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Hard Clam/Northern Quahog

Mercenaria mercenaria

Illustration of a Hard Clam, also known as Northern Quahog

Also Known As

  • Hard clam
  • Quahog
  • Round clam
  • Chowder clam

U.S. farmed Hard clams are a smart seafood choice because they are sustainably grown and harvested under U.S. state and federal regulations.

Environmental Impact

Clams provide net environmental benefits by removing excess nutrients and improving water quality.

Feeds

Growing clams require no feed – they filter phytoplankton directly from the water column.

Farming Methods

Clams are grown in tidal areas. They can be grown directly on the beach bottom or in mesh bags, trays, or pens that are secured to the bottom.

Human Health

Shellfish toxins and bacteria occur naturally in the environment and can cause food-borne illnesses. State and federal regulations require monitoring of farmed clams to ensure they are safe to eat.

  • Availability

    Available year-round.

  • Source

    Primarily Virginia, Washington, Florida, Connecticut, and New Jersey.

  • Taste

    Mild flavor, sweet, and briny.

  • Health Benefits

    Clams are excellent sources of omega-3 fatty acids.

U.S. Farming

Management

  • Permitting for shellfish aquaculture is governed by federal, state and local governments.
  • The federal agencies involved are NOAA, the Army Corps of Engineers, Fish & Wildlife Service, U.S. Department of Agriculture, the Environmental Protection Agency, the Food and Drug Administration, the Bureau of Ocean Energy Management, and the Coast Guard.
  • Shellfish farms must adhere to federal regulations including those in the Magnuson-Stevens Fishery Conservation & Management Act, the Endangered Species Act, the National Environmental Policy Act and the Clean Water Act.
  • Information on shellfish aquaculture permitting can be found in the Shellfish Growers Guide.
  • A variety of shellfish aquaculture tools, including maps and models, are available to coastal managers.

Farming Methods

  • Juvenile clam (seed) production:
    • Clam larvae are bred in hatcheries and are fed a diet of algae until they reach about 1 mm in size.
    • Larvae develop shells after 2 days. After 7-14 days they lose their swimming ability, settle out, and are moved to containers with mesh that allow water flow over the seed.
  • Nursery stage:
    • Clams are supplied with seawater containing natural plankton from a nearby estuary.
    • Clams are grown in upweller systems until they reach 2-5 mm and can then be grown in raceways, mesh bags in the field, or upwellers until they reach 8-5 mm.
  • Final grow-out:
    • Clams are planted in plots or placed in trays, pens, or bags secured to the bottom of intertidal or subtidal areas.
    • Clams are grown until they reach harvest size of 50 mm.

Production

  • In 2012 the United States produced 10 million pounds of clams, valued at $99 million.

The Science

Environmental Considerations

  • Habitat:
    • Clam farming has a benign ecological footprint, with little disturbance of sediments or aquatic vegetation during grow-out.
    • Some clam growing methods may temporarily cover aquatic vegetation.
  • Feeds:
    • Once past the larval stage, clams do not need to be fed because they filter their food from the water column.
  • Genetics:
    • Hard clams are native to the Atlantic and Gulf coasts where they are mainly farmed.
    • Juvenile clams are produced in hatcheries that can have limited genetic variation compared to wild populations, but there is little evidence of negative impact on wild clam populations.

Ecosystem Services‎

  • Water quality improvements:
    • Clams are filter-feeders, removing algae, organic matter and excess nutrients from the water column as they grow and improving water quality.
    • When clams are harvested, excess nutrients, such as nitrogen and phosphorus, are removed from the ecosystem.
  • Providing habitat:
    • Clams and the gear used to farm clams provide habitat for marine organisms.
    • Clam beds stabilize coastal sediments and help minimize impacts from storm surges.

Human Health

  • Shellfish toxins:
    • Shellfish poisoning is an illness that can occur from eating contaminated shellfish.
    • Clams can assimilate the toxins that causing shellfish poisoning from the algae on which they feed.
    • Early warning systems exist to detect harmful algal blooms that produce toxins.
    • New technologies, such as the Environmental Sample Processor, provide near real-time detection of harmful algal species.
    • For more information on identification, prevention and monitoring of harmful algal blooms, read about the NOAA Ocean Service Harmful Algal Bloom programs.
  • Pathogenic bacteria:
    • The bacteria Vibrio parahaemolyticus (Vp) and Vibrio vulnificus (Vv) occur naturally in the environment and can cause food-borne illness from consuming raw shellfish.
    • Ingestion of undercooked or raw shellfish with Vp or Vv can lead to gastrointestinal illness.
  • Public health officials monitor shellfish from growing areas to ensure they are safe to eat.

Physical Description

  • Adult hard clams are just less than 3 inches, but can reach up to 5 inches.
  • The shell is thick, grey to white in color, and has outer concentric growth rings.
  • The inside of the shell is white with violet markings.
  • Some hatchery raised clams have dark, zigzag stripes across the shell known as “notata”.

Biology

  • Are of the shellfish family. Like oysters, mussels, and scallops they are bivalve mollusks, and have a hinged shell.
  • Clams have slow growth rates and can live 12-20 years on average, and up to 40 years.
  • Adults are sessile – they stay in one place – and inhabit both intertidal and sub-tidal areas.
  • Clams burrow into the sediment, leaving only their siphons expose to feed.
  • Hard clams prefer saline water and cannot survive if the salt content is too low.
  • Each female can produce between 1 and 5 million eggs during a spawning event.

Research

  • Growth & reproduction:
  • Ocean acidification:
    • Acidification causes a number of changes in water chemistry that may be stressful to estuarine organisms.
    • Ocean acidification and its impacts on shellfish are being investigated by NOAA and other labs. For more information, visit NOAA’s Ocean Acidification Program.

Last updated: 06/05/2018