I am very surprised to see that hokkigai is not so prominent in the US.
Hokkigai clam is a specialty item in Hokkaido Japan and you can buy frozen ones. Surf clam has a crispier texture than the akagai and abalone and the meat that you are served in sushi is the top part of the cleaned surf clam, with all the rest of the meat(the intestines and the himo) detached.
In my experience, the hokkigai surf clam is served raw, but it will taste okay even if you poached it in a hot pot or shabu-shabu. Usually you do that for low grade hokkigai, but you should save it for sashimi for the good ones.
The profile picture to the right shows a natural hokkigai surf clam. The surf clam should look pinkish white in the lower body and it should look dark red on the tip of the clam. I have see some hokkigai with bright red top and a white bottom and I highly suspect they are counterfeits.