Re: Why do some ocean beaches have many shells while others have none?

The answer has a bit to do with biology, and a bit to do with physics. (Which is probably why I, a physicist, was asked to answer your question.)

I don't know much about the mollusks which produce seashells, but I'm sure that they prefer different habitats. Some like sandy, muddy areas, while others prefer rocky shores. Wave action will carry the shells away from the spot where they formed, but you're more likely to find shells near places where the creatures that produce them live.

Equally important, I think, is wave action. The churning effect of waves on the shore tends to keep sediment (silt, sand, gravel, shells, and rocks) suspended in the water, preventing it from settling. The stronger the waves are, the bigger a chunk of sediment must be to avoid getting moved around by the waves. This means that the waves sort the sediment: fine silt tends to collect where the waves are weak, and gravel and rocks will be deposited where the waves are large. Seashells are relatiely large and heavy, but they're also flat and non-"aerodynamic", so the surf sorts them with relatively coarse sand and fine gravel. I rarely find shells sorted with fine sand or silt, and only very large shells end up with cobblestones.

Is the beach in Rhode Island in a protected harbor? Is the beach in New Jersey more exposed to the surf? If so, wave sorting could explain the difference. If the waves and sediment on the beaches are very similar, then perhaps differences in mollusk habitat offshore are the cause.

I'll close by mentioning something from a scientific talk I went to last week. It seems that at the mouth of the Colorado River in the Gulf of California, clams have been living in, dying in, and being buried by silt carried by the river for many thousands of years. But in this century, the Colorado River has been dammed and diverted by humans, so almost no water (or sediment) reaches the sea anymore. With no more silt or nutrients coming in, the clams have all died, and the silt shoreline is now being eroded by the ocean waves. The clam shells are unearthed by the waves and tossed up on shore. In some places, you can find vast beaches made of nothing but trillions of seashells! These beaches are hundreds of meters wide, meters deep, and many kilometers long. The speaker called this sad story the "Silence of the Clams".