5 Fun Facts About Crabs

Crabs are crustaceans, marine animals which have a thick exoskeleton made of a chemical called chitin (which is chemically derived from glucose). Crabs belong to the class Malacostraca, which means “soft shelled animal” and to the order Decapoda, which mostly includes marine crustaceans (like lobster, shrimp and prawn) that scavenge for food, as opposed to hunting them. This makes crabs soft-shelled scavengers.

Here are five fun facts about them: 

1. There are two types of crabs in the world – true crabs and false crabs – classified so because of their differing physiology. True crabs have the traditional body structure of a crab – a short and shallow abdomen curled underneath the shell and 4 pairs of legs excluding the pincers. False crabs on the other hand, look a little like crabs, but not completely. They have longer abdomens and less than 4 pairs of legs. True crabs include spider crab, blue crab and ghost crab. False crabs include king crab, hermit crab and porcelain crab. There are a total of 5000 crabs in the world – 4500 true crabs and 500 false crabs. 
2. The largest crab in the world is the Japanese Spider Crab, which measures 13 feet or almost 4 meters from one end of the body to another. In comparison are the Coral Gall crab, Pea crab, Marsh Fiddler crab and Flattop crab – all of which measure in at a teeny-tiny half an inch at adulthood. If you kept 4.5 standard sized mail boxes one-on-top-of-the-other on one side and a small pea on the other side…well, that’s how the size difference would look between these crabs.  
3. A small species of crab called Lybia or boxer crab, carry stinging anemones in their pincers anywhere they go. Why?  Lybia are very small in size and they don’t have venom to protect themselves from predators. They use the anemones in a mutually-beneficial partnership where the anemone acts as their defensive, venom-filled gloves. If an animal were to attack the Lybia, the anemone would sting the predator, protecting the crab. In return, the crab takes the anemone to different water bodies, allowing it to feed-off various sources and gaining valuable nutrients not found in its native environment. 
4. If a crab loses its limbs in a fight, it can grow them back in a matter of months. This is a feature that is also found in starfish and lizards. 
5. Crabs walk sideways because their legs are positioned to the sides of their body and their joints bend outwards and sideways. The reason for this type of evolution traces back to the crabs’ feeding behaviour. As sand-digging scavengers, crabs never needed to move forwards or move fast. This meant they didn’t need forward bending legs (which are one of the reasons animals can walk or run fast) and could make-do with sideways legs and sideways walking. However, not all crabs walk sideways. Frog crabs and spider crabs belong to the handful of crab species that walk forwards. 

 

 

Bonus

There is a type of parasitic barnacle called the Sacculina, which injects itself into the crab’s body, takes control of the crab’s will and makes it do its bidding. Crabs infected by Sacculina can’t control their own body mechanisms and are forced to become walking, breathing incubators of Sacculina eggs. Read this highly-informative article to learn all about the relationship between the Sacculina and its crab host. 

Here’s what a crab infected by Sacculina look like: 

 

Sacculina before it expels its shell

Sacculina after it expels its shell

 

Video: Coconut tree crabs are the only type of crabs that can climb trees. Watch this monster of a crab climb a tree, bend coke bottle caps and more. 

 

 

A Lybia with anemone in its pincers (image source)

 

True crab – 4 sets of limbs & shallow abdomen

False crab – 3 sets of limbs & rounded abdomen

 

 

 

-NISHA PRAKASH 

P.S: Featured image – The Sally Lightfoot crab from the Galapagos Islands. Sacculina – Mental Floss & Wikipedia.