Shellfish FAQs

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General Information

pile of scallops on deck

Most of them are filter feeders that have two shells or valves and therefore are called bivalves, with the exception of abalone. 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. They are called sea snails and are not filter-feeders but grazers. They are particularly known for their inner nacre with many shades – from blue to green to pink – and are used widely for producing ornaments, especially in New Zealand.

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.

Because bivalves are filter-feeders, they remove particles from the water column by actively pumping water past their filtering apparatus and capturing and assimilating nutrients from the plankton they eat. Accordingly, they can mitigate excess run-off of nutrients 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.

Certain kinds of shellfish possess a gland that produces a thread-like material (byssus) that serves to anchor them to hard substrate such as rocks. Other types lack a byssal gland and use the foot to burrow deep into the seabed.

Eggs and sperm are released into the water seasonally, generally in late spring and mid-summer when water is warm and planktonic food is abundant. After fertilization of an egg, cellular division produces larvae and eventually tiny shellfish that settle to the bottom.

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 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.

Shellfish Consumption

mollusc under microscope

Offshore are exposed marine areas, usually relatively far from the coast and characterized as high-energy environments. Aquaculture is the cultivation of aquatic organisms. Therefore, offshore shellfish aquaculture is the cultivation of shellfish far from the coast in the sea. Taking into account the increasing demand for fisheries products and the fact that the US imports more than 90% of its seafood, moving shellfish to offshore areas can contribute to national food security and relieve the pressure on wild stocks while optimizing marine space and decreasing conflicts of use in coastal environments. For that to happen in a sustainable way and without causing undesired consequences for other species and activities, research and management need to be transparent and informed by sound science.

If 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 US 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.

The red algae they sometimes consume, often composed of the microscopic one-celled dinoflagellates which appear in planktonic mass.

They are dangerous to man, causing mild to severe illness, sometimes death. Both sewage and industrial wastes can affect shellfish.

Yes. Sewage-polluted shellfish transplanted to clean water purify themselves rapidly and become safe to eat.

Not entirely. Cooking will kill bacteria that cause some diseases, but it is not known whether certain virus diseases, such as infectious hepatitis, can be prevented by cooking.

No. Tests of shellfish to date have shown mercury levels to be below those considered dangerous to humans.


chestnut astarte clam

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.

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.

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.

The geoduck (pronounced gooey-duck) clam, caught and farmed in Northwest Pacific waters, weighs an average three pounds and yields over a pound of flavorful meat. It is mainly exported to Asia.


tiny sea scallop

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.

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


oyster on ice

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 commences a growth period that is classified into juvenile and adult phases.

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 (a radula) and a shell-dissolving secretion, the gastropod drills a hole in the oyster shell, usually the beak of the oyster, and eats the soft body within.

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 nacreous (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 Pinctada fucata, native of the Indo Pacific, and cultivating them in a marine farm for a couple of years until the pearl has achieved commercial size.

Yes. Fresh oysters are nutritious throughout the year. They 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 and their visual aspect is thin in warm months (typically the ones without R), 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 (triploid oysters), so their quality is high and the meat looks plump throughout the year.

The small crabs that infest the Eastern oyster are popularly called “pea crabs” (Zaops ostreum), but these crabs can also be found in mussels and jingle shells. They are parasites, using the host (in this case, the oyster) to obtain food from its gill and also for 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.

More Information on Oysters

Mackenzie CL Jr. 1983. A History of Oystering in Raritan Bay, with Environmental Observations. In: Proceedings of the Walford Memorial Convocation, Sandy Hook Lab Tech Ser Rep. 30; p. 37-66.

Contact and credits

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Taken in part from “Being a bouillabaisse of fascinating facts about fish: the most-asked questions,” NOAA Magazine, April and July of 1973.

Unless otherwise noted all color artwork is © Ray Troll 2002, © Terry Pyles, colorization 2002. These images may not be utilized in whole or in part by any entity other than NMFS without the express permission of NMFS' Office of Constituent Services and the copyright artist, Ray Troll.