Re: Why do some crabs have hair on their claws? ie: Alaskan King & Kona

Hi Jodi,

Question: Why do some crabs have hairs on their claws?

First of all, it might be useful to review what kind of animals crabs are. Crabs are crustaceans, mostly aquatic members of a larger group of animals called arthropods. Arthropods are the most numerous and successful group of animals, with a huge amount of diversity and exploiting just about every habitable place on earth. As a matter of fact, arthropods, represented by insects, are the kings of the air.

All arthropods are covered by an outer layer, or cuticle, called an exoskeleton. The exoskeleton provides support and protection for the animal's body, similar to the way that a an old-fashioned suit of armor protects a knight. In many arthropods, including the crustaceans, the cuticle is impregnated with minerals and decorated with few to many "hairs" which are technically called setae (pronounced SEE-tee). If you look closely at a crab, you will see that there are setae on all of the legs, not just the claws, and some on the body as well. Here is an example of a particularly hairy crab.

Although these setae look sort of like mammalian hairs, they are completely different. In fact, setae are actually extensions of the cuticle. If you imagine grabbing a small section of your skin with tweezers and pulling it out to form a hairlike thread, you'd have something very like a crustacean seta. Now the cuticle itself is inert and, unlike your skin, not very good at sensing external stimuli such as touch, chemicals, heat/cold, etc. Setae and other structures of the cuticle are responsible for sensing what is going on in the outside world. Setae can detect vibration or touch, and some can detect chemicals in the water. A nerve inside the seta itself transmits the information to the rest of the nervous system, and that's how the crab knows when it's being touched, or when something brushes past it, or whether or not there's food nearby.

One last interesting tidbit of information about crustacean exoskeletons: They do not grow, and thus must be molted as the animal gets bigger. And since the setae are part of the exoskeleton, they too are molted. This means that a crab gets new sensory setae every time it molts its cuticle. Pretty cool, huh?

I hope this answers your question!

Allison J. Gong
Mad Scientist