11 November 2005

Snail shells are made of this

Yesterday, an inquisitive mind prowling the Internet using the search string "what are land snail shells made of" ended up on this blog. I don't think that question was specifically addressed on this blog before. Therefore, we must answer it now.

Land snail shells are made of calcium carbonate. Well, that's the quick and simple answer. In reality, however, nothing that evolution has ever created can be explained away so simplistically. And snail shells are no exception. So, here is a slightly more detailed answer.

Snail shells are indeed made of mostly calcium carbonate, but that's not all they contain. They also contain small amounts of protein. Basically, the shell consists of calcium carbonate crystals organized within a matrix of protein. But the reality is still more complicated than that. Calcium carbonate crystallizes in 2 principal forms, aragonite and calcite, and what form is present in a particular shell may depend on several factors. For example, the crystal type in the shell of the land snail Helix pomatia is normally aragonite, but calcite has also been found in repaired areas of shell1. The shells of many marine snails and bivalves normally contain calcite.

Moreover, the shells of many aquatic as well as land snails are covered on the outside by a hard, skin-like layer of protein that is called the periostracum. Geerat Vermeij in his book A Natural History of Shells2 has a discussion on how the periostracum of marine snails, sometimes growing quite thick, helps protect the underlying shell from dissolving, especially in cold water in which the solubility of calcium carbonate increases.

Here is a simple way to demonstrate that the composition of snail shells is mostly calcium carbonate. When calcium carbonate (CaCO3) dissolves in any common acid, carbon dioxide (CO2) is generated. So, for example, the dissolution of CaCO3 in acetic acid (CH3COOH) can be represented as follows.

CaCO3 + 2CH3COOH ----> Ca(CH3COO)2 + H2O + CO2

Calcium acetate (Ca(CH3COO)2) is soluble in water and that's why a shell placed in acetic acid will eventually completely dissolve away. What happens to carbon dioxide? It bubbles out of the solution. And that's how one tells—by noticing the bubbles forming on the surface of the shell—that the shell is made of calcium carbonate.

In fact, a geologist's standard test for calcium carbonate is to place a drop of dilute hydrochloric acid (HCl) on a rock suspected of containing calcium carbonate (for example, limestone or marble or even a conglomerate containing calcium carbonate besides other minerals). If bubbles form in the drop of acid, the rock contains calcium carbonate.

But don't take my word for it. Try it yourself and be amazed. If you have a shell, a piece of marble or a limestone pebble from your yard, put a drop of vinegar, which is about 5% acetic acid, on it and look for bubbles. Moreoever, if you have powderized limestone and if you drop a bunch of it in a glass of vinegar, you will get a violent reaction with lots and lots of bubbles. Do it in your parents' kitchen, just don't tell them that you read it on Snail's Tales.


1. Saleuddin, A.S.M. & Wilbur, K. M. 1969. Shell regeneration in Helix pomatia. Canadian Journal of Zoology 47:51-53.
2. Vermeij, G. J. 1993. A Natural History of Shells. Princeton University Press.

11 comments:

Anonymous said...

it helps alot but how do the snails acctually make the shells?

Anonymous said...

Please - we too wish to know about how the snail makes its shell. :)

Anonymous said...

thank you so much for writing this! i'm basing part of my research paper on snail shells and this helps fabulously!!!

Anonymous said...

I am growing some fresh water Apple snails (Ampullariidae) and want to provide the necessary minerals. What puzzels me is how you can be sure that Calcium Carbonate is the only Carbonate that fizzes in an acid. Wouldn't Magnesium Carbonate also? I can test roughly for general hardness and provide hard water but I believe this water could have a variety of alkaline-earth carbonates. If the shells are only calcium componds then I would be helpful to me to know that for sure.

AYDIN ÖRSTAN said...

The acid test demonstrates that the snail shell is a carbonate salt. To demonstrate that the cation is indeed calcium as opposed to magnesium, one needs to do more sophisticated tests.

Bruce Berman said...

I think that your searcher may be one of my students. Thanks for this, Aydin.

Anonymous said...

Heyy..i was wondering if the snail's shell was considered to be alive or an organism?? thanksss

Anonymous said...

hi so how do you get calcium from snails' shell for your experiment?

Anonymous said...

How can extract calcium from a snail shell taking into account that the shell contains a compound of calcium

Anonymous said...

as a geologist I was asked this question by my child and answered calcium carbonate, but then I thought so what happens in areas where soils are very low in CaCo3? Do you get more snails in chalk lands and fewer in sandstone areas?

AYDIN ÖRSTAN said...

Yes, as a general rule limestone areas have more snail species & more shells than non-limestone areas. In places where there is no limestone snails get their calcium from the plants they eat & also gnaw on each other's shells even while the shell's occupant is still in it.