what is this
It looks to me to be a pholad clam (family Pholadidae), also called piddocks, which are deep burrowing and hence the long siphon. The ususally have teeth-like edges to their shell to facilitate burrowing. Of course, it could be something similar to a geoduck, which obviously also has a long siphon that is not retracted into the shell (as with pholads).
Having said that, can't say I know much about Alaskan bivalves (since I'm in Florida!!). I suggest you contact Dr. Ray Ralonde at Alaska Sea Grant, surely he can narrow down your ID. email@example.com