A shell is an outer structure that serves a variety of protective functions for animals. For invertebrates like snails, it acts as an exoskeleton, housing internal organs and muscles. For land-dwellers like armadillos, it allows for easier navigation through harsh environments. And for soft-bodied creatures like hermit crabs, it’s a necessary supplement for survival.
The external armor comes in many shapes and forms, from the snail's spiral shell to the armadillo’s scaly plates.
Turtles & Tortoises
Turtles and tortoises are the quintessential shelled creatures. Their shell is not merely a external covering, but part of their skeletons, formed from the fusion of many bones. The shell is attached to the spine and rib cage, making it a permanent structure. Plates of keratin called scutes cover the surface.
The shell has three major parts: the carapace, the plastron, and the bridge. The carapace is the upper half that covers the back—the part we usually see. The plastron is the bottom half that covers the underside. The bridge, located on the side of the turtle or tortoise, joins the carapace and plastron together.
Though similar in composition, the shells of turtles and tortoises are not identical. Turtles generally have a flat, lightweight shell suited for swiftly navigating underwater. Tortoises, on the other hand, usually possess a heavier, more dome-shaped shell.
Some turtles can retract their head, tail, and legs inside their shell. These are known as “hidden-necked turtles.” Most have a hinge that enables the carapace and plastron to shut once the turtle has tucked itself in. Others, called “side-necked turtles,” are unable to completely retreat, but can tuck their heads sideways into the shell.
The function of turtle shells may seem obvious: protection from predators. However, this was not always their main role. The earliest turtles formed their broad, ribbed shells primarily for the purpose of burrowing underground more easily. They did this because the South African environment they lived in at the time was becoming too barren and dry for survival.
Modern-day turtle and tortoise shells play a role in self-defense, regulation of body temperature, and storage of important minerals.
Mollusks are soft-bodied invertebrates that usually reside in the sea. Many species of mollusks are shelled, including snails, clams, oysters, chitons, and nautiluses. More than 50,000 varieties of mollusk shells exist.
The mantle—the outer wall of a mollusk’s body—is responsible for the development, maintenance, and repair of the shell. It secretes a tissue of calcium carbonate and proteins that eventually hardens into a protective covering. The formation of the shell typically takes place during the larva stage, and the mantle continues to secrete calcium carbonate as the mollusk grows in size over its lifetime.
Snails are univalves, meaning they possess a one-part shell. This trait is shared with turtles and tortoises. Most snail shells are spiral in shape, but some varieties, such as limpets, have cone-shaped carapaces.
During gestation, snails form a protoconch—the earliest component of their shell. As the snail grows, the shell expands and coils around the protoconch, leaving it at the center of the shell’s whorl.
Snails can retract completely into their shells to guard against predators and, in the case of land-based gastropods, survive extreme heat. Some desert snails have a white-colored shell that reflects sunlight and secretes a mucus coating to prevent dehydration.
The abalone, a type of marine snail, has one of the toughest shells of any other mollusk. The calcium carbonate crystals that make up the shell are bound together with a polymer adhesive that is both hard and elastic, providing them with optimal protection.
Aside from snails, bivalves and chitons are two other common shelled mollusks.
Clams, oysters, mussels, and scallops are all bivalves—mollusks with a two-part shell. The shell halves are hinged together by an elastic ligament, and two adductor muscles allow the shell to open or close as needed.
Chitons are marine mollusks with a shell divided into eight plates. Unlike snails, they cannot completely retreat into their shell. Instead, their valved shell structure allows them to curl into a ball, not unlike the defense mechanism of three-banded armadillos.
Additionally, chitons can harden their plates to latch onto surfaces, preventing predators and strong currents from nabbing them.
Most crustaceans have a hard, calcified exoskeleton, but not an actual shell. The hermit crab is an exception.
Hermit crabs cannot develop their own shells; instead, they seek out and scavenge the abandoned shells of mollusks. Why? Hermit crabs have softer and longer abdomens than their crustacean relatives, so they require more protection.
The soft, curved exoskeleton of a hermit crab allows it to safely fit its body into a spiraled shell. Appendages at the end of the abdomen called uropods help hold the crab securely within the shell.
When retreating into its shell, hermit crabs may use one claw to block the shell’s opening and thus prevent predators from easily pulling them out.
The name “armadillo” translates to “little armored one” in Spanish, an apt description for these unique creatures. They are the only mammals with shells, and their makeup is distinct from that of turtles and mollusks.
Armadillos have a leathery, armor-like shell that covers most of the body. The shell’s top layer is made up of plates of keratin, or scutes, that are fixed in place within the skin. The base of the shell is formed by solid, fossilized bones called osteoderms.
Armadillos are born with soft keratin shells, similar to human fingernails. The shell grows and hardens over time, ossifying into a solid carapace by adulthood.
Though the shell protects a large portion of the body, it leaves one area exposed—the underside. Only one species, the three-banded armadillo, is able to curl itself into a ball and guard this vulnerable area. Others must flee or dig to safety when threatened by predators.
It is believed that armadillo shells initially evolved in response to their subterranean lifestyle. They spend much of their time burrowing and scavenging underground, and their shells act as a safeguard against abrasion.
Armadillos have few natural predators, but their shell is a handy self-defense mechanism for those who do pursue them—including humans. In 2017, a man attempting to kill an armadillo with a handgun ended up in the hospital when the bullet ricocheted off the creature’s shell and hit him instead. A similar incident occurred in 2015 when a bullet from a pistol bounced off an armadillo's back and struck a nearby woman.