Crabs are a type of crustacean, meaning they have a hard exoskeleton and no inner skeleton -- like insects. Although there are several different species of crabs, dungeness and blues are popular choices for steaming. Much of the crab is inedible -- a 7-ounce crab may yield less than 3 ounces of meat. In general, male crabs are larger and meatier than females and are more desirable for steaming.
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Steamed crabs contain protein and fat but no carbohydrates. Three ounces of steamed crab meat contains 87 calories, 17 grams of protein and about 1 gram of fat. The majority of fat -- 0.6 grams -- is polyunsaturated; 0.2 g is monounsaturated and 0.2 grams is saturated fat. In other words, 83 percent of crab meat is protein and 17 percent is fat. Like all saltwater seafood, there is some naturally occurring sodium -- about 235 milligrams -- more than 10 percent of the USDA recommended sodium intake for the general population -- 2,300 milligrams.
Steamed crabs are an excellent source of vitamin B-12; just 3 ounces of crab meat contains more than 100 percent of your RDA of B-12, which your body uses to produce red blood cells and DNA synthesis. Adequate B-12 intake may help prevent heart disease, cognitive and age-related brain decline and boost your energy levels, according to the National Institutes of Health. A B-12 deficiency can cause neurological disorders.
A 3-ounce serving of crab meat also contains 27 percent of your RDA of copper and 24 percent of zinc. Copper performs several roles in the body, including energy production, iron metabolism and the formation of connective tissues, according to the Linus Pauling Institute. Copper may also help boost your immune system and prevent osteoporosis, Alzheimer's and Parkinson's disease. Zinc is also important in keeping your immune system strong and may reduce the severity and duration of the common cold, according to MedlinePlus.com. Other benefits of zinc include reducing your risk of macular degeneration, ostoporosis, stomach ulcers and muscle cramps.
A Dozen Steamed Crabs
Generally when ordering steamed crabs, you order by the dozen or the bushel. A dozen crabs is always 12, but the number in a bushel varies depending on crab size. Twelve large crabs -- about 7 ounces each -- should provide 630 calories, 125 grams of protein and 11 grams of fat. Unfortunately, they will also provide more than twice the recommended amount of dietary cholesterol -- 618 milligrams -- and 150 percent the recommended amount of sodium -- more than 3,700 milligrams.
Crabs are divided into five size categories, but category standards change based on the region. The categories -- small, medium, large, jumbo and colossal -- are based on size measurement across the body, not weight. Male crabs are called "jimmies" and female crabs are called "sooks." Jimmies have more meat and are preferred for steaming. When buying live crabs, ask for heavy or fat crabs, which are near the end of their molting period. Crabs that have just molted will be full of water and not meaty.