![Crab on the ocean floor photo taken by HabCam.]()
HabCam image of a crab on the ocean floor.
Can crabs swim?
Most crabs "walk" or run across the ocean bottom. Some, such as the commercially-caught blue crab of the Atlantic coast can swim. Their rearmost pair of legs is modified for swimming and their legs are paddle-shaped.
How do crabs grow?
Crabs grow by shedding their shell once they've outgrown it. The rigid shell imprisons the crab and limits growth. Once the shell is shed, the crab can absorb water and expand into its new-grown shell.
How much does a blue crab grow after molting?
Under normal conditions, about a one-third increase occurs with each molt.
What's 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 which have just shed their shell hide in rocks or bury themselves in sand and mud to escape predators. They emerge after the new shell hardens, a quick process.
![Orange northern stone crab]()
Northern stone crab on measuring board.
How old do blue crabs get?
A female may live 2 years, a male as long as 3 years.
Does it hurt a stone crab when its large claw is broken off?
Fishermen often break off the large claw and throw the crab back into the water. If they make the break at the first joint, the crab is not harmed. The stone crab can and does sever its own claw at the first joint (by muscular contraction) to escape from danger.
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 people.
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 6.5 years, others live only a year.
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.
How many eggs does a shrimp produce in one spawning?
About 500,000.
How much shrimp is produced in the U.S.?
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 U.S. east coast?
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's the difference between an oyster, a mussel, a scallop, a clam, and an abalone?
Most of them are filter feeders that have two shells or valves, so they are called bivalves. Bivalves include:
- Oysters are grayish in shell color and usually shaped as a tear drop.
- Mussels are deep purple, brown or green and are elongated and roughly triangular in shape.
- Scallops are roughly circular with two "ears" on the either side of the extremity where the valves are connected, deep marked shell lines and colorful shells ranging from purple, yellow to orange hues. They are most recognized by the “Shell” company symbol.
- Clams are gray to brown and have thick, triangular shells.
Abalones have only one oval shell with a sequence of holes along its edge, so they are not bivalves. They are called sea snails and are not filter-feeders but grazers. They are particularly known for their beautiful inner shell with many shades—from blue to green to pink. They are used widely for producing ornaments, especially in New Zealand.
What do bivalves like oysters and clams eat?
Called filter-feeders, bivalves eat plankton—microscopic organisms and algae in the water column. By pumping water through their bodies, the mollusks filter water with their gills—just like a sieve—and capture food.
Do shellfish really help clean the water around them?
Because bivalves are filter-feeders, they remove particles from the water by pumping water past their filtering apparatus and capturing nutrients from the plankton they eat. They can help filter pollution from land and may be used in water quality restoration projects. If shellfish are removed from marine environment, such as when harvested for food in aquaculture, it will also recycle nutrients from the sea back to land.
How do shellfish establish themselves on the sea bottom?
Some shellfish have a gland that produces a thread-like material that anchors them to hard surfaces such as rocks. Other types lack this gland and use their foot to burrow deep into the seabed.
![Shellfish on ocean bottom as captured by HabCam.]()
HabCam image of scallops on the ocean bottom.
How do bivalves reproduce?
Bivalves release eggs and sperm into the water seasonally. This generally happens in late spring and mid-summer when water is warm and food is abundant. After the egg is fertilized, cellular division produces larvae and eventually tiny shellfish that settle to the bottom.
What's the most commercially important shellfish species for the United States?
The oceanic surf clam is the most important commercial species. The largest clam of the East Coast, it sometimes reaches a shell length of more than eight inches. Landings of surf clams in New Jersey and Georges Bank 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.
What is offshore shellfish aquaculture?
The term "offshore" means exposed marine areas, usually relatively far from the coast. Aquaculture is the cultivation of aquatic organisms. Therefore, offshore shellfish aquaculture is the cultivation of shellfish far from the coast in the sea. The demand for seafood is increasing, and the U.S. imports more than 90% of its seafood. Moving shellfish to offshore areas can help meet the demand for seafood and relieve the pressure on wild stocks. For that to happen in a sustainable way and safe way, research and management need to be transparent and informed by sound science.
How can you know when shellfish are safe to eat?
Oysters in experimental flow-through tanks during ten-week ocean acidification experiment.
If they are not sold as a shucked meat (canned or in brine), all shellfish should be alive at the moment of purchase. That means their shells should be closed tightly or they should close when the mollusks are tapped. Bivalves with opened and cracked shells should be discarded. The U.S. Public Health Service, in cooperation with the states, has a sanitation control program that covers the labeling and shipment of shellfish. These shellfish may be harvested only from non-polluted waters and processed for shipment in sanitary plants inspected by state shellfish inspectors. Authorities periodically test water for sewage pollution and ban catches from polluted areas.
