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About a Dungeness crab

Life stages

Dungeness crab are relatively short lived crustaceans with a maximum life expectancy of about 10 years. Most commercially caught Dungeness are 4 years old when they are between 6 ΒΌ and 7 inches wide across their carapace. To view images in a larger size, click on the image.

zoea
Dungeness crab zoea

Zoea: The beginnings of a crab

After hatching, Dungeness crab take on a larval form similar to many other crustaceans. The microscopic zoea live pelagically (suspended in the water) and are generally swept out to sea with currents, as they have very limited swimming abilities.

The zoeal period includes 5 stages and lasts from winter to spring.
megalope
Dungeness crab megalope

Megalopa: Becoming a crab

The megalopal stage is characterized by a reduced tail and increased size. Dungeness megalopa, like zoea, continue to live pelagically. Reproduction events of adults have evolved to time the occurrence of this stage in the late spring. By this time, ocean currents tend to move towards shore where these young crabs can settle in nearshore areas. Dungeness crab megalopae are an important prey item for vast numbers of larger animals such as coho salmon and grey whales. In late spring/ early summer, megalopae can be seen throughout the nearshore areas of Oregon, sometimes in fantastic numbers. Megalopae can even be observed doing some ingenious hitch-hiking on a small purple jellyfish known as velella velella (or "by the wind sailors), who sail to Oregon's beaches in spring months from offshore areas. The Megalopa stage occurs in the spring and summer.
juv dungeness
Juvenile Dungeness crab

Juvenile crab: Settling to their home

As juveniles, they prefer shallow estuarine areas with protective structure such as pilings, woody debris, and eelgrass. They usually avoid habitat overlap with adult crab, since they are known to cannibalize. Crabs remain in the juvenile stage for their first 2 years.
adult dungeness
Adult Dungeness crab

Adult crab: Living in the marine environment

Adult Dungeness crab forage on a number of fish and invertebrate species. They can be found throughout the sandy and muddy areas in the shallowest parts of lower estuaries all the way to depths of 2,000 feet.

Molting

crab molt
Dungeness crab molt

Growing larger

Growing larger In their first 2 years, Dungeness crab molt as many as 6 times a year! When they become mature, between ages 2 and 3, this slows to about once a year.

When preparing to molt, a crab's old exoskeleton separates from the new one beneath. At this time the new exoskeleton absorbs water and become larger. This causes a split at their "molt line", located mid laterally on the carapace. The new and extremely soft crab now has the flexibility to back out of its old shell. During this incredible feat, crabs are extremely vulnerable to predators and for that reason it's done quickly. Soon after the live crab has exited (the larger, green crab in adjacent photo) they bury themselves in sand to allow their new shell to harden.

dungeness bury
Burried Crab

Buried crab

During swift tidal exchanges, crab often bury themselves. This is why crabbing is best during the least amount of tidal exchange or, even better, at slack tides when crabs are out foraging.

Crab also bury themselves after a molt to allow their shell to harden enough to provide adequate protection.

crab molts on beach
Mass molting

molt line
The molt line on a live crab

 

Mass molting

Adult crab populations tend to molt simultaneously, females in the spring, males in the late summer.
It often produces a scene like this one.

Nearly all of these crabs are NOT dead, they are molts. Since molting is such a tricky undertaking of physics some percentage of crab will die during the process and turn up among the molts.
crab exuvia
Juvenile Dungeness crab exuvia

Exuvia!

Observant beachgoers are familiar with crab exuvia, known more commonly as "crab molts". Determination on whether beached crabs are dead or simply the shell left behind from this process can be made with a quick check at the molt line. Since, the exuvia is very whole, as the crab leaves even its old gills, antennae, and mouthparts behind, this determination usually requires a good look. In summer months, when Dungeness crab molting activity peaks, ODFW usually receives calls reporting "many dead crab on the beach" which are almost always these exuvia or "molts".

Reproduction

pre-mating embrace
Pre-mating embrace of Dungeness crab

Timing

Mating and spawning of Dungeness crab occur at two different times: in late spring and summer. This is when females are molting and males are still in a hard shelled condition. Dungeness crab can only successfully mate when the female is newly molted. Pre-mating embraces of crab occur when males detect pheromones exuded by females indicating that they are ready to molt. At this time, male crab will carry females protecting them from other males.
Protective mate
Protecting his mate

Protecting their mate

Crabs will stay in a pre-mating embrace until the female molts. This can go on for as long as two weeks. When crabs are in a pre-mating embrace, they are still capable of protecting themselves and their mate. Males can move their mate towards their back legs in a position, like the one shown, which allows use of their claws for protection.

Pre-mating embrace times are synchronous among Dungeness crab, red rock crab go through the same process later in the year (late summer). Since they are eating less during that period, this may partially explain why crabs are often hard to attract to bait during that time.

dungeness female molt
Female Molting

Female crab molting

Unlike male and juvenile female crab, mature female crab (~3 inch carapace width and larger) most often molt under the protection of a male crab.

In the adjacent photo, the female can be seen beginning to molt. The old shell is beginning to be tipped forward and the "new" crab is exiting backwards via the "splitting line".

Males will juggle the female and protect her while in this vulnerable state. After females molt, male spermatophores are deposited and the embrace often continues for protection of the newly molted female.

female dungeness with eggs
Female Dungeness crab carrying eggs

Reproductive success

In the fall, females begin extruding eggs and fertilizing them with spermatophores stored from spring mating. When first extruded, these eggs are bright orange, this condition is commonly referred to as "berried up". Females hold the eggs in place with a wide abdominal flap equipped with haired pleopods (a feature absent in male Dungeness crab). As the eggs develop and become closer to hatching, they turn blackish, at which time they are released into the water to begin the cycle anew.

 

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