Crab shells, along with the exoskeletons of lobsters, shrimp, and other crustaceans, are made up primarily of a substance called chitin. In this article, we’ll take a closer look at chitin – what it is, how it works, and why crabs need it.
What is Chitin?
Chitin is a natural polymer – a long chain of similar molecules linked together. Specifically chitin is made up of chains of N-acetylglucosamine, which is a derivative of glucose. The chemical structure of chitin makes it very tough and rigid. This gives it the strength and hardness needed to form the protective exoskeletons of crabs and other crustaceans.
Chitin is the second most abundant natural polymer on Earth after cellulose. It can be found in the cell walls of fungi and the exoskeletons of arthropods – a group that includes insects spiders and crustaceans like crabs.
While cellulose is made up of linked glucose molecules, chitin’s molecular structure uses N-acetylglucosamine instead This difference gives chitin some unique properties that make it well-suited as a structural material for growing shells.
Why Do Crabs Need Chitin Shells?
Crabs, like other arthropods, don’t have an internal skeleton made of bone like we do. Instead, their “skeleton” is on the outside of their bodies in the form of a hard, protective shell. For crabs, this chitin exoskeleton serves several important functions:
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Protection – The hard chitin shell acts as armor, shielding a crab’s soft inner body from predators and environmental hazards like sharp rocks or coral. This protection is vital for a small animal with otherwise soft, vulnerable flesh.
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Support – On the inside, the rigid chitin shell provides attachment points for muscles and tissues, giving the crab structural support and shape. This allows crabs to move, walk, grasp food, and perform other functions.
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Growth – Rather than expanding, a crab’s chitin exoskeleton has to be shed and replaced as the crab grows. After molting its old shell, a soft crab will have space to expand a bit as its new shell hardens.
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Reducing Water Loss – Crabs live in the ocean but still face the threat of dehydration, especially on land. Their impermeable chitin shell helps retain bodily moisture.
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Camouflage – Crab shells come in an array of colors and patterns that help them stay hidden from predators in their environments.
Without their tough chitin exoskeletons, crabs would be defenseless and unable to survive in the wild. Their shells provide the structural support, environmental protection, and other functions they need to thrive in their habitats.
How is Chitin Organized in Crab Shells?
The chitin in crab shells is precisely arranged in alternating layers of chitin chains called α-chitin and proteins called β-chitin. This structure makes their shells strong and flexible while still allowing for growth through periodic molting.
Here’s a look at the layered structure of crab shells:
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The Epidermis – This thin cellular layer produces the shell and connects it to the crab. Pigment cells in the epidermis give crab shells their unique colors.
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The Exocuticle – Made of several horizontal α-chitin chains bonded with protein, this is the hardest, outermost part of the shell. It provides the main protective barrier.
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The Endocuticle – Below the exocuticle, this area has chitin fibers oriented randomly for flexibility. It also contains some protein and mineralization.
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The Membrane Layer – This thin boundary separates the shell from the inner body tissues it supports.
This carefully engineered arrangement allows crab shells to be lightweight yet extremely tough. Their complex structure provides an ideal balance of hardness, durability, and flexibility.
How Do Soft Shell Crabs Fit In?
Soft shell crabs are simply regular crabs that have recently molted their hard exoskeletons and are still covered in a temporary soft shell as their new shell hardens. As they grow, crabs must periodically shed their rigid shells. For a short time after molting, they are extremely vulnerable with only their soft inner membrane and beginnings of a new exoskeleton for protection.
Once a crab molts its old shell, its new one will take time to harden and develop the typical hardness and coloration. This process can take several days, depending on the species. During this time, the crab is called a “soft shell” – and considered a delicacy by some seafood lovers! Eventually their new shell will fully harden and they will no longer be soft shells.
Chitin – Abundant and Useful
With so much chitin available in the discarded shells of harvested crabs and other crustaceans, scientists have been looking for ways to make good use of this abundant natural material. Chitin has shown promise for a wide range of applications:
- Producing chemical derivatives like glucosamine, a popular supplement.
- Serving as a raw material for bioplastics and biodegradable films for food packaging.
- Providing a scaffold material for tissue engineering and medical implants.
- Acting as a coagulant/binder in wastewater treatment processes.
- Improving soil health as a fertilizer or seed treatment.
Researchers continue to find new potential uses for chitin extracted from crustacean shells that would otherwise go to waste. One day, chitin-based products could provide environmentally friendly replacements for plastics, styrofoams, and other items currently made from fossil fuels. Crab shells may be able to provide abundant raw materials for manufacturing more sustainable products.
