Crabs are instantly recognizable by their unique sideways walking motion. While most animals walk forwards, crabs have evolved to move in this peculiar fashion. But why do crabs walk sideways instead of forwards like other creatures? There are some fascinating biological reasons behind this quirky form of locomotion.
The Anatomy of a Crab Dictates Sideways Movement
Crabs belong to a group of animals called decapod crustaceans. This includes shrimps, lobsters and crayfish. They all share a common body shape and anatomy. Decapods have five pairs of legs for walking as well as a large muscular tail for swimming. The front pair of legs is modified into large claws used for defense and feeding.
Early decapod ancestors of crabs walked forwards just like lobsters do today. However, over millions of years of evolution, crabs underwent some key changes:
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Their bodies became flatter and wider, giving them a distinct crab-like shape. This allows them to squeeze into narrow spaces and makes them faster burrowers.
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Their tails shrunk dramatically into a tiny flap on their underside. This reduces weight and frees up energy for walking.
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Their legs moved into positions along the sides of their bodies. This wider stance increases stability on the seafloor.
These adaptations allow crabs to move effectively in their coastal and seafloor environments. But they also restrict the motion of their legs to sideways only. The sideways alignment of their legs, combined with the lack of tail for propulsion, means crabs have no choice but to walk sideways.
The Mechanics of Sideways Movement
Crabs move using a simple but efficient system. Their legs are attached to their bodies by ball and socket joints. This allows motion in every direction. Each leg has two segments connected by a hinge joint, similar to our knees. But while our knees bend forwards, a crab’s leg joints only bend sideways.
When walking, crabs lift and lower their legs in coordinated waves. They move their back legs on one side first, then the front legs on the same side. The legs on the opposite side move in the next wave. This alternating gait gives them stability while allowing for rapid movement.
Crabs can actually shuffle forwards slowly if needed. But sideways is their fastest and most efficient option. Some species, like ghost crabs, can sprint at up to 10 mph! Their legs are specialized for this type of motion.
The Advantages of Sideways Walking
At first glance, moving sideways seems inefficient compared to walking forwards. But this method of walking offers some key benefits for crabs:
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Speed – The sideways leg motion is quick and coordinated. Crabs can dart out of harm’s way faster than if they walked forwards.
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Adaptability – Walking sideways allows crabs to move well on land as well as underwater. They can crawl into tight spaces and burrows.
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Stability – The wide stance provides more stability and balance than walking forwards. This helps them navigate uneven and slippery terrain.
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Vision – With eyes positioned on stalks that can swivel 360 degrees, sideways motion gives crabs a panoramic field of view to spot danger.
So while it may look strange to our eyes, walking sideways is the best way for crabs to move around effectively in their environments. After hundreds of millions of years, they’ve perfected the sideways scuttle.
A Few Exceptions: Crabs That Walk Forwards
While most true crabs move sideways, there are a few exceptions:
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Hermit crabs – Since they carry heavy coiled shells on their backs for protection, hermit crabs evolved a forward-walking motion to help balance the extra weight.
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Spider crabs – These large crabs use the long spiny tips on their legs to help climb vertical rock walls. Forward walking gives them better leverage.
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Fiddler crabs – Males have one giant claw they use to attract females and defend burrows. Walking forwards helps counterbalance the heavy claw.
So in species where sideways walking has some disadvantage, forward locomotion is retained. But the vast majority of crabs are fully adapted for quick and stable sideways scuttling.
The Next Time You See a Crab, Appreciate Its Sideways Genius!
Crabs are marvelously adapted for life in intertidal zones, estuaries, and seafloors worldwide. Their quirky method of walking sideways is an ingenious evolutionary solution that serves them well in their environments. The next time you see a crab sidling along a beach, take a moment to appreciate the sideways genius!
Have You Ever Wondered…
- Why do crabs walk sideways?
- Do all crabs walk sideways?
- How do crab knees differ from human knees?
Today’s Wonder of the Day was inspired by Jesse. Jesse Wonders, “Why do crabs walk sideways” Thanks for WONDERing with us, Jesse!
