – Alaska jingle – is the name of your mystery bivalve.
http://www.wallawalla.edu/academics/departments/biology/rosario/inverts/Mollusca/Bivalvia/Ostreoida/Anomiidae/Pododesmus_macroschisma.html
This link should provide information, if you still have a need for that.