Crabs, like all crustaceans, appear to be covered in fine hair, but these structures are biologically distinct from the keratin-based hair found on mammals. Crabs do not possess true hair. They are instead covered in specialized, hair-like appendages that are extensions of their hard outer shell. This superficial resemblance is a product of convergent evolution, where two unrelated organisms evolve similar traits.
The Structure Crabs Possess
The hair-like structures on crabs are scientifically termed setae. These structures are not grown from follicles beneath the skin like human hair, but are actual outgrowths of the crab’s exoskeleton, or cuticle. Setae are primarily composed of chitin, a tough, nitrogen-containing polysaccharide that forms the bulk of the arthropod shell.
The structure of a seta is essentially a hollow, flexible extension of the rigid, mineralized exoskeleton. A nerve cell is typically housed within the body of the crab, connected to the base of the seta to transmit information. Since these structures are part of the outer shell, they are shed and regenerated every time the crab molts.
How Crabs Use These Structures
The primary purpose of setae is to serve as a sensory interface between the crab and its aquatic environment. These structures are highly specialized and can be broadly categorized into mechano- and chemosensory types, acting like tiny biological antennae. Mechanosensory setae detect water currents, vibrations, and physical contact, allowing the crab to orient itself and sense predators or prey moving nearby.
The chemosensory setae, which are often thin-walled and tubular, function similarly to taste and smell receptors. They are especially abundant on the mouthparts and antennae, detecting chemical signals in the water to help the crab locate food sources. Setae also perform numerous mechanical functions, such as aiding in grooming, filtering food particles, and assisting with grip and locomotion. Specialized hooked setae on decorator crabs, for example, allow them to attach camouflage materials like algae to their shell.
Setae Versus Mammalian Hair
Mammalian hair is made of keratin, a fibrous protein, and is produced by living, cellular follicles embedded deep in the skin. In contrast, crab setae are made of the carbohydrate chitin and are non-cellular extensions of the non-living exoskeleton.
This difference means that hair is a soft tissue appendage, while a seta is a rigid, external projection of the skeleton. While hair in mammals is primarily for insulation and display, setae in crabs are focused on sensory perception and various mechanical tasks. The visual similarity is simply an example of nature utilizing a bristle-like form to interact with the external world.