We were making a sand castle on the Wonderopolis beach the other day when we saw a group of crabs gathering nearby. The group appeared to consist of an adult crab and several younger crabs. Our curiosity got the best of us, so we decided to eavesdrop to try to hear what they were talking about.
Kid Crab: Mrs. Crab? What are we going to learn about in class today?
Mrs. Crab: Well, Patty, Im so glad you asked. Today, were going to learn why human beings walk frontways.
Kid Crab: Thats awesome, Mrs. Crab! Ive always WONDERed about that. They look so funny when they walk down the beach like that!
Apparently, we stumbled upon a class of crabs with their teacher about to learn more about human beings and the way we move. From their perspective, we must look as funny to crabs as they do to us. Lets take a closer look at why crabs move the way they do.
If youve ever seen crabs moving along the shoreline, youve probably noticed that they move differently than human beings. Instead of walking forward on two legs, they move quickly sideways in a flurry with their multiple legs.
Crabs move sideways for the same reason humans move forward: thats how their bodies were built to move. Human beings have knees that bend forward. Therefore, its only natural that we would move forward with each step we take.
Crabs, on the other hand, have multiple legs and theyre located on the sides of their bodies. In addition, their leg joints bend outward. Given this particular design of their bodies, its most efficient and natural for crabs to move sideways.
Not all crabs move sideways, however. There are a few species of crabs that can move forward, because their bodies are shaped in a way that makes them longer than they are wide. Examples of these crabs include raninids, Libinia emarginata, and Mictyris platycheles.
In addition, most crabs have a front pair of legs that are mainly used for defense and grasping food. These front legs can be used to move forward, but doing so is not very efficient and doesnt allow the crab to move quickly. Thus, most crabs move sideways since they can move quickly and most efficiently that way.
We’d like to thank:
Suzanne and Lindsey from MD for contributing questions about today’s Wonder topic!
Keep WONDERing with us!
Did you get it?
Tomorrow’s Wonder of the Day is a real breath of fresh air!
Feeling crabby? We hope not! Ask a friend or family member to help you explore the following activities:
- Instead of crabby, we hope youre feeling crafty! Jump online to check out 12 Crab Crafts for Kids To Make. Choose one or two crafts to try at home. Be sure to check the instructions and obtain any supplies you need before you get started. Have fun!
- Could you survive as a crab? Give it a try today! Head outside and practice moving sideways. Ask a friend or family member to tell you to move to different, random places. The catch? You can only move sideways to get there! How much longer does it take you moving sideways compared to moving normally? Check and see. Use a stopwatch to measure how long it takes you to walk across the room sideways and then the same distance normally.
- Have you ever eaten a crab? Seafood lovers all over the world prize the crab for its delicious meat. Ask an adult friend or family member to take you to a local store that sells seafood. If possible, purchase some crab to use to make one of the following recipes: + Crab Cakes + Garlic Crab Legs + King Crab Appetizers
Why do Crabs walk Sideways? | Mocomi Kids
FAQ
Why do crabs only go sideways?
Crabs have four pairs of walking legs, plus a set of claws. In walking sideways, the crab avoids the danger of tripping over its own feet.Jun 23, 2022
Why do crabs walk sideways on Reddit?
The tides generally go east and west, so the crabs developed the ability to walk sideways against the tides, then evolved to where it is their standard …Oct 11, 2024
Why do crabs walk sideways?
In addition, crabs have a specialized joint known as the lateral flexor that enables them to move their legs in a coordinated manner. The sideways gait of crabs is so distinctive that it has even been incorporated into popular culture, with the term “crab walk” often used to describe a similar sideways motion in humans.
Do crabs walk backwards?
Yes, crabs are capable of both forward and backward movement. While their natural preference is to walk sideways, they can easily switch direction and scuttle in reverse if necessary. This ability to move in multiple directions adds to their remarkable agility. Do all crabs walk the same way?
How do crabs walk?
Some crabs, like hermit crabs, have a unique walking technique where they carry a protective shell on their backs. These crabs walk forward, while keeping their soft abdomen protected within the shell. Other crab species, such as fiddler crabs, have specialized claws that allow them to walk forward with ease, albeit with a slight sideways tilt.