Crustaceans FAQ

Updated February 21, 2017 | Factmonster Staff
Source: National Marine Fisheries Services' Northeast Fisheries Science Center

Golden Crab

What is the difference between soft and hard shell crabs?

They are the same species. A soft-shell crab is one that has just discarded its shell. Crabs that have just shed their shells hide in rocks or bury themselves in sand and mud to escape predators. They emerge after the new shell hardens, a quick process.

What are the small crabs found inside oysters?

These are “pea” crabs. They live, often in pairs, inside the oyster shell, eating food collected on mucous strands in the oyster. Because they do cause damage to oyster mantle and gills, the crabs are considered parasites. Pea crabs are not harmful to man.

How do prawns, crayfish, and shrimp differ?

As so often happens, common names are used loosely and inconsistently in the shrimp family. The “prawn” of Great Britain and other countries is essentially the same animal as the shrimp of the United States, the only biological difference being that prawns have their second abdominal flap (counting from the head towards the tail) overlapping the first and the third. In this country, the term “shrimp” applies to all crustaceans of the Natantia group, regardless of size. “Crayfish” or “crawfish” are names given to both a common freshwater crustacean and to the saltwater spiny lobster.

Is there more than one kind of shrimp?

Numerous varieties exist, among them brown, white, pink, royal red, brine, and rock shrimp.

How big do shrimp grow?

Depending on the species, size ranges from about 1/2 inch long on the west coast of the United States, to almost 12 inches elsewhere.

How long do shrimp live?

The life cycle varies geographically and by species. Some live as long as 6-1/2 years, others live only a year.

How much shrimp is produced in the United States, and where is the largest catch taken?

The annual catch has been running close to 400 million pounds for several years. The Gulf States usually lead in shrimp catches, with Texas and Louisiana the leading States. Alaska has been an important shrimp producer for the past several years. The shrimp fishery has the highest market value of all U.S. fisheries.

What are the commercially important shrimp on the east coast of the United States?

Three shrimp species are of primary commercial importance: Pink shrimp from Chesapeake Bay through the Gulf of Mexico and the West Indies to Brazil; white shrimp from Fire Island, New York, to Cape Kennedy, Florida, in the Gulf of Mexico from Pensacola, Florida, to Campeche, Mexico, in Cuba and Jamaica; brown shrimp from Massachusetts down the east coast through the Gulf of Mexico, and the West Indies to Uruguay.

What do oysters and clams eat?

Called filter feeders, oysters and clams eat plankton. By pumping water through their bodies, the mollusks strain microscopic organisms through their gills, which act as sieves.

What purpose does a clam's siphon serve?

Three main purposes: breathing, obtaining food, and eliminating waste products. Since clams are relatively immobile and movement is usually limited to burrowing in the sand, their double-tubed siphon—which operates much like a snorkel—is their lifeline. Inflowing water is pumped through the siphon, passed over the gills, and strained to remove food particles. After receiving carbon dioxide from the gills and other waste products from the digestive tract, the water is expelled through the outgoing siphon. Constant circulation of the water is maintained by the beating of a multitude of microscopic hairs (called cilia) located inside the tube and in the gill chamber.

How does a clam shell grow?

A thin tissue that adheres to the inner surfaces of the shell, called the mantle, and a thickened rim of muscular tissue at the mantle edge deposit new shell material at the shell edge. Rings on the shell indicate how many years old a clam may be.

How do clams establish themselves on the sea bottom?

Certain kinds of clams in early stages of life possess a gland that produces a thread-like material (byssus) that serves to anchor them to grains of sand or rocks. Other types of clams lack a byssal gland and use the foot to burrow into the seabed. As the clam grows, its wedge-shaped foot, which expands and contracts as it moves, becomes more important as a burrowing tool.

How do clams reproduce?

Eggs and sperm are released into the water seasonally, generally in mid-summer when water is warm and planktonic food is abundant. After fertilization of an egg, cellular division produces larvae and eventually tiny clams that settle to the bottom. In a few species, the final stage is completed within the mantle cavity of the parent.

Which clam species is of greatest commercial importance to the United States?

The oceanic surf clam is the most important commercial species. The largest clam of the U.S. east coast, it sometimes reaches a shell length of more than eight inches. Landings of surf clams in New Jersey and Virginia account for about half the total U.S. annual landings of all clam species. The surf-clam catch in recent years—in shucked meats—ranged from about 41 to 63 million pounds.

How are soft-shell clams harvested?

They are dug from the intertidal flats of bays and estuaries at low tide in New England, using a short-handled fork to obtain clams living in burrows six to ten inches below the surface. In Chesapeake Bay, because the beds are mostly subtidal, a hydraulic dredge washes clams from the bottom and onto an endless belt that conveys the clams to the dredge boat.

How are scallops caught?

A large dredge scrapes scallops off the bottom and carries them aboard fishing vessels.

What is the biggest clam caught and eaten in the United States?

The geoduck (pronounced gooey-duck) clam caught in Northwest Pacific waters weighs an average of three pounds and yields over a pound of flavorful meat.

What are oyster borers?

An oyster borer, or drill, is an aquatic snail that preys on oysters, especially thin-shelled young oysters. Using a band of scraping teeth (a radula) and a shell-dissolving secretion, the gastropod drills a hole in the oyster shell and eats the creature within.

How do oysters produce pearls?

Pearls begin with the presence of a foreign substance, such as a grain of sand, that lodges in the shell. The oyster's body reacts by depositing layers of nacreous (pearl-like) material around the foreign body to wall it off and reduce irritation. Many oysters—as well as some clams and mussels—manufacture material like the pearl-producing substance. True pearl-producing oysters, however, inhabit waters of the Indo Pacific.

Is it safe to eat oysters during the months without R's?

Yes. Fresh oysters properly refrigerated are wholesome and nutritious throughout the year. They spoil rapidly at high temperatures, however. The belief that oysters were unsafe to eat in May through August arose in earlier days when refrigeration was less prevalent than it is today.

How does a scallop move?

It compresses the valves of its shell and forces water backward in jets near the shell hinge. The force drives the scallop in the direction of the shell opening. The bivalve appears to be clapping the two sides of its shell together.

What is chitin?

Chitin is the structural material that forms the shells of crustaceans, such as crab, lobster, and shrimp.

How does the consumer know that shellfish are safe to eat?

Clams and oysters in the shell should be alive and the shells should be closed tightly or should close when the mollusks are tapped. The U.S. Public Health Service, in cooperation with the States, has a sanitation control program that covers the labeling and shipment of clams, mussels, and oysters. These shellfish may be harvested only from non-polluted waters and processed for shipment in sanitary plants inspected by State shellfish inspectors.

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