Native to estuaries of Japan and Korea, These clams were brought to Victoria Island, Canada around 1988, probably in ship's ballast water. They have spread north from there, and south along the northern coast of Washington State. A similar introduction now infests most of the coastal estuaries of the state of Oregon. They grow to about 2.75 inches measured the long way, but the largest of the photo specimens was 2.0 inches, weighing 0.88 ounce (25 gm). They are not currently farmed, but are harvested from high in the intertidal zone (above the Manilla Clam zone), where there can be more than 1400 clams per square yard.
More on Bivalve Mollusks.
Purple Varnish Clams have a good strong clam flavor and tender flesh, if not overcooked. Because they are smaller and lighter than Manilla clams, I consider them good for fancy seafood soups where whole live clams are included. Note: depending on harvest location, many of these clams can contain a parasitic Pea Crab. Do not be frightened, these don't harm the edibility of the clam, and many consider them a gourmet treat, raw or cooked. It is reported that George Washington liked them scattered over his oyster soup as a garnish.
Buying: Labeled "Savory Clams" (a name made up by the Canadians), these clams can now sometimes be found in the live tanks of big Asian markets. They come from Canada, where a commercial fishery has been established. The photo specimens were purchased from a large Asian market in Los Angeles (San Gabriel) for 2016 U.S. $3.99 / pound. Since they are harvested wild, a batch will vary quite widely in size. The specimen batch included clams from 2 inches and 0.88 ounce (25 gm) to 1.3 inches and 0.25 ounce (7 gm). They averaged 34 clams per pound.
These clams can be harvested recreationally in Washington and Oregon states, but check with the local fishery authorities about safety (they can retain shellfish toxins under some conditions), and mind the quotas.
Storing: These clams are better than Manilla Clams for dry storage. They are able to survive even 30 days in the refrigerator. They should be in an open container so they can breath, and covered with a damp cloth to keep humidity high.
Yield: A pound of clams will yield about 4.2 ounces of meat (26%). At the current price (2016) of $3.99 / pound that comes to just over US $15 per pound of meat, significantly better than Manilla Clams due to their lighter shells.