Everything You Wanted to Know About Crabs

by Jim Rapp

Jim Rapp tells us about Delmarva's Crabs: They're not all blue!

Delmarva is famous far and wide for our Chesapeake Bay blue crabs. No summer is complete without spending a few lazy evenings with friends and family sitting around a picnic table covered in newspaper with a hot, steamy bushel of Old Bay and vinegar-infused blue crabs in the middle, picking apart these savory crustaceans one by one until only a trash can full of empty shells remains, next to a recycling container full of empty beer cans.

While steamed crabs may fill our bellies with their tasty, tender white meat, live crabs fill an important ecological role in Delmarva's estuaries, marshes and beaches. The Peninsula is home to more species than just the blue crab, and while all are coastal creatures and share similar features, each has adapted to life in their respective habitat.

Eating a steamed blue crab is very much like a dissection exercise in a high school science lab. We rarely get as intimate with our food as we do when picking crabs, and you can learn a lot about crab anatomy during dinner.

A live blue crab is actually dark olive-green on top and white underneath. Their name comes from the bright blue color on their large claws. The dark green color changes to red when they are steamed due to the breakdown of pigment-protecting proteins in their shells.

To get at the delicious meat inside of a hot-red steamed crab, you must crack open the hard exoskeleton that protects the soft organs inside. The exoskeleton does not grow, so crabs shed, or molt, enabling the crab to grow as big as ten inches across the shell. When molting, the crab's shell is squishy soft, and they are vulnerable to predators -- including other crabs, and humans who eat soft-shell crabs whole-bodied and lightly pan-fried. In the brackish waters of Delmarva, soft-shelled crabs hide in the aquatic grasses at the bottom of our bays until their shells harden.

The upper shell is called the carapace. The belly has an abdomen, or apron, with a distinctive shape. If you know your Washington, D.C. landmarks, you can easily determine males from females. The apron on a male crab is shaped like the pencil-straight Washington Monument, and the female has a rounded apron that looks like the Capitol dome.

On each side of the blue crab shell are five legs, and each is modified to help the crab survive in its aquatic environment. The two back legs are paddle-shaped, and can rotate up to 40 times per minute to help the crab swim quickly through the water. The scientific name of the blue crab is Callinectes sapidus, which means "beautiful swimmer that is savory." I guess their name implies that they don't swim THAT quickly.

The three thin middle legs on each side help the crab walk along the bay bottom, which they do in a sideways motion. As they walk, blue crabs clear a path with the sharp spines that protrude laterally from the upper shell.

Blue crabs are most famous for their two front legs, which have adapted into impressive, powerful claws. The male crab, called "jimmies" by watermen, have blue claw tips, and females, called "sooks," have red tips. When threatened, blue crabs extend their claws in a defensive posture. There's no doubt that these weapons can inflict damage, as anyone who has suffered a painful crab pinch can attest.

In addition to the ten legs, two eyestalks and two sets of antennae protrude from the front of the shell. The eyestalks stick out so the crab can see forwards, backwards and sideways. The antennae pick up vibrations and sense chemicals in the water that help the crab find prey and avoid predators. The crab's mouth is located between the antennae, and has jaws that can hold and push food into the esophagus.

After you've separated the legs, claws, and carapace from the crab's body, you'll find the soft, fleshy parts protected by the hard exoskeleton. Gills help take up oxygen in the water, and the heart pumps oxygen-rich blood to the body tissues. The yellow substance known as "mustard" is the crab's hepatopancreas, the organ that filters impurities from the crab's blood.

The white meat found in the interior chambers along the back and sides of the crab is the muscle that moves the claws and legs. The most prized meat -- the jumbo lump -- is the muscle that moves the powerful swimming legs.

Each of these specialized adaptations helps the blue crab survive in our wild estuaries, where they are an important link in the food web. Blue crabs are both predator and prey. Their diet mostly consists of clams, which they dig up from the bay bottom and crack open with their claws. They also eat fish, worms, and crustaceans, including other blue crabs.

