The sun sets on a hot summer day in Crisfield, Maryland — the crab capital of the world. You're out to dinner at a seafood restaurant, and you order a steaming plate of real, mouthwatering crab legs.

Size is the most obvious difference between the two types, with king crabs being the larger specimen. The biggest among them can reach 20 pounds and measure 5 feet across. Most store-bought king crabs ...