What causes a reddish color in the liquor of shucked oysters and clams?
This is caused by the red algae they sometimes consume, often composed of the microscopic one-celled dinoflagellates which appear in planktonic mass.
How unsafe are shellfish from polluted waters?
They can be dangerous to people, causing mild to severe illness, and sometimes death. Both sewage and industrial wastes can affect shellfish.
Is it possible to purify shellfish from sewage-polluted water for safe eating?
Yes. Sewage-polluted shellfish transplanted to clean water purify themselves rapidly and become safe to eat.
Will cooking make sewage-polluted shellfish safe to eat?
Not entirely. Cooking will kill bacteria that cause some diseases, but we don't know whether certain diseases, such as infectious hepatitis, can be prevented by cooking.
Do shellfish contain mercury levels dangerous to human health?
No. Tests of shellfish to date have shown mercury levels to be below those considered dangerous to humans.
What purpose does a clam's siphon serve?
There are three main purposes to the siphon: breathing, obtaining food, and eliminating waste products. 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. 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 located inside the tube and in the gill chamber.
How does a clam's 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 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. Clams live in burrows six to ten inches below the surface. In the Chesapeake Bay, where the beds are mostly subtidal, a hydraulic dredge washes clams from the bottom and onto an endless belt that brings the clams onto the boat.
What is the biggest clam caught and eaten in the United States?
The geoduck (pronounced gooey-duck) clam, caught and farmed in Northwest Pacific waters, weighs an average three pounds. It yields more than a pound of flavorful meat and is mainly exported to Asia.
HabCam image of a scallop swimming over the ocean bottom.
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 as it swims.
How are scallops caught?
A large vessel with a dredge scrapes scallops off the bottom and carries them aboard fishing vessels.
What is oyster spat?
The life cycle of the oyster begins with a free-swimming larval stage that eventually attaches to a hard substrate, forming an oyster spat. The spat begins a growth period that is classified into juvenile and adult phases.
What are oyster borers?
An oyster borer or oyster drill is an aquatic snail that preys on oysters, especially thin-shelled young oysters. Using a band of scraping teeth and a shell-dissolving secretion, the snail drills a hole in the oyster shell, usually the beak of the oyster, and eats its soft body.
How do oysters produce pearls?
![Close-up of pile of oysters.]()
Oysters in a tote after being cleaned, sorted, and graded.
Pearls are a defense response of the animal to the presence of foreign substance, such as a grain of sand, that lodges in the shell. The oyster's body reacts by depositing layers of pearl-like material around the foreign body to wall it off and reduce irritation. Pearls can be perfectly round or irregular-shaped. Many oysters, as well as some clams, mussels and abalones, can produce a pearl-like substance. Nowadays large-scale commercial pearl production is performed by artificially implanting a small shell/plastic bead inside oysters, native to the Indo-Pacific. They are cultivated in a marine farm for a couple of years until the pearl has achieved commercial size.
Should you really only eat oysters during the months with an "R" in their name?
Fresh oysters are nutritious throughout the year. They do spoil rapidly at high temperatures, however. The belief that oysters were unsafe to eat from May through August arose in earlier days when refrigeration was less prevalent than it is today. Additionally, as high temperatures triggers reproductive spawning (release of eggs and sperm), meat content is low. They also look thin in warm months, resulting in what is considered as low oyster quality—and maybe the reason it would be avoided. However, many cultured oysters sold nowadays do not reproduce, so their quality is high and the meat looks plump throughout the year.
Why did I find a tiny crab inside an oyster?
The small crabs that infest the Eastern oyster are popularly called “pea crabs." These crabs can also be found in mussels and jingle shells. They are parasites, using the host (in this case, the oyster) for food and shelter. Although crabs do affect oyster meat quality, they do not seem to increase mortalities in their hosts. They are found throughout the coast of Massachusetts all the way south to Brazil.
Other Fun Facts Pages
Humpback whales swimming underwater. Credit: Kogia.org
An endangered leatherback sea turtle swimming at the ocean surface. Once prevalent in almost every ocean, the leatherback population is rapidly declining in many parts of the world. As one of NOAA Fisheries’ Species in the Spotlight, we have made it a priority to focus recovery efforts on stabilizing and recovering the leatherback sea turtle to prevent its extinction. Photo credit: NOAA Fisheries
Gray whale breaching. Credit: NOAA Fisheries/Merrill Gosho.
NOAA Administrator, Dr. Richard Spinrad, and NOAA Fisheries Assistant Administrator, Janet Coit, with representatives from the Intertribal Agriculture Council at Seafood Expo North America 2023 in Boston. Credit: NOAA Fisheries.