1. Crab Leg Joints Bend in Only One Direction
Crab legs have joints similar to human elbows. A key feature is that these joints only bend in one direction [1]. One explanation notes that a crab’s body is wide and flat. This shape limits the range of motion where the legs attach to the body, which corresponds to the shoulders in humans [2]. The joints act like simple hinges, bending only sideways [2]. The legs used for walking are arranged with four walking legs on each side of the body. Because crabs also have large claws, moving forward seems difficult [1].
2. Why Crabs Can Walk Forward but Move Faster Sideways
Crabs can walk forward, but only slowly. However, using the second joint of the leg to walk sideways allows them to move much faster [2]. According to a children’s science guide, crab legs are lined up along the sides of the body, and the spaces between a crab’s legs are very narrow [3]. If a crab tries to go forward, its legs bump into each other. When moving sideways, gaps open up between the legs, allowing for long, fast strides [3]. It is like trying to walk forward in a crowded train without bumping shoulders, though real crab legs move in a more complex way.
3. Spider Crabs and Box Crabs Can Walk Forward
Not all crabs follow this rule. Spider crabs and box crabs have thin legs with extra room at the base, and their shells are not angular, so they can walk forward [1]. Other species, such as ark shells and box crabs, move backward [1]. The Japanese giant spider crab is known for using its long legs to walk forward, backward, and sideways [1].
4. Soldier Crabs Walk Forward Across Mudflats
Among crabs that mostly walk sideways, the soldier crab is distinct because it walks forward [4]. When they walk in groups across mudflats during low tide, their formation looks like a military column, which is why they are called "soldier crabs" in English [4]. Scientists do not yet know exactly why this crab walks forward [4].
5. A Nagasaki University Study of How 50 Crab Species Walk
A research team from Nagasaki University observed how 50 species of crabs walk. They reported that 35 species walk sideways and 15 walk forward [5]. This means 70% walk sideways and 30% walk forward. Using genes from 344 crab species, the team performed a "phylogenetic analysis" to check how closely related the species are. They estimated that sideways walking evolved once from an ancestor that walked forward, about 200 million years ago, at the start of the Jurassic period [5]. NHK News also reported that it is highly likely this happened around 200 million years ago [6]. This study was released in April 2026 as a preprint, which is a paper that has not yet completed peer review. Therefore, the conclusion is not yet final [6].
6. Possible Benefits of Walking Sideways for Escaping Predators
The research team suggests that walking sideways might help crabs escape predators quickly to the side. It may also make it harder for predators to predict which way the crab will run [5]. The team also proposed that this new way of moving might have helped crabs split into many different species [5]. The children’s science guide also states that it is thought to have evolved to help crabs escape enemies [3]. All of these ideas are hypotheses using words like "might" or "possible." We cannot decide on just one reason for why crabs walk sideways.
7. Try Bending Your Elbow to Understand Crab Leg Joints
At home, you can slowly bend and straighten your own elbow to see which way it moves. Do not force it to bend the other way. Crab leg joints work on a similar system, bending only in a set direction [1]. If you visit an aquarium or the beach, watch where the crab’s legs bend and which direction it walks. When exploring mudflats or rocky shores, watch your step and go with an adult.