Besides hungry humans, adult blue crabs are also eaten by fish, such as rockfish and red drum; birds, such as gulls and herons, and -- as mentioned -- other adult blue crabs. Newly hatched crabs are microscopic, and are eaten by filter feeders such as clams, oysters, barnacles, and menhaden.

Mating season for blue crabs living in Delmarva's bays runs from May to October. After mating, the female crab will migrate to the saltier waters at the mouth of the bay near the ocean. Egg fertilization can take nine months after mating. After the eggs are fertilized, the female crab will develop an egg mass, called a "sponge," under her dome-shaped apron. The sponge can contain 8 million eggs on a large female blue crab. Over a two-week period, the orange sponge will turn brown and then black as the crab larvae develop inside the eggs.

The microscopic hatchlings drift about for a while in the current, but will eventually return to the seagrass beds of their parents. They molt several times before they begin to look and live like an adult blue crab. Once they're able to
swim, the young crabs migrate towards the fresher water at the top of the bay.

When the water gets cold, most blue crabs will bury themselves in the muddy bottom of the bay. Adult females prefer the salty water of the lower bay, and males will winter in the deeper water of the upper bay. They remain mostly dormant from December through March, and become more active as the water begins to warm in early spring.

Callinectes sapidus is not the only crab in Delmarva's bays. The common spider crab looks similar to its namesake: it is a large, spindly-legged, sluggish crustacean that excels in camouflage. To blend in on the bay bottom, it covers its bumpy, spiny shell in algae, debris and tiny, living invertebrates to hide from predators. Spider crabs are one of the few bay species that is tolerant of polluted, low-oxygen water.

Another aquatic crab in Delmarva's bays is the tiny black-fingered mud crab. It only grows to an inch-and-a-half long, and prefers life on oyster reefs and under shells at bottoms of marshy creeks. Their muddy brown color provides camouflage, and they have chunky, black-tipped claws. One claw is slightly larger than the other, and they use their pincers to extract snails from their shells.

Some invasive crabs are causing problems in our already stressed bays. Chinese mitten crabs have been introduced to Delmarva's waters from East Asia. Mitten crabs are named for their fuzzy-looking white-tipped claws. The mitten crab was confirmed in the Chesapeake Bay in 2005, and became a concern to bay scientists due to their omnivorous diet and tolerance of fresh, brackish and salty water, which can cause them to spread quickly and compete with native crabs and other animals. The Chinese mitten crab is the only crab species found in fresh water in the United States.

Some crabs prefer life at the edge. Our coastal salt and brackish marshes are teeming with small marsh crabs that live in the mud along the shoreline. Marsh crabs grow to just one inch long, and live communally in interconnected, watery burrows 25 to 30 inches deep in the mud. Burrow entrances are located near the high-tide line, and male marsh crabs will defend their burrows by making a "rapping" sound. Marsh crabs specialize in eating the outer leaves of marsh grasses.

Three species of fiddler crabs colonize Delmarva's marshes, mud flats and beaches. They only grow to one-and-half inches long, but the males have one enlarged claw that can grow to two inches long. The large claw is used to attract females, which the male does by waving and drumming it at the entrance to his burrow. Females' claws are equal in size.

Fiddler crabs that prefer the beach are sandy in color, while those that live in the mud are dark brown. Like marsh crabs, fiddler crabs dig burrows up to two feet deep. They feed by sifting through sand or mud for algae, bacteria and decaying plants, and will often eat in a puddle to help separate food from sand.
The most terrestrial of Delmarva's crustaceans is the Atlantic ghost crab. It lives along our wild beaches, where it scavenges mostly at night for decaying plants, insects, mole crabs, and turtle eggs and hatchlings. The ghost crab is sandy colored and has a pair of white claws, making it almost invisible when standing still on the beach. They grow to three inches long and dig burrows in the sand up to four feet deep, where they seek shelter from the sun during the day.

So remember: Delmarva's blue crabs should not just be appreciated when steamed, pan-fried, or made into delicious cakes. Blue crabs and their cousins fulfill critical roles in the ecology of our bays, beaches and marshes. Next time you're picking apart a pile of these savory, beautiful swimmers, think about their need -- and ours -- for clean, healthy bays that support crabs, communities and commerce.

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