Dungeness crab

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"Nature Guide Journal"

23 August 2001

Water cascaded from the crab ring as we pulled it out of the water and dropped it onto the dock.  The people from my Elderhostel group gathered around as the scuttling crabs were quickly examined to determine if we had landed any keepers.

Crabbing is a popular–and rewarding–sport in Oregon's estuaries.  The most popular quarry, Dungeness crab (Cancer magister), is as fascinating as it is appetizing.

Measuring the largest crab in our ring told us it was large enough to keep:  5 ¾" across the back (just in front of the longest spines) for sport-caught.  Since only males can be kept, we flipped the crab over to ascertain its sex.  Like most crab, Dungeness have a jointed flap on the underside:  this flap on females is broad and rounded; on males it's narrow.

Crabs are arthropods (joint-legged invertebrates) that have jointed, exterior shells rather than interior skeletons for structure and support.  Crabs shed their shells periodically to grow larger.  The old shell splits along a seam around the main part of the body (the "carapace") and the animal pulls its body out of the now-too-tight shell.

By the time the crab molts, a new shell has developed under the old one.  The larger shell is quite soft when new, and the crab pumps up with water to stretch it.  A mature animal can gain an inch or more across the carapace with the molt.

It will take several weeks or a month for the new shell to harden well.  In the meantime, the crab are at greater risk to being eaten or damaged.  Soft-shell crab should be released:  From the crabbers' point of view, soft-shell Dungeness are less valuable since the meat hasn't yet grown to fill the shell, making it sparse and watery.

Molting is especially significant for mature Dungeness crabs–it is only when the female molts that she can be fertilized by a male.  Male Dungeness crab seek females that are about ready to molt and trap them in their legs until the old shell is shed.  The male assists the female in casting off the old shell, then inserts a packet of sperm under her flap.  Mature females usually molt in the spring and store the live sperm until the eggs are developed in winter.

Up to two million fertilized eggs are carried under the female's wide jointed flap until hatching.  Small enough to float in the water with other plankton, the microscopic young crab larvae look more like curled, horned shrimp than like adult Dungeness.  The size of a small pea, the last larval stage of the Dungeness looks like a crab, but still moves in the water column.

By summer, the young crab metamorphose into thumbnail-sized juveniles that settle onto the bottom.  Dungeness crab will mature in two or three years, and reach harvestable size in about four.  By the time they're large enough for a legal catch, Dungeness crab will have molted about 15 times since settling to the bottom.

Mature crab generally molt once a year, with females molting in late spring and males molting in late summer.

Because molted shells have all the external parts intact–including legs, mouthparts, antenna, eyes, and gill supports, they look like whole, dead animals.  People often mistake large numbers of molted shells on the beach as evidence of widespread disease or disaster.  (Visit the Oregon Department of Fish & Wildlife's informative page on this topic.)

Dungeness crab are predators, not scavengers.  Adults feed on clams, worms, shrimp, smaller crab, and other fresh meat–which is why fresh bait is more effective than rotten bait.  The prey is caught or snipped off and pulled apart with the claws, then further shredded by the assemblage of mouthparts before being eaten.

In addition to people, octopus and cabezon are major predators of Dungeness crab.

Although our group didn't catch our limit of 12 male crab per person, what we did land was delicious!

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