- Both crabs and lobsters are decapods, or crustaceans with 10 limbs. Other decapods include crayfish, prawns, and shrimp.
- While lobsters have a long, segmented abdomen that sticks out at the back of their bodies, crabs have a similar but smaller abdomen that is curled up underneath the main shell.
- Most crabs have flat bodies that enable them to squeeze into very narrow crevices.
- A crab’s shell is really a skeleton on the outside of its body. Insects and spiders also have external skeletons.
- A strong waterspout may sweep up animals, such as crabs, that live near the water surface and then rain them down over land.
- The largest crab in the world is the giant Japanese Spider Crab, which can measure up to 13 feet across.
Horseshoe crabs are often called "living fossils"
- Horseshoe crabs are not crabs at all. They are not even crustaceans. Instead, they have their own separate class named Merostomata. They have hardly changed since the age of the dinosaurs, and their closest-living relatives are spiders.
- There are two kinds of crabs. First are true crabs, or brachyurans, which have a very short abdomen and use four pairs of long legs for walking. True crabs include blue crabs, spider crabs, and ghost crabs. Second are false crabs, or anomurans, which have a longer abdominal section and fewer walking legs. False crabs include hermit crabs, king crabs, and squat lobsters.
- There are about 5,000 species of crabs. Only about 4,500 are true crabs. The other 500 types are false crabs and include hermit crabs, king crabs, porcelain crabs, horseshoe crabs, and crab lice.
- Crabs are also known as “spiders of the sea” because, like crabs, spiders have legs that bend at joints.
- All crabs have claws on their two front legs.
- All crabs have one pair of pincers (chelipeds) and four pairs of walking legs.
- Crabs have large compound eyes made up of hundreds of tiny lenses.
- A crab can use its claws as a vice for crushing or like scissors for cutting. They can also be used like chopsticks to pick up food.
Some crabs use anemones as camouflage
- The small boxer crab carries a pair of stinging anemones in its claws as protection.
- A hard shell called a carapace covers a crab’s body.
- All crabs have maxillipeds and other paired structures on their heads that are used for feeding.
- The Sally Lightfoot (a.k.a. red rock) crab is the most colorful crab in the world. It is red, orange, yellow, and white.
- Crabs live in more different places than any other sea animal. They are found almost everywhere in the ocean, including smoking volcanic vents thousands of feet below the surface. They also live under the ice in Antarctica. One type of crab even lives on land and climbs trees.
- Crab lice, also known as “pubic lice,” are parasitic insects that live on humans. They can live any place where there is hair, including pubic hair and eyelashes. They feed only on blood.
- The word “cancer” is related to the word “crab” in Latin. Greek physicians Hippocrates and Galen, among others, noted the similarity of swollen tumors with veins to crabs.
- The coconut crab is a giant hermit crab that lives on islands in the Pacific Ocean and the Indian Ocean. It weighs up to 10 pounds, making it the world’s largest land invertebrate.
- Male and female crabs can be distinguished by looking at their abdomens. In most male crabs, the form of the abdomen (pleon) is narrow and triangular, while the females have a broader, rounder abdomen. Additionally, female crabs have smaller claws than male crabs.
- Crabs hatch as tiny larvae, just the size of the head of a pin. Crab larvae float for several weeks before settling to the ocean floor to hatch.
Female crabs lay millions of eggs at one time
- A female crab lays millions of eggs at one time. She carries them underneath her body until the eggs hatch.
- Crabs communicate by flapping their pincers or drumming their claws.
- A group of crabs is called a cast.
- Crabs are crustaceans (have an exoskeleton) and arthropods (have segmented appendages).
- While early research declared that crabs feel no pain, new research suggests crabs not only suffer pain but they also remember it. Consequently, scientists say crustaceans should be treated with the same care as vertebrates.
- True crabs can walk slowly in any direction, but when they need to hurry, they usually move sideways.
- Pea crabs are the smallest of all crabs and, not surprisingly, are about the size of a pea.
- Some crabs, such as the spider crab, disguise themselves by attaching living things, such as anemones, to their bodies. These living decorations not only camouflage the crab, but the anemones’ stings also discourage predators.
- The Japanese Blue Crab or the Horse crab is the most consumed crab in the world.
People consume about 1.5 million tons of crab annually
- Humans eat about 1.5 million tons of crab every year. Crabs make up 1/5 of all creatures that are caught from bodies of water around the world.
- Crab meat is very high in vitamin B12. Just 2–3 ounces of crab meat will supply an adult with the daily B12 requirement.
- Crabs have eyes that are set on eyestalks. Eyestalks can move in different directions and allow a crab to see all around. The stalk helps a crab see while hiding under water, rock, coral, sand, or mud.
- Crabs are omnivores. They are usually not picky eaters and will eat bits of dead plants and animals, algae, worms, detritus, or other crustaceans.
- Crabs have several predators, including seagulls, sea otters, octopuses, and humans.
- A crab may lose a claw or leg in a fight. In time, the claw or leg grows back.
- Crabs can live on land as long as they keep their gills moist.
- A crab’s shell does not grow or stretch. When a grab gets bigger, it must climb out its shell in a process called molting. When a crab molts, a crack forms along the shell and then the crab backs out of it.
- Most crabs molt six or seven times during their first year of life, and then they molt just once or twice a year.
- Crabs are invertebrates, which mean they don’t have a backbone. More than 95% of all kinds of animals are invertebrates.
Crabs may live socially for protection
- Crabs often work together to get food and to protect their families.
- While crabs do not have teeth inside their mouth, some crabs—such as the decorator and brown crabs—have teeth in their stomachs. These grind against one another when the stomach contracts to mash up food.
- Crabs can contract a terrifying parasite called a sacculina, which effectively castrates a crab and makes it incubate and hatch the parasite’s eggs. Additionally, due to loss of nutrition, the crab’s natural ability to grow another claw for defense purposes is also lost.
1“Claws for Concern? ‘Crabzilla’ is 12ft across, 40 Years Old . . . and He’s the Biggest Ever to Go on Display.” Mail Online. Updated August 3, 2010. Accessed: August 17, 2014.
2“Crabs ‘Feel and Remember Pain’ Suggests New Study.” CNN. Updated March 27, 2009. Accessed: August 17, 2014.
3Gilpin, Daniel. Lobsters, Crabs, & Other Crustaceans (Animal Kingdom Classification). Minneapolis, MN: Compass Point Books, 2006.
4Lunis, Natalie. Crawling Crabs (No Backbone! The World of Invertebrates). New York, NY: Bearport Publishing, 2008.
5Rhodes, Mary Jo and David Hall. Crabs (Undersea Encounters). New York, NY: Children’s Press, 2007.
6“‘Sharknado’ Got One Thing Right: Aquatic Animals Sometimes Do Fall from the Sky.” National Geographic. July 19, 2013. Accessed: August 17, 2014.