Red king crabs are among the largest crustaceans, with males reaching up to five feet in leg span and 11 inches in body length. Females are generally smaller. As the name suggests, the species is dark red to burgundy. It has a fan-shaped abdomen. The posterior legs are smaller than the others; females use them to clean their embryos while males use them to transfer sperm during mating. Male red king crabs have a narrower abdominal flap than females.
Females may keep their embryos protected underneath their tail flaps for a year. After hatching, the swimming larvae start to feed on plankton and undergo several molts as they grow. Later, the larvae settle to the ocean bottom, preferring waters less than 90 feet deep. Young crabs are often smaller than a dime. They reach sexual maturity four to five years after hatching.
Like other crustaceans, the red king crab has a skeleton of calcium, which does not grow. Juveniles molt or change their carapaces several times to allow their bodies to grow. Adult female crabs molt every year; adult males can molt every two years. Red king crabs mate and molt in shallow waters but go to deeper waters offshore to feed. Some adult males migrate up to 100 miles annually and can move as fast as one mile per day.
Red king crabs are members of the family Lithodidae, which includes more than 100 other species of king or stone crabs found in cold seas. The genus Paralithodes comprises five species, including the blue king crab (Paralithodes platypus) and California king crab (Paralithodes californiensis). Red king crabs often are mistaken for golden king crabs (Lithodes aequispinus).