A shell is a hard or firm external covering produced by many animals that serves primarily to protect soft tissues, support the body and sometimes assist with locomotion or buoyancy. Shells occur in multiple animal groups and have evolved independently in response to similar ecological pressures. They range from thin organic plates and flexible keratinous shields to thick mineralized casings and composite bony structures.

Composition and structure

The materials and microstructure of shells differ between lineages. Many mollusc shells are composed mainly of calcium carbonate (aragonite or calcite) arranged in layered crystals within an organic matrix. Crustacean exoskeletons combine the polysaccharide chitin with mineral salts. Turtle and tortoise shells are formed from fused ribs and vertebrae overlaid by keratinous scutes. Echinoderms such as sea urchins build tests from interlocking calcite ossicles. These differences influence growth patterns, repair capacity and mechanical properties.

Major groups with shells

Shells appear across diverse groups of animals as examples of convergent form. Typical representatives include molluscs (snails, bivalves, cephalopods with external shells), echinoderms such as sea urchins, various crustaceans (crabs, lobsters), and reptiles like turtles and tortoises. Some mammals, notably armadillos, have developed bony or keratinized armor that functions similarly to a shell. The term often overlaps with 'exoskeleton' but can denote non-segmented or non-molting protective coverings as well; see evolutionary origins for broader context.

Functions and life history

  • Defense against predators and abrasive environments.
  • Protection from desiccation and control of internal microclimate in terrestrial species.
  • Support for muscle attachment and, in some cases, structural integration with the skeleton.
  • Camouflage, signaling and, for some marine organisms, buoyancy regulation.

Shells grow by accretion: new material is secreted at the margins or internally, producing growth lines or rings that can record life history and environmental conditions. In taxa that molt, such as many arthropods, the hard covering is periodically shed and replaced. In contrast, bony shells like those of turtles grow with the animal and may show different patterns of repair after damage.

Evolutionary and human relevance

Shells provide important information for comparative anatomy, paleontology and ecology because their durable components preserve well in the fossil record and record past environments. Humans have used shells as tools, decoration and currency in various cultures, and modern scientists use shell chemistry to infer water chemistry and climate history. Conservation concerns include overharvesting, habitat loss and acidification, which can impair shell formation in sensitive species.