Researchers do not fully understand the biology and life cycles of any of the king crab species, but red king crabs have been the most extensively researched. Scarlet king crabs live very deep where they are challenging to study, so biologists know little about their life history. The following describes the life cycle of the red king crab, except where noted.

Before mating, a female king crab must molt by shedding her shell. A few weeks before molting, the female begins releasing pheromones into the water, signaling to males in the area she will soon be ready to mate. When the male finds the female, he grasps her first pair of legs in his claws and holds her facing him for several days. Meanwhile, the female begins to molt. Her old shell separates, and it takes her only 15 minutes to climb out of the shell. A new, soft carapace now covers her, and she absorbs water and swells, making her appear larger.

After the female has molted, the male turns her upside down and places her beneath him. He inserts his ventral surface under her abdominal flap, where he releases strings of sperm. The female releases her ova from paired openings on the underside of her second walking legs. As soon as each ovum is exposed to seawater, a sac forms around it, and the sperm fertilizes the ovum. This process can be repeated several times over the next few hours. Once he finishes, the male releases the female and shows no further interest in her.

The female incubates the eggs under her tail flap for eleven to twelve months. A female king crab, depending on her species and her age, will carry between 45,000 and 500,000 eggs. Blue king crabs have bigger eggs and a lower fecundity than red king crabs. The female releases her larvae between February and April over a period of approximately 29 days. When they first hatch, the larvae resemble tiny shrimp. The larvae pass through four zoeal instar stages, each lasting between ten days to two weeks, and they finally transition into the stage which resembles a small crab. The larvae eat both phytoplankton and zooplankton and become more carnivorous as they age. When the young crabs finally settle to the bottom, they are about the size of a dime and are very susceptible to predation. The larvae settle from July through early September.

Young king crabs migrate to depths of 150 ft. or deeper. Red king crabs are known to form giant pods, and biologists believe they assemble in these pods to protect against predators. Other king crab species have not been observed forming pods. Around age four or five, king crabs move to shallower water during the spring migration to join the adults.

Red king crabs spawn every year, but blue king crabs reproduce every two years. After spawning, adult red king crabs settle at depths between 90 and 200 ft. for the remainder of the year.  Red king crabs seem to prefer soft sand. Red and blue king crabs are known as shallow-water species, while golden king crabs settle at least 300-feet deep, and scarlet king crabs seek out even deeper habitats.

King crabs are opportunistic feeders, and they eat sponges, barnacles, sand dollars, brittle stars, sea stars, worms, clams, mussels, snails, crabs, and other crustaceans. What they eat depends on their size and available prey species.

King crabs have several predators, including fishes such as Pacific cod, halibut, sculpins, and yellowfin sole. A king crab will prey upon a smaller king crab, and octopuses and sea otters also eat king crabs. Nemertean worms consume king crab embryos.

King crabs are also susceptible to parasites and many diseases. The Rhizocehpalan barnacle invades a king crab’s internal tissues, producing an immunosuppressive agent to cloak its presence. The barnacle eventually castrates the crab and stunts its growth. Liparid fish parasitize king crabs by depositing their eggs in the gill chambers of the crabs. The egg mass interferes with respiration and can lead to death.

Biologists estimate king crabs can live twenty to thirty years.

Robin Barefield is the author of three Alaska wilderness mystery novels, Big Game, Murder Over Kodiak, and The Fisherman’s Daughter. To download a free copy of one of her novels, watch her webinar about how she became an author and why she writes Alaska wilderness mysteries. Also, sign up below to subscribe to her free, monthly newsletter on true murder and mystery in Alaska.