Lobsters have a hard outer covering, an external skeleton known as an exoskeleton. This specialized structure supports the lobster’s body and enables its movement. Its unique properties and growth mechanism are fundamental to a lobster’s survival in its marine environment.
What is a Lobster’s Shell?
A lobster’s shell is an exoskeleton, an external skeletal system that encases the animal, unlike the internal bones of vertebrates. This complex biocomposite material is primarily composed of chitin, a fibrous carbohydrate polymer, and calcium carbonate minerals. The organic matrix of the exoskeleton is mostly chitin, with a smaller percentage of protein.
Calcium carbonate provides rigidity and strength, existing as amorphous calcium carbonate (ACC) and crystalline calcite. This combination gives the shell its characteristic hardness and structural complexity. The lobster grows this shell as part of its body, rather than secreting and adding to it like a snail’s shell.
Why Lobsters Have Shells
The lobster’s exoskeleton serves several functions, primarily offering protection from the environment and predators. This hard armor acts as a physical barrier against impacts and provides defense against animals. It also helps prevent physical damage from its surroundings on the seafloor.
Beyond protection, the exoskeleton provides structural support for the lobster’s body. It acts as an anchor for muscles and internal organs, necessary for movement like walking and swimming. The shell also prevents desiccation, or drying out, in its aquatic habitat and forms a barrier against pathogens.
How Lobsters Grow Their Shells
Since a lobster’s shell is rigid and does not expand, lobsters must shed their old shell to grow larger, a process called molting or ecdysis. This process is triggered by hormonal changes that soften the existing exoskeleton. The lobster absorbs water, causing its body to swell and put pressure on the old shell, typically causing it to split along a seam between the carapace and tail.
The lobster then pulls itself out of its old shell, emerging with a new, soft, and pliable exoskeleton formed underneath the old one. Immediately after molting, the lobster is in a vulnerable “soft-shell” state, as its new shell offers little protection from predators and is susceptible to infection. During this period, which can take several weeks, the lobster often hides.
The new shell hardens by absorbing minerals, particularly calcium carbonate, from the seawater, a process that can take days to weeks. Lobsters may even consume their old shells to replenish lost calcium and accelerate the hardening of the new shell.
Molting frequency varies throughout a lobster’s life. Younger, faster-growing lobsters molt more often, sometimes multiple times a year. As lobsters age and grow larger, molting events become less frequent; adult males molt about once a year, and adult females once every two years. This continuous molting allows lobsters to increase in size, typically by about 20% in length with each molt.