From the appearance and size I think your clam is a wild collected Maxima from French Polynesia.
About 10 years ago I went there and set up the collection station and helped start a clam farm. These clams are found naturally in the remote islands distant from Tahiti and occur in the hundreds of millions. They are mostly embedded in rock and must be carefully chiseled out. When we first set up the station the plan was to collect and sell wild clams in a effort to fund the farming. My partner there hand collected each clam. Its slow and difficult work when done correctly, I've done it myself and can attest to the difficulty. The clams are found less than 6 feet deep under intense tropical sunlight, with strong water flow, in pristine water on occasional rock outcroppings. In some areas we collected clams where the water temps were in the 60s although mostly in the low 70s.
Over the next few years we imported as many as 1000 or more at a time, you can check our videos on YouTube and also learn more in my blog:
https://pacificeastaquaculture.com/blogs/coral-farming-in-the-solomon-islands
We varied the islands we collected because the coloration was different from each. After a time my partner became more intetested in collecting fish and less so in collecting clams so he turned over that aspect to local fisherman. Unfortunately they are not as careful to take the time to chisel them from the rocks and usually damage the foot. Also, they started to ship them from the remote Islands to Tahiti dry instead of in water. You see, the clams are harvested there as food and this type of rough collection is common. Once these changes began we started seeing increaseed mortality and central mantle bleaching as seen in your clam.
Later it was discovered that the Perkinsus parasite has commonly been found in these clams in the wild and presumably the stress of dry shipping within the islands and rougher collection stresses the clam to the point that it can no longer deal with the parasite. I stopped dealing with the wild collected clams several years ago and now only deal with smaller cultured where we find no mortality or stress signs such as mantle retraction or bleaching.
Freshwater dips could be of some benefit, but in my experience an already stressed clam can't tolerate the additional stress of the dip.
Sorry for being long winded, but thats the story on these clams.
The rare Chimera or so-called Two Faced clam, see about 1 in every couple million in the wild. Actually culturing some now.
