All you need to know of the little nippers of the sea aka crabs.
The crab is a very interesting crustacean normally found around coastlines and beaches. But do you know why some crabs use Sea anemones as pom poms? And did you know that some crabs live in a certain animal’s poo?! Find out more in the science of Crabs.
Start
Crabs are crustaceans defined by their thick exoskeleton or shell. They have 10 legs, which makes them decapods. Side note: The prefix deca, having originated from Greek, refers to the number 10. For example, a decathlon would have 10 events and a decade has 10 years. Anyway, the legs are divided into 5 sets, and the front two are adapted as pincers.
The crab’s shell is armour which provides protection for the soft tissue underneath it. The hard-upper shell is called a carapace, which is the same type of shell as a tortoise has.
You can distinguish a male and female crab by looking at their abdomens. Males generally have a narrow and triangle abdomen, whereas females have a larger, rounder one. Female crabs also usually have smaller claws. Crabs use their claws as a vice for crushing or cutting objects. A crab may also lose a claw in a fight but this is ok because it has the ability to grow a new one!
Crabs eyes are on stalks, which can move in all directions and even retract when needed. The stalks can help them see when they are hiding below water, around rocks or in sand.
Most Crabs breathe with gills, like fish. These gills extract the oxygen that dissolves in water as water flows over them. But they can still absorb oxygen on land as long as the gills are kept moist, which might be why you see them blowing bubbles on land sometimes. There are also crabs with lungs as well as gills, and this enables them to live on land, in places like the jungle.
Crabs typically walk sideways, this is because their legs are located on the sides of their bodies and the joints point outwards. So sideways movement is quicker and more efficient.
The smallest crab in the world is the Pea crab. It is less than a third of an inch or… the size of a pea. They are parasitic and live in molluscs like oysters that provide oxygen and food in the form of zooplankton.
The biggest crab in the world is the Japanese Spider Crab, which lives deep on the ocean floor and is believed to have a leg span of up to 12 feet. That’s over twice the length of the average human adult lying horizontally.
What the Fact!? (Or just did you know?)
Did you know?
The remote Christmas island in the Indian Ocean is home to the Christmas Island red crab. These crabs are drawn by the moon to make a yearly migration from the forest to the beach to breed. What is special about this is that around 40 million crabs make this journey, that’s more crabs than there are people in Canada! (Estimated at around 36 million in 2016)
The boxer crab is also known as the pom pom crab because it wields sea anemones on each of their claws, like an angry cheer-leader. They use the sea anemones to fight against other crabs or predators as anemones sting with venom-filled tentacles. In turn, the sea anemones get fed by leftover scraps of food from the crab.
The poo-dwelling crab we mentioned earlier lives in a giant cave in Borneo called Deer cave. It lives in a giant 100m mound of Guano formed of the droppings of the millions of Bats who also live there.
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