The family Lithodidae, known as the stone or king crabs, has 16 genera and 95 known species. Four species are commercially fished in Alaskan waters. These are the red king crabs (Paralithodes camtschaticus), the blue king crabs (Paralithodes platypus), the golden king crabs (Lithodes aequispinus), and the scarlet king crabs (Lithodes couesi). Of these, red king crabs are the most abundant and extensively studied species. Scarlet king crabs are much smaller than the other three species, and because they live in very deep water, researchers know little about their life cycle. Since scarlet crabs are smaller than red, blue and golden king crabs, they are not commercially significant.
All four species have different but overlapping distributions throughout the Gulf of Alaska, Bering Sea, and the Aleutian Islands. Red king crabs range from British Columbia to Japan and north to the Bering Sea. They are most abundant in Bristol Bay and the Kodiak Archipelago. Red king crabs exist from the intertidal zone to 600 ft. (183 m) or deeper.
King crabs receive their common names from the color of their carapaces. All king crabs are decapods, meaning they have ten legs. Unlike brachyuran crabs, which are considered “true” crabs, king crabs are not symmetrical but have an asymmetrical abdomen, asymmetrical first pair of walking legs, and modified fifth pair of walking legs. Biologists think king crabs are more closely related to hermit crabs than they are to brachyuran crabs such as Dungeness crabs.
King crabs have tails or abdomens which are fan-shaped and are tucked underneath the rear of the shell. Of their five pairs of legs, the first is their claws or pincers. The right claw is usually the largest. The next three pairs are their walking legs, and the fifth pair of legs are small and usually tucked underneath the rear of their carapace. Adult females use these specialized legs to clean their embryos, and males use them to transfer sperm to females during mating.
A crab’s skeleton is its external shell made of calcium. In order to grow, a crab must periodically shed and grow a new, larger carapace, during a process called molting. Juveniles molt frequently during their first few years but less often when they reach sexual maturity at the age of four or five years. Adult females must molt in order to mate, but a male does not need to shed his shell to mate. Adult female red king crabs molt and mate once a year, but males often keep the same shell for two years. King crabs shed their shells by absorbing water, causing the shell to crack.
Red king crabs are the largest of the king crab species. Blue crabs are the second largest, and golden king crabs are the third largest. Female red king crabs reach a maximum weight of 10.5 lbs. (4.8 kg), and males grow as large as 24 lbs. (10.9 kg). A large male has a leg span of nearly five ft. (1.52 m) and a carapace as long as 11 inches (27.9 cm). King crabs can live 20 to 30 years.
Red, blue, and golden king crabs migrate annually from nearshore to offshore. They migrate to shallow water in the late winter or early spring where the female’s embryos hatch. Adult females and some adult males then molt, and mating occurs before the crabs return to deep water. Once they have mated, adults segregate by sex. Biologists studying male red king crabs near Kodiak noted some males migrate up to 100 miles (161 km) round-trip annually, and at times, they move as fast as a mile (1.6 km) per day. While depth ranges and habitats overlap, red, blue, gold, and scarlet king crabs rarely co-exist.
In my next post, I will cover the lifecycle and feeding habits of king crabs as well as the status of king crab populations and the threats they face.
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I am looking forward to learning more about crab, and my next post will delve into the types and ranges of the four commercially valuable king crab species in Alaska.