Underwater World

Dungeness Crab

North America

At a glance | Vital stats | Curiosities | Closeup

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At a glance

The Dungeness crab is a highly prized sport and commercial shellfish well familiar to people living along North America's west coast. This crab is one of the 35 true crabs living in Canada's Pacific waters. Its scientific name, Cancer magister, means "chief crab." Its common name, Dungeness, was inspired by the name of a fishing port near Puget Sound, Washington.

Vital stats

Curiosities

People often mistake empty Dungeness shells strewn along beaches for dead crabs. Crabs shed and grow new shells regularly as part of their growth process. The old shell splits at the back and along the sides so the crab can back out. The shell the crab leaves behind is an almost intact replica of the crab.

Closeup

About the Dungeness crab

Dungeness crabs are typically light brown in colour. These creatures have one pair of claws and four pairs of walking legs. Their claws are serrated and so are the edges of their shells-from the eyes down to the middle of the body.

Crabs are measured by the width of their shell, which is also called the 'carapace'. A male Dungeness crab can grow to a width of about 230 mm and can weigh up to about 2 kg.

The Dungeness crab's slender, light-coloured claw tips distinguish it from other crab species, as does its relatively large size.

As a predator, the Dungeness crab eats clams, mussels, crabs and other crustaceans as well as some small fish. Crabs pursue prey more actively at night, tending to bury themselves in the sand during the day. When moving along the sea bottom, these crabs find and capture prey by probing the sand with their legs or claws. Dungeness crabs can move in any direction-quickly enough to give a scuba diver a run for his money!

Lifecycle and reproduction

Crabs don't grow at the same gradual rate that other species with internal skeletons do: crabs grow through a special process called moulting, in which they shed their shells. Each time a crab sheds its shell it grows by 15 to 25 percent. A new, larger shell replaces the discarded one.

Once a crab moults it absorbs water into its body tissue, expanding its new soft shell. It takes about three months for the new shell to fully harden.

Males mate with females in the hours following a female's moult. The mating itself lasts about 30 minutes but the male crab often carries the female crab around with him for several days to make sure no other male attempts to mate with her.

During the fall months, female Dungeness crabs release fully developed eggs-up to a million at a time-from a sack below their abdomen. The females fertilize these eggs and carry them until they hatch at the end of the winter or in early spring. Females can store sperm for up to two years and use it at any time, and can fertilize several batches of eggs following a single mating event.

When eggs first hatch, young crabs go through what is called a zoea stage. During this phase the tiny crabs float in the water column, beginning the moulting process and starting to develop limbs. After about four months, these creatures move into what's called the megalopae phase. Here they transform into what look like small crabs. They become strong swimmers and settle on the sea floor. Juvenile crabs moult many times through the year and move into deeper waters as they grow. Once they reach adulthood, they tend to moult only about once a year.

It takes about two years-and more than 10 moults-for a juvenile crab to reach maturity. After maturing, females grow more slowly because most of their energy is devoted to egg-production rather than growth. Dungeness crabs tend to live to between six and eight years of age.

The underwater world of Dungeness crab

The Dungeness crab is distributed along the west coast from Mexico to Alaska. It inhabits waters up to depths of about 180 metres. Although these crabs can sometimes be spotted on mud and gravel, they prefer sandy bottoms and shallow cool waters around eelgrass.

Predators of the Dungeness crab include octopus, halibut, dogfish, sculpin, birds and other crabs. Crabs are most vulnerable immediately after they've moulted when their bodies are soft and lack the protection of a hard shell.

Harvesting the Dungeness crab

Dungeness crabs are an important part of the diet of coastal Aboriginal peoples. There are also healthy commercial and recreational Dungeness crab industries. The crabs are captured through trap fishery: traps are baited and left either individually or together on a line over night or for several days.

In Canada, Dungeness crabs are fished year round, although the period from May to October tends to be the most fruitful for fishermen. Fisheries and Oceans Canada regulates the Dungeness crab fishery primarily through minimum catch size limitations-a way to protect male crabs until they become sexually mature, giving them the chance to spawn at least once before they are harvested. The department also restricts the harvesting of female crabs, closes fishing seasons to protect soft shell crabs when necessary, limits the number of fishing licenses, and regulates gear.

Revised: October 2009