I don't normally go out of my way to collect marine mollusks, but if I am at a beach and there are shells everywhere, then I don't miss the opportunity either. This complete specimen of the Atlantic surfclam (Spisula solidissima) at the beach at the Assateague Island National Seashore last Friday afternoon was too good to leave behind.
Subsequent close inspection revealed a prominent varix across the top of each valve. The older shell above the varix had lost most of its periostracum and thus was white, while the younger shell below it still retained the yellowish-brown skin. This color difference and the growth of new shell material out from under the old shell make it look like a smaller pair of shells had been glued on top of a larger pair.
Of course, on the inside surface of the shells there is no break, but only a lumpy ridge corresponding to the position of the varix on the outside.
According to this NOAA review of life history and habitat characteristics of S. solidissima, growth of these clams is not uniform over the year, which is not surprising since lower water temperatures in the winter are expected to slow down or completely stop growth. The varix on these valves indicates that the occupant clam did stop growing at least once. I am otherwise not familiar with these clams and don't know if the presence of such a prominent varix on their shells is a common occurrence. However, I have noticed that the specimen pictured on Plate 32p of Abbott's American Seashells (1954) seems to have a similar looking lighter colored section.
The above magnified picture was taken from below the varix; the umbo was towards the top. The edge of the varix sticks up ~0.3 mm above the shell surface. Clearly, the new shell started growing out from under the old shell.
In these 2 posts, here and here, I explained that when a land snail resumes shell growth following a break, new shell starts growing below the old shell and slightly behind the break. Now we know that the same thing happens in bivalves. This too is not surprising, since gastropods and bivalves are evolutionarily related and even though they diverged from each other hundreds of millions of years ago, they still seem to use similar shell-building mechanisms and are therefore subject to more or less similar constraints and limitations when it comes to building their homes.