Dungeness crabs (Metacarcinus magister) live nearshore along the coast of North America from the Aleutian Islands to Magdalena Bay, Mexico. The species derives its common name from a favorite habitat in a shallow, sandy bay inside the Dungeness Spit on the south shore of the Straits of Juan de Fuca in Washington state. Dungeness crabs prefer a sandy bottom. They usually inhabit depths less than 100 ft. (30 m), but they sometimes live as deep as 656 ft. (200 m). They can tolerate a wide range of salinities and sometimes live in estuarine environments. Juvenile Dungeness seem to favor estuaries where they can hide from predators amid the eel grass and other plants.

A Dungeness crab has a wide, oval, body covered by a hard brownish-orange shell made from chitin. Unlike a tanner or a king crab, a Dungeness crab has a smooth carapace, lacking spines. The legs of a Dungeness crab are much shorter than those of a king or tanner crab. A Dungeness has ten legs, four pairs of walking legs, and two claws. The crab uses the claws for defense and to tear apart its food. You can distinguish between a male and a female Dungeness by examining their abdomens. Females have a rounded abdomen, while a male’s abdominal flap appears triangle-shaped. An adult Dungeness with a carapace width of 6.5 inches (16.5 cm) weighs between two and three pounds (1 kg). A large male Dungeness can measure more than ten inches (25.4 cm) in width.

Dungeness crabs shed their shells nearly every year in a process called molting. Mature females molt between May and August, and males molt later. A male mates with a female only after she molts and before her new exoskeleton hardens. Scientists believe a female attracts a male and signals her readiness to mate by releasing pheromones in her urine. Male Dungeness are polygamous, meaning each male may mate with more than one female. After mating, the female stores the sperm in internal pouches and holds it until her shell hardens. A female can store sperm for up to two years, and older females sometimes used stored sperm to fertilize their eggs rather than molting and mating. Research shows many older females mate less than once a year. When the female is ready to fertilize her eggs, she extrudes the eggs through pores on her ventral surface. The eggs are fertilized as they pass through the stored sperm. The fertilized eggs then adhere to hairs on the abdominal appendages, and the female carries the eggs inside her abdominal flap until they hatch. An old, large female Dungeness can carry 2.5 million eggs.

When the eggs hatch, the planktonic larvae swim free. Larval development takes between four months and a year, and the larvae pass through several stages before they finally resemble a crab and settle on the bottom. During their first two years, male and female Dungeness grow at a similar rate and may molt as many as seven times, growing with each molt. Adult Dungeness molt only once a year. After two years of age, males begin to grow more quickly, and they grow larger than females. Dungeness crabs have a maximum lifespan of eight to thirteen years.

Dungeness eat live clams, worms, fish, and shrimp, and they also scavenge dead fish and invertebrates. Predators of Dungeness include sea otters, and several species of fish, including halibut. Many species of fish, marine mammals, and invertebrates prey upon juvenile crabs. Dungeness are susceptible to pollution, ocean acidification, habitat damage, and overfishing.

In my next post, I’ll describe the commercial fishery for Dungeness crabs and explain how they are managed.

Robin Barefield is the author of four Alaska wilderness mystery novels, Big Game, Murder Over Kodiak, and The Fisherman’s Daughter, and Karluk Bones. You are invited to 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, and listen to her podcast, Murder and Mystery in the Last Frontier.