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Name: Rachel
Status: student
Grade: 9-12
Location: IL
Date: August 2007

Question:
My family and I collected some seashells on vacation, and when we were going through them we noticed that some of them had a ittle hole at the top and some of them did not. What is the reason or this? What is the little hole for?



Replies:
Hello Rachel,

My wife and I used to collect shells while living near the ocean. We had exactly the same question. The holes often looked so very deliberate and circular that we figured there had to be a reason.

It turns out that the hole has a definite purpose, but it's not for the benefit of the creature living inside the shell. Instead it's usually drilled there by something else looking to eat the animal inside the shell. I think there are several species of mollusks and snails that do this in order to feed.

best wishes,

Michael S. Pierce
Materials Science Division
Argonne National Laboratory


One mechanism is the rubbing of the shell against sand. Shells get rubbed against sand as they are washed up on shore. Remember, in the wild, the shell of a living bivalve is rough. After it dies, it's polished by the grinding action of the sand. Due to the shell's shape, the top and bottom get ground down first. As the bottom (open end) gets ground, it just gets shorter. However, since the shell is hollow, when the top gets ground down, it can make a small hole in the top. Take a whole shell and grind it down on a flat surface -- see how a hole can form? If you look, you can also find 'rings' or 'c-shapes' of shell on the beach too. These are bits of shell where the top and bottom have been ground down substantially. There may be other mechanisms as well.

Hope this helps,

Burr



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