The Importance of a Shell for Crustaceans and Mollusks

There was once a glass lamp that we owned, filled with seashells, that went with nothing in my home. My husband loved it. I hated it. I accidentally, “broke” it. A conchologist, a studier of shells probably would’ve gripped their chest in horror at my little accident, screeching as he watched the former homes of sea creatures crumble to dust on our kitchen floor.

Shells, serve more than a decorative purpose for the creatures that inhabit them, primarily mollusks and crustaceans. These purposes can range from protection, locomotion, defense, structure or feeding.

There are 112,000 species of mollusk, ranging from miniscule snails smaller you’re your pinky to gigantic squid. Mollusks are divided into two different types and their shells look quite different. Their names sound complicated, like most scientific names, but broken down, they aren’t so bad.

First, there’s the Gastropoda, which we’ll call univalves. Univalves, means that these guys only have one side. These are mollusks like snails. Their shell is attached to their body through a muscle and it is used for a couple of things. Primarily it serves as an exoskeleton, but also as protection from predators. It also helps keep the sun from drying them out. It’s made up of several layers of calcium carbonate that’s secreted by a part of their body called the mantle.

An interesting tidbit about their shells is that they are mostly righties. This means that they mostly coil to the right because of their asymmetrical shape. This phenomenon is called chirality.

The second kind of mollusk is called a Bivalva. If a uivalva had only one side, guess how many sides a “bi”valve has? Of course, two. These are clams oysters, and scallops. Their shells are made up of two calcareous valves. The sides are composed of calcite (as in the case of oysters) or calcite and aragonite (aragonite being the inner layer). The outermost layer is a horny organic substance called periostracum.

These mollusks use their shells for the same things as their univalve brothers, but they can also use their shells for locomotion. They can flip them open and closed and dig into the sand. Or, some like the scallop can open and close them rapidly, creating a jet of water behind them and swim. This kind of motion though is very tiring for a mollusk and they cannot do it for long. Their shells also serve as a filter for food, as they open them to let the water in, sifting it for their meals.

The crustacean is an aquatic arthropod. These are lobsters, crabs, crayfish and even barnacles. The main purpose of their shell is that they have no internal skeleton. It’s an exoskeleton, allowing them to be mobile. Their shell also provides them protection against predators.

Their shell, or exoskeleton is made up of a series of segments. The segment behind their head is fused with several ligaments behind it to form the cephalathorax. Each segment beyond that bears a pair of branched limbs. Their exoskeleton must be shed in order for them to grow.

Mollusks and crustaceans, while giving conchologists a great deal to look at and study, also have found a great deal to do with their shells. Their shells come in a wide variety of colors and shapes and while being utilitarian are also beautiful and an example of nature’s bounty.