Why do shells form in different shapes? The environment and the biology of each living organism has caused their evolution to go in a slightly different direction.
Most of the creatures that live in the sea and make shells are mollusks. They are soft-bodied creatures and use their shells as an exoskeleton. It gives them shape, allows them to attach to things, and gives them protection from predators and general wear and tear. The shell is far stronger than the mollusk that lives inside it, which is why we find so many sea shells on the beach. In fact, some beaches are made almost entirely of crushed sea shells.
Mollusk shells are different to the shell of a tortoise because that is a part of the tortoise’s skeleton while the mollusk shell is secreted from within the mollusk. Mollusk shells are made up of calcium carbonate and a small amount of protein. They are made of calcium, just like our bones, except our bones contain calcium phosphate. They are different to bones in other ways as well. Bones are made of cells and they have nerves and a blood supply. If bones break, they can regrow at any point and they are constantly being added to and reformed. Mollusk shells do not have cells and they grow only from one edge. They can repair themselves but the process is very different.
Mollusks have an area of their body called the mantle. This is the place where the shell is grown and it is where the organism is connected to the shell. Snails, for example, might look like they are loose inside their shell, but they are connected to it by the mantle, which is at the opening to the shell. The mollusk secretes proteins and crystals of calcium carbonate, which harden to form the shell. The proteins and calcium are always released from the same place and the shell grows from the bottom up in the same way that our fingernails do. The shells have three layers. The outer layer is called the proteinaceous periosteum. It is a layer of dark protein on the edge of the shell that is very hard and protects the edge of the shell as it grows. It helps the shell keep its structure and is a frame that further layers of shell can be built on. We very rarely see this because it rots off the shell after the organism dies. The next layer is called the prismatic layer. This is the middle layer, and it has a very hard calcium carbonate structure. It is white and chalky. The last layer is the inner layer, and it is made of nacre. Nacre is the material that gives the inside of shells their pearly appearance. Nacre is very strong and elastic, making the shell strong and less likely to break.
So, why do shells form in different shapes? There are many different shapes of shells, but they are divided into bivalves and snail type shells. The snail type shells are the ones that organisms live inside, and bivalves are hinged shells that the organism can open and shut if it needs to. However, they all have something in common and that is their patterns. Many creatures that have an exoskeleton have to shed it every time they get larger and then grow a new one. Shellfish can’t do that. When they grow, they add another layer to their shell. If you look at a conch shell, each spiral of it is a stage in the conch’s life. Every time it grows, its body produces a larger piece of shell at the mantle, pushing the smaller piece further out. This carries on until the conch finishes growing. Bivalve shellfish, like mussels, have the same pattern, but it is circular and on the shell. When mussels grow bigger, they add another layer to the shell. With each extra layer, the shell gets thicker and stronger as well.
The shells of all shellfish are different because of a huge number of factors in their environment. The kind of water they live in and the available minerals will affect the color of the shell. The amount and the speed at which they grow will affect the size and the number of sections on their shell. The temperature in the water will affect the size and the shape of the shell. Whether they float in the sea, bury themselves in the sand at the bottom of the sea, or attach themselves to a rock will affect the design on the outside of the shell. And how they avoid, hide from, or escape from predators will affect the shape of their shell. And this is what I learned today.
Photo by Karolina Grabowska: https://www.pexels.com/photo/close-up-of-shells-and-starfish-6825476/
Sources
https://en.wikipedia.org/wiki/Mollusc_shell
https://www.scientificamerican.com/article/how-are-seashells-created/
https://www.scientificamerican.com/article/how-seashells-take-shape/
https://www.science.org.au/curious/earth-environment/sea-shells