Dungeness Crabs and Crab Fishing

We would like to tell you about some of the interesting biology of the Dungeness crab, so when, for example, you clean a crab you can recognize some of its parts, or when you catch soft-shell crabs you will know why they are soft-shelled and have little meat.

Crab Anatomy.
The Dungeness crab is a member of the ten-footed crustaceans, the decapods. Its two forward legs, the pincers, technically called chelipeds, are used for both defense and feeding. The remaining eight legs are for walking.

The first thing a crabber should know is how to tell a male from a female, since it is currently unlawful to keep females. Look at the underside of the crab . If the abdomen, the flap of shell that folds under from the rear end of the crab, is wide and rounded, the crab is a female. If the abdomen is narrow, it is a male.

Still looking at the crab from the underside, but forward between the chelipeds, you can see the overlapping parts of the mouth that are used for grasping and biting off chunks food. Turn the crab over. The large back is called the carapace. On the anterior or front part of the carapace you can find the stalked compound eyes and the antennae. The antennae are chemorec eptors. They allow the crab to taste and smell, to find food and mates.

When you pull the back off a boiled crab, you will see a number of anatomical structures and some mushy yellowish stuff, some of which is clinging to the inside of the shell. You may notice first the new cuticle that is forming for the next molt. It has the appearance of shell, but is soft and fleshy. The yellow mush, sometimes called crab butter, is the hepatopancreas, the digestive gland, equivalent to our liver and pancreas. It is used for energy storage and the secretion of digestive enzymes. Some people consider this a delicacy, but if it is from crabs taken from polluted areas, it may contain contaminants such as PCBs, heavy metals, or in our area the neurotoxin that causes Amnesic Shellfish Poisoning (ASP). Chemical pollutants are not a problem in pristine Netarts Bay, but ASP can be.

The long, triangular structures on each side of the center of the body are the gills. Each gill consists of a stack of lamellae, thin plates with a single layer of cells surrounding a blood sinus, each lamella resembling a miniature hollow pita bread. The lamellae are connected to blood vessels that travel the length of the gill. The gills lie in a pair of cavities called the branchial chambers. Water is moved through each chamber by a fringed, paddle-like appendage -the gill bailer, technically called the scaphognathite. Its ore-like motion moves water backward along the bottom side of the chamber, up along the gills and between the lamellae where oxygen is extracted, then forward where it is expelled out the mouth region. Other fringed, paddle appendages in the branchial chamber clean the gills.

Centered in the crab between the two sets of gills is the heart, a rectangular sack connected to arteries that feed blood to capillaries and tissues. The blood of the Dungeness crab is colored blue due to its copper-based respiratory pigment, hemocyanin. If you see a bluish colored liquid in the bottom of a bucket of live crabs, that is crab blood. Just forward of the heart is part of the stomach called the cardiac chamber, which contains the gastric mill, teeth that grind food. Dungeness crabs typically eat clams, snails, crustaceans (including baby Dungeness crabs), marine worms and small fish.

Molting. 

The exoskeleton, a characteristic of crustaceans and all arthropods, is comprised of the cuticle, a complex of proteins and a polysaccharide called chitin. In the Dungeness crab, it surrounds the outside of the animal, and also some internal structures such as the gills. It also lines the gut, where it is thin and porous. Parts of the exoskeleton are calcified and ridged and form the shell, giving the crab a suit of armor that is hinged at the joints. The shell can not expand as the crab grows, so the crab must periodically shed its old shell to allow for a new, larger one. This is the process of molting. Secretion of hormones starts the molt cycle. A new cuticle forms, separates from the old cuticle, and some calcium is reclaimed from the old shell. The crab quits feeding before molting and does not resume eating until well after. It relies on energy reserves stored mainly in the hepatopancreas. When the exoskeleton is shed during the molt, a fracture opens along the back of the crab, just below the carapace along what is called the molt line, and the crab backs out of its old cuticle - shell, gills, gut lining and all. The new cuticle, soft and wrinkled, fills with water and swells up to thirty percent, adding up to an inch to the width of the crab. The crab at this time is extremely vulnerable to attack and predation. It hides buried in the sand several days until the shell starts to harden. It then begins to feed to restore its energy reserves, fill out its muscles, and completely harden its shell, which may take up three months. It is during this recovery period that we find soft crabs with poor quality meat.

Young crabs may molt several times a year, but by the time they reach about four inches in width, the frequency is reduced to about once a year. Females molt before the males so they can mate (see below). Males usually molt during summer and early fall, but this can vary. Dungeness crabs will live to about eight years.