Manila clams are generally sold live year-round, although some quantities may be frozen whole. These small clams tend to be soft and sweet in taste. Buyers generally recommend avoiding buying clams by the bushel, a common measure used on the East Coast, because the definition of a bushel can vary among suppliers. If buying by the bushel, check to make sure that the shipments are what was paid for. The best Manila clam shelf life and meat yield is in the winter time, which coincides with reduced prices due to lower demand from coastal resorts, according to some buyers. After the clams spawn in the summer, shelf life drops off.
Manila clams are a bivalve species that are oval in shape. Their hinged shells narrow at the anterior end. The color of the shells ranges from off-white to yellow, brown, or gray. Manila clams often have stripes of red, blue, or black on their shells when small. The surface of the shells is variegated and often have concentric or radial brown lines. The inside of the shell is white or yellow with a purple stain along the edges. This species usually measures around two inches wide. Manila clams can live up to 14 years.
This species grows quite rapidly and reaches sexual maturity at one to three years of age when it is around half an inch to an inch in width. Manila clams are broadcast spawners. They spawn during the summer when water temperatures reach around 55-60° Fahrenheit (13-16° Celsius). Female and male clams will release their eggs and sperm into the water column where fertilization will occur. A female can release over two million eggs during a spawning event, depending on its size. Once hatched, larvae are planktonic for three to four weeks, after which they settle onto suitable substrates.
Manila clams are filter feeders. Using their siphons, they take in water and filter out suspended particles using their gills. Larvae and juveniles feed mostly on phytoplankton, while adults will feed more on microalgae, such as diatoms. Manila clams are preyed upon by moon snails, starfish, crabs, shorebirds, and sea otters.
Manila clams are found throughout the Philippines, South and East China Seas, the Yellow Sea, Sea of Japan, Sea of Okhotsk, and around the southern Kuril Islands. During the 1930s they were accidentally introduced to Washington State as part of larger shipments of Pacific oysters. They have since spread throughout Washington and the Puget Sound area to Oregon, California, and British Columbia where wild populations now occur. Manila clams have also been successfully introduced, amongst other locations, to Hawaii, Tahiti, France, the United Kingdom, Ireland, Germany, and Italy.
Manila clams are shallow burrowers – usually burrowing between two to four inches under sand, mud, or gravel surfaces. They inhabit a variety of different habitats including intertidal zones, brackish water, estuaries, eel grass beds, and even under ice. Manila clams can tolerate a range of salinities, but grow best in salinities of 24-31 ppt, with optimal salinity levels being between 24-25 ppt for spawning. Likewise, they can also tolerate an array of temperatures, but grow best in water temperatures of 59-82° Fahrenheit (15-28° Celsius) with spawning occurring in waters above 57° Fahrenheit (14° Celsius).
Science & Management
Research funded by the USDA and National Institute of Food and Agriculture has looked at potential causes for decreasing manila clam populations outside of Bellingham, Washington. It is possible that increased carbon deposition in the area can lead to bacterial growth that reduces oxygen supply. This reduction in oxygen supply causes bacteria to produce hydrogen sulfide which is toxic to many species and can impact the ability of clams to grow. The research project is also looking into altered food supplies and whether that can affect manila clam populations. The team is using biomarkers, such a stable isotope, and fatty acid analyses, to determine what clams digest and how that varies during the year.
Fisheries and Oceans Canada is conducting ongoing research to determine the effects of both mechanical and hand harvesting for manila clams. This research is focused on finding any risks to valued habitat productivity as a result of harvesting. Findings can provide information needed to develop regulations for these types of harvest methods.
Numerous local, state and federal agencies are involved to some degree in the permitting process and regulation of manila clam aquaculture in the United States. While there is no national oversight agency for aquaculture in the US, there are extensive regulations in place regarding predator controls, therapeutant use, and disease management. Permitting varies by location with numerous federal agencies providing some degree of oversight. These include:
The United States Department of Agriculture (USDA) – which is responsible for coordinating national aquaculture policy and providing the industry with research, information, and extension services;
The Environmental Protection Agency (EPA) – which regulates waste discharge from aquaculture facilities;
The Fisheries and Wildlife Service (FWS) – which regulates the introduction and transport of fish; and,
The Food and Drug Administration (FDA) – the FDA’s Center for Veterinary Medicine is responsible for approving and monitoring the use of drugs and medicated feeds used in the aquaculture industry.
Additionally, the National Oceanic and Atmospheric Administration (NOAA), the US Coast Guard, the Bureau of Ocean Energy Management (BOEM), and the US Army Corps of Engineers are involved in the permitting and management of manila clam aquaculture. Amongst regulations, shellfish farms must adhere to include the Magnuson Stevens Fishery Conservation and Management Act, the Endangered Species Act, the Clean Water Act, and the National Environmental Policy Act.
Water quality is monitored by a national shellfish sanitation program. Manila clam aquaculture production is well-managed, according to the Monterey Bay Aquarium.
Clam farms are usually located in protected beaches, inlets, and estuaries that have been registered with a shellfish authority. On the U.S. Pacific coast, Manila clams are farmed from cultured beds that have received a national permit through the U.S. Army Corps of Engineers. In British Columbia, Manila clams are cultured and harvested from wild beds by hand.
Shellfish are filterfeeders so they generally don’t require additional feed beyond seawater. Some farmers may add some algae as feed but clams can actually lower the levels of nitrogen, phosphorous, and other particles in water, effectively cleaning it. As a result, no controls are necessary for effluent from Manila clam farming operations.
Disease, Pathogen and Parasite Interaction
The majority of farming for clams occurs with the native range of individual species of clams, and although the grow-out phase for clams occurs in open systems (coastal areas and estuaries), the risk to wild stocks is therefore considered low. Additionally, there is little chance of escape by juvenile or adult clams since they are usually secured by netting &/or bags.
Escapes and Introduced Species
Although the species is native to Japan, Manila clams have been farmed along the Pacific coast of U.S. and Canada since the 1930s so there are no negative impacts on native ecosystems. Manila clams are farmed at high densities, but these volumes do not surpass what the beaches can handle. However, outside pollution and contamination from bacteria as well as brown tides have forced beach closures, causing farms to close as well. The mesh netting used to deter predators is not considered harmful. Manila clams are usually collected with tongs, rakes, and handheld dredges that don’t harm the seafloor the way large hydraulic dredges used for collecting other clams can, according to the Seafood Choices Alliance and the Monterey Bay Aquarium.