Lobsters are crustaceans and members of the phylum Arthropoda, meaning they do not possess scales. Their outer covering is a hard, rigid structure known as an exoskeleton. This specialized outer shell provides both protection and structural support for the animal’s soft tissues.
The Anatomy of a Lobster’s Outer Layer
The lobster’s protective shell is a complex biocomposite material. It is primarily composed of the polysaccharide chitin embedded within a protein matrix. This organic structure is hardened by biomineralization, where mineral crystals are deposited. Calcium carbonate is the main mineral used for this reinforcement, providing the shell with its characteristic rigidity and resistance to compression.
The largest part of this exoskeleton is the carapace, a single, shield-like structure covering the lobster’s cephalothorax (the fused head and mid-body section). While the shell is generally thick, the exoskeleton between the body segments, such as the joints and the abdomen, is thinner and more flexible. This enables the necessary movement for walking, swimming, and flexing the tail.
The Process of Molting
Since the exoskeleton is a hard case, it does not grow with the lobster’s internal body tissues. To increase in size, a lobster must periodically shed its old shell in a process known as molting. This cycle is regulated by hormones, which signal the growth of a new, soft cuticle layer underneath the rigid exoskeleton.
The lobster prepares for molting by absorbing water, which increases its internal hydrostatic pressure. This pressure causes the old shell to split, typically along a seam between the carapace and the abdomen. The lobster then carefully backs out of its entire old casing, including the lining of the gills and stomach. Shedding the shell takes about 30 minutes and reveals the new shell underneath, which is extremely soft.
Immediately after molting, the lobster is in a vulnerable “soft shell” state. It absorbs more water to rapidly inflate the new shell to a larger size. This allows room for tissue growth before the new shell begins to harden, a process that takes days to weeks as calcium carbonate is redeposited. Younger lobsters may molt multiple times per year, while older lobsters may only molt once every one or two years.
Defining Scales and Exoskeletons
The lobster’s outer covering is structurally and biologically distinct from scales found on other animals. Scales are small, overlapping plates that grow out of the skin (dermis) of vertebrates like fish and reptiles. These scales are generally made of keratin or bone, and they grow continuously with the animal.
In contrast, the lobster’s exoskeleton is a continuous, external skeleton that encases the entire body. It is a defining feature of the arthropod phylum, providing leverage for muscle attachment and protection from predators. Unlike scales, the exoskeleton must be shed periodically to accommodate growth.