20th June: The Night of the Crab

Tonight is known as the night of the crab, because at some point during the night, the sun will enter the zodiac sign of Cancer, the crab. A good time to tell you 10 things you didn't know about crabs.

1. Crabs belong to the Brachyura, which contains 6,793 species in 93 families. Hermit crabs and horseshoe crabs are not true crabs. Neither are pubic lice.
2. Crabs have four pairs of legs to walk with and a pair of pincers. They do walk sideways - it's the most efficient way for them, especially if they're in a hurry, although they can walk forwards and backwards too, and some species have legs which are adapted to allow them to swim, rather than walk along the ocean floor.
3. The pincers have a number of uses - crushing things, picking up food, and communication. Male crabs living on land may wave their claws to attract females.
4. A crab's eyes are on eyestalks which can move in different directions so the crab can see all around. They can see colours and have better eyesight than humans.
5. A group of crabs is called a cast.
6. The smallest type of crab in the world is the pea crab, which is about the size of a Pea. The largest is the Japanese spider crab, which can have a leg span of up to 4 metres (13 ft), the largest leg span of any arthropod. The most colourful is the Sally Lightfoot or red rock crab which is red, Orange, Yellow, and white.
7. Crabs are often aggressive, fighting over mates and holes to hide in - but they can also be co-operative and work together to provide food and protection for their family, and during mating season to find a comfortable spot for the female to release her eggs.
8. Crabs make up 20% of all marine crustaceans caught, farmed, and consumed by humans. Crab meat is high in vitamin B12. Just 2–3 ounces of crab meat will supply an adult with the daily B12 requirement.
9. Most crabs moult six or seven times during their first year of life, and then they moult just once or twice a year.
10. In Greek mythology, Karkinos was a crab that came to the aid of the Lernaean Hydra as it battled Heracles. Heracles kicked the crab with such force that it was propelled into the sky, creating the constellation of cancer.