"Do you know," said my friend, Stevie, jabbing a somnolent crab with the end of his fishing rod,"that a crab can break a fishing rod with its claws." whereupon the crab woke up and promptly demonstrated the truth of his observation. So, if you get a live one, take care how you handle it.
The crab we eat most of is the Edible or Common crab. You also see spider crabs for sale, which are very good, and much loved by the Spanish, and very occasionally swimmer crabs, which make splendid stock. If you come across soft-shelled crabs they're likely to be the American Blue Crab caught during the season when it's shed it's old casing and before the new one has hardened, although Venetians use the Shore or Green Crab for a similar ends. But for most of us, the Common Crab is the crab of choice. And the bigger the better, and certainly never under 12cm across, or it'll be illegal. Before you boil them, you're supposed to dispatch them by driving an awl or something similar into the brain just below the eyes before being boiled in salted water for 20 to 25 minutes, depending on size. And mind those claws. Males are preferred to females because they pack bigger claws. I make no comment.
Crabby characteristics vary with the seasons. According to the king among fish chefs (and Great British Chef), Nathan Outlaw: "In north Cornwall the best time for cock Common crabs is February to May, for spider crabs May to early July, and then for Common Crabs, July to September”. And he should know.
I've always preferred crab to lobster. Its flesh is every bit as sweet, but it's lighter, more delicate and I can eat a lot more of it. Of course you get white meat and brown meat from the crab. The white meat is purer and light. The dark crab meat is more pungent and more potent. Actually, if you must know, it's mainly the digestive gland and the reproductive organs. That should make you feel better.