Hermit crabs are sometimes called false crabs or Anomura, and the likes of the common shore crab are known as true crabs or Brachyura. There are numerous differences between these animals in how they look and behave.
Unlike hermit crabs, who need to locate a shell, true crabs are born with a rigid exoskeleton that covers their entire body. Despite this, hermit crabs have a longer lifespan than some true crabs if they can avoid predators or capture and are usually more docile.
Both species have 10 legs, but those of a hermit crab bend back and forth, while true crabs shuffle from side to side. True crabs will also have two claws, of chelipeds, of equal size, and Hermit crabs have one large claw and one smaller.
Despite their name, hermit crabs aren’t technically crabs. In terms of appearance and biology, hermit crabs are closer to lobsters. Hermit crabs and true crabs rarely live together in the wild. An aggressive species of true crab may prey on hermit crabs.
Are Hermit Crabs Really Crabs?
Hermit crabs and true crabs are decapod crustaceans but are very different animals. There are seven core variations between hermit crabs and true crabs in terms of appearance and lifestyle:
Shells
The biggest difference between hermit crabs and true crabs is the presence of a shell.
Hermit crabs have a hard exoskeleton under their abdomen but a soft exoskeleton over the back. This means they must track down a vessel to use as a shell to protect themselves.
True crabs are born with hard exoskeletons over their stomach and back, so they boast an organic shell. The shell that covers a true crab’s back is twice as tough as the exoskeleton that coats its abdomen.
Legs
Both hermit crabs and true crabs have ten legs, including claws. These claws are chelipeds, while the remaining eight legs are called pereiopods.
The biology of these legs differs between the species. Hermit crabs have knee joints that bend forward and backward. This means that hermit crabs prefer to walk in a straight line, ideally moving forwards.
True crabs scuttle sideways to cover terrain because their knees bend in this direction. A true crab can walk forwards, but it’s a slow and laborious process that could result in tripping over its own feet. Moving sideways is a faster way to travel.
As per American Zoologist, true crabs can regrow lost limbs, much like hermit crabs. This ensures that a crab can shed a leg while fleeing a predator or escaping a difficult scenario and continue to live safely.
Claws
Hermit and true crabs have two prominent claws, or chelipeds, at the front of their body. In a hermit crab, one of these claws will be much larger than the other. True crabs have identical claw sizes, except for the male fiddler crab, which has one prominent cheliped.
For hermit crabs, these varying claws fulfill different purposes. The large claw is used for self-defense, pinching a threat if fleeing and hiding is not an option. This large claw can also be beneficial for climbing and cutting food morsels into smaller portions.
The small claw of a hermit crab is most useful while the animal is eating. Once food has been broken down into an appropriate size, the small claw is used to guide it into a hermit crab’s mouth.
True crabs will also use their claws for eating. Unlike hermit crabs, which are strictly scavengers, some true crabs will actively hunt living prey, including snails, mussels, and even small hermit crabs. The large dual claws of a true crab can be used to incapacitate prey and break open any protective barrier.
Eating and Digestion
True crabs are likelier to stalk live prey than hermit crabs.
Both species will usually scavenge, and both are omnivorous by nature. Eating and digesting also differ between hermit crabs and true crabs.
Hermit crabs don’t have teeth. Food is cut into morsels and guided toward the mouth. Small mouthparts, known as the maxillipeds, take in this food and pass it to the gut for digestion.
True crabs have teeth, though not in their mouth, and some species boast small ridges and spines that resemble teeth on their shells. As per Marine Organisms as Model Systems in Biology and Medicine, they’re predominantly used as a defense mechanism rather than for chewing.
Other species of crab have teeth in their stomach. After ingesting food, the crab contracts its stomach, causing these teeth to grind against each other. This means that morsels of food found in the gut are mashed up and easily digested.
Molting
Hermit crabs and true crabs both undergo a periodic molting process. In both instances, they’ll bury themselves under sand or a form of substrate and won’t wish to be disturbed. Molting is an intensely personal and potentially complicated process.
A molting hermit crab will leave its shell to molt. The hermit crab innately understands that molting will increase its body mass, and the shell will likely no longer fit. This leaves the hermit crab vulnerable until it finds a new, appropriately sized shell.
True crabs face a slightly different challenge when molting. As the crab increases in size, the shell will no longer fit. Unfortunately, unlike a hermit crab, a true crab can’t evacuate its shell and choose a replacement. The new shell must be grown.
Before molting, a true crab will absorb as much calcium as possible from a shell. Enzymes are also released to loosen the shell from the body. The crab gradually crawls out of the shell, a few millimeters at a time. This process can take weeks.
While this occurs, the crab is steadily growing into a new shell. Initially, this will be as thin as paper. It hardens with time, and the crab will wander into the ocean just before completing a molt.
Absorbing seawater makes the crab expand and swell. The old shell will eventually open up like a lid on a kitchen trashcan. The crab can step out of its old shell and move freely again. The new exoskeleton quickly toughens up and is fully hard within a month.
Temperament
Neither hermit crabs nor true crabs are likely to pick a fight with humans willingly.
Both would rather stay out of your way and not interact. If you disturb and upset either species in the wild, their reactions will likely differ slightly.
Hermit crabs would always rather hide or flee than fight. They’ll only pinch as a last resort and usually seize the opportunity to retreat.
An adult hermit crab must pinch firmly to break the skin, and the pinch of most hermit crabs isn’t severe. According to PLoS One, an exception is the coconut crab, which has a pinching force of 3,300 newtons.
A true crab would rather stay away from humans. If you get too close, you may receive a light warning pinch. If the crab feels cornered or threatened, it’ll pinch and fight with all its worth, refusing to let go.
Life Expectancy
Hermit crabs have a reputation for being short-lived animals. However, a wild hermit crab left to live its days on the beach could live as long as 30 years. Sadly, the twin stresses of captivity and substandard husbandry end many hermit crab lives prematurely.
Of course, the natural world is fraught with danger for hermit crabs, and the same applies to true crabs. While true crabs are larger and tougher than hermit crabs, they’re just as likely to fall victim to wild predators and potentially caught by human fishing expeditions.
If a true crab avoids nature’s pitfalls, life expectancy varies wildly depending on the species. The blue crab has a lifespan of 4 years, while the spider crab could live for up to 100 years.
True crabs and hermit crabs share a name but little else. If you’re keen to take in the wildlife found on the beach, understand the fundamental differences between these animals.