With its specialized composition and structure, the chitin found in crab shells provides these animals with the armor-like exoskeletons they need to survive. Strong and rigid yet flexible, crab shell chitin perfectly fulfills the requirements of an external supportive framework for soft-bodied crabs. Learning more about this key biological material can give us further insight into the anatomy and lifestyle of crabs along with revealing new possibilities for utilizing crab shells. The ubiquitous natural polymer chitin continues to provide an intriguing and abundantly available material for new discovery.
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Lithium Batteries: The nanostructures of crab shells provide a cheap, renewable template for sulfur and silicon electrode materials
Most Popular in Materials
Credit: Shutterstock
Crab shells usually are just a nuisance that you have to crack and dig through to get the delicious meat inside. But one team of materials scientists thinks the shells could help them fabricate materials for long lasting batteries. The team used the nanostructures found in the crustacean shells as templates to make sulfur and silicon electrode materials for lithium-ion batteries (Nano Lett. 2013, DOI: 10.1021/nl401729r).
Most lithium-ion batteries on the market have lithium cobalt oxide cathodes and carbon anodes. If those batteries used sulfur as the cathode or silicon as the anode instead, the batteries’ theoretical energy storage capacity would be 10 times greater, says Yi Cui, a materials scientist at Stanford University.
Unfortunately, electrodes made from those materials have structural and chemical problems that lead to short battery lives. For example, as the battery discharges and recharges, the electrodes expand and contract as the sulfur or silicon reacts with lithium ions in the battery electrolyte. This repeated expansion and contraction cracks the electrodes.
To counter this problem, some researchers, including Cui, have tried to contain the sulfur or silicon inside nanostructures such as carbon nanofibers (Nano Lett. 2011, DOI: 10.1021/nl2027684). These structures have enough room for the electrode materials to expand and contract freely, Cui says. Researchers make these nanofibers by growing them on templates, often made from aluminum oxide.
Cui and his team thought crab shells could serve as an inexpensive, environmentally friendly alternative to those templates. Previous studies showed that the shells consist of about 70-nm-wide calcium carbonate nanochannels—just the right size to contain electrode materials. Every year, the food industry throws away about 500,000 tons of crab shells, making the crustacean waste a low-cost, renewable source of these templates. Crab Shell Secrets
Credit: Nano Lett.
To test their idea, Cui and his group bought stone crabs from a local Chinese grocery store. After collecting the crabs’ shells, they prepared the templates by first burning off organic matter on the shells and then grinding the calcium carbonate shells into a powder. Next they mixed the powder in a solution of dopamine to coat the calcium carbonate in a dopamine polymer. Heating the powder at 800 °C turned the polymer into a layer of carbon. Finally, using dilute hydrochloric acid, the scientists etched away the calcium carbonate shell. What remained were hollow, 65-nm-wide carbon nanofibers grouped into bunches, Cui says.
The researchers stuffed these nanostructures with sulfur by heating them with the element under argon, or filled them with silicon via chemical vapor deposition. They then made electrochemical devices to test each material’s performance as an electrode.
As a test of the electrodes’ lifetime, the team put them through 200 charge-discharge cycles. After those cycles, 60% of the energy storage capacity remained in the sulfur-containing electrode, and 95% remained in the silicon one. Cell phone battery electrodes retain about 80% of their capacity after 500 cycles, Cui says. However, he points out that it’s difficult to make a full comparison with commercial electrodes because the test devices were not full batteries.
A cheap, renewable template for high-capacity electrode materials is an important advance, says Galen D. Stucky, a materials scientist at the University of California, Santa Barbara. But he says that the researchers’ materials don’t retain enough of their high energy-capacity over time to be ready for practical applications, such as use in a hybrid car.
Cui says his group is now working on other biological templates, such as ones made from rice husks, and trying to improve the ability of the electrodes to store charge. Chemical & Engineering News ISSN 0009-2347 Copyright © 2025 American Chemical Society
What’s Inside a Hermit Crab Shell?
FAQ
What are crab shells made out of?
Their shells are made of chitin, proteins, and calcium carbonate.
Is crab shell made of keratin?
The base material in a crab’s shell is chitin which is a polysaccharide compound that is functionally but not chemically similar to the keratin found in …Feb 17, 2018
Are crab shells healthy to eat?
Crab shells are a good source of nitrogen and phosphorus but contain little or no potassium, which means they have a NPK of (4-3-0), (N – nitrogen, P – phosphorus, K- potassium, the numbers you see on fertilizer packages in the US). Crab and crustacean meals are also good sources of calcium and magnesium.
How often do crabs get new shells?
A crab will molt about six times during each of its first two years of life. Once a Dungeness crab reaches about 4 inches in width, they are a mature adult and typically molt only once per year and increase is size from 1 to 1 1/3 inches in width.