"I thought oysters produce pearls!" is a phrase I hear a lot, especially when I talk about freshwater pearls. It is true, oysters do produce pearls! But, they are not the only species that produce pearls! Mussels, clams and scallops can produce pearls too. So can sea snails!
Oysters are part of a larger phylum called Mollusks. Remember that word from science class? Phylum is a rank of classification in Biology. Mollusks (also knows as molluscs or mollusk) are the largest marine phylum. Almost one-quarter of all marine organisms are known as mollusks. There are also mollusks in freshwater and land environments too!
What makes a mollusk a mollusk? All mollusks have a mantle. The word mantle means cape so think about the mantle as a cape of muscular tissue that covers a mollusks internal organs. What is cool about the mantle (well, to me, a Pearl Girl) is that some mantles have an outer layer of cells that produce calcium carbonate and conchiolin, the essential ingredients that form a shell and a pearl. This does not mean that all mollusks have a hard shell, though. Slugs and octopus are considered mollusks too. So, they are examples of mollusks with a mantle but without a hard shell.
This cloak of a mantle that covers up a mollusks internal organs forms a body cavity. This cavity is another essential part that makes a mollusk a mollusk. In other words, all mollusks have mantles and they all have mantle cavities. This mantle cavity serves different functions in different species. This cavity is usually responsible for breathing, and in most mollusk it is responsible for eating, reproductive functions, excrement and movement.
All mollusks can be divided into different classes. There are bivalves characterized by two valves and a hinge, gastropods (abalone, snails, slugs..) and cephalopods like octopus and squids. All of these classes are invertebrates. Two classes, gastropods and bivalves can produce pearls. We mainly look at bivalves to produce pearls and in that category of bivalves we have clams, mussels, oysters and scallops. They all CAN produce pearls, whether they do or not is a different story.
So, what is a pearl? It is the result of a foreign substance entering the bivalve. Most bivalves spend their days burrowed in mud (it is safe!) or attached to a rock or other hard surface. They keep their shell open up to about 4mm and they suck in water. In the water is nutrients which they digest, they purify the water and expel it again. All day long. Now, what happens when an organism invades their shell or gets sucked into their open valve? Well, most of the time they try to expel the foreign substance. If that does not work, they might die. But, if they survive, they utilize the epithelial cells in their mantle to cover that foreign substance up. And that is a pearl.
Now, just because the pearl is there, that doesn't mean it is a marketable pearl! Pearls are made of the same calcium carbonate the bivalve uses to make its shell. Calcium carbonate is composed of aragonite and calcite. A pearl made of just calcite is considered a non-nacreous pearl. There are some very beautiful non-nacreous pearls. And some very expensive ones too! But, a lot of non-nacreous pearls are called calciferous concretions. Basically, they are like a calcite deposit. There are not anything you want to spend your money on! Put the big bucks on the pearl with aragonite which has beautiful layers of interlocking crystals and has a beautiful reflection of light.They are the truly gorgeous pearls and they can be formed by mussels as well as oyster. I hate to give clams a bad name but they just are not out to produce a pretty pearl! Look at their shells. Pretty but not shimmering with beautiful nacre.
So, to wrap things up, oysters produce pearls but so do other mollusks, both freshwater and saltwater alike. So, when I say a mollusk produced a pearl, you know that could include any of the many many mollusks in the world! Oyster, clam, mussel or scallop alike!