Blue Crabs are found from Nova Scotia to northern Argentina but are most abundant from Massachusetts on down and around the coast to Texas. They are particularly important to the economy in the Chesapeake Bay area of Maryland and Virginia. They can grow to about 9.1 inches across the points of the shell, but the photo example was 7 inches and weighed 8-3/4 ounces.
This is a highly mobile "swimming crab" which will travel a couple hundred miles seasonally. It is very active and feisty and you don't want to get pinched because the claws are very strong. Populations in the Chesapeake Bay have been declining and major experimental hatcheries have been established to try to reverse the decline. The demand is so great crabs are now being imported from as far away as Texas.
More on Crabs.
Blue Crab flesh is very tender and succulent, but it's a crab you eat for the adventure not to fill you up. Eating them is quite a project and yield is extremely low, 8% for smaller females to 14% for larger males. A 4-1/4 inch 3.8 ounce female crab, carefully picked, yielded 0.3 ounces of edible crab (8%). Figure 6 or 7 crabs per person and have plenty of other stuff to eat.
Females have red claw tips and males blue claw tips. On the underside immature females have a triangular "apron", mature females have a wide rounded apron and males have a narrow pointy apron. Males are called "Jimmies", immature females "Sally" or "She Crabs" and mature females are called "Sook".
Blue crabs are sold by two grading methods, a number system on the docks and a size system in commerce (with typically exaggerated size designations).
Most of the male crabs go to the East Coast or are sold to fancy restaurants elsewhere. #3s (all female) are sent to Southern California where they are sold in Philippine fish markets, often for less than $1.00/pound.
There are two generally accepted ways to eat these crabs: