Nuttalia obscurata (Purple varnish clam,varnish clam, or dark mahogany clam)

Animal Molluscs
Family: Psammobiidae
Classification: Regulated

Purple Varnish Clam
Varnish clam found in Willapa Bay.

The mahogany clam has been observed in northern Puget Sound for about ten years now. The clam apparently was introduced via ballast water from Asia. It tends to inhabit the upper one third of the intertidal zone, decreasing in the middle and lower intertidal zone. However, on beaches where no other clam species were present it ranged into the lower tidal levels. The varnish clam is more freshwater tolerant than other species, and the most abundant populations are found in areas where there are significant influxes of fresh water runoff. The clam has rapidly established densities as high as 1,780 clams per square meter in the southern portion of the Georgia Straits, and densities exceeding 800 per square meter on some beaches in the San Juan Islands. The clam is now found throughout Puget Sound, and may be harvested as part of the 40 clam limit.

Fisheries Potential of the Purple Varnish Clam

Subject: Nuttalia obscurata
Taxonomy: Nuttalia obscurata, Soletellina obscurata, Psammobia olivacea, Nuttallia solida, and was once thought to be a subgenus of Sanguinolaria (Gillespie et al., 1999)
Common names: Purple varnish clam,varnish clam, or dark mahogany clam


The purple varnish clam Nuttallia obscurata was first observed in British Columbia waters in the early 1990’s. The most likely means of introduction are larval or juvenile release in ballast water, or as fouling organisms transported on the hulls of vessels. By the late 1990’s the clam was well established throughout the Georgia Strait, had spread into north Puget Sound and the Strait of Juan de Fuca, and along the west coast of Vancouver Island. In 1999 the clam was found at Nehalem Bay State Park in Oregon. This is thought to be a separate introduction. The clam now occurs in most Oregon estuaries. In February of 2002 a single live specimen was found on the southern Washington coastline, in Willapa Bay.

Both Oregon and British Columbia have established recreational fisheries for the clam, and in Washington they have been classified as shellfish as of 5/1/04 and may be harvested as all or part of the 40-clam per day limit. A few were commercially marketed in British Columbia in 1998. This was halted pending further study regarding human health risks and the status of the resource under the government’s Pacific Fisheries Adjustment and Restructuring program. The program utilizes a rigorous three-phase scientific review process for developing new fisheries.

The first phase requires collection and review of available information on the species, and identification of any information gaps that may limit assessment and development of a potential management framework. Fisheries and Oceans Canada (DFO) completed this phase in 1999. In 2000-2001 funding was provided for the second phase, consisting of detailed biological assessments and distribution surveys. DFO researchers worked with Aquametrix Research, the Community Futures Development Corp. of Powell River, and clam diggers from the Area C Clam Harvesters Association to conduct population dynamics studies. Their research was designed to determine the effect of varnish clams on beaches in the Gulf of Georgia, potential competition issues between Varnish clams and other species (in particular commercially harvested Manila clams), and harvest characteristics. Marketability studies have also been conducted. Twenty thousand pounds of clams were harvested under a scientific license and marketed as the "Savoury Clam." These studies are complete, and the results provided to the Pacific Scientific Advice Review Committee for review and recommendation in January of this year.British Columbia is ready to proceed with phase three, the implementation of a monitored small-scale commercial fishery.

The opinion of the Aquatic Nuisance Species Committee has been solicited regarding the development of this fishery for the Varnish Clam. This opinion paper considers the Canadian stock assessments, the work of University of Washington researchers at Friday Harbor, and the opinions of the Washington Department of Health, shellfish growers, and others.

Taxonomy and Biology

The purple varnish clam, or varnish clam, derives its name from the shiny brown coating (periostracum) on the outer surface of the shell. The interior surface of the shell is usually a uniform shade of purple. Occasionally small specimens will have purple coloring showing on the outer surface. The clam is also sometimes referred to as the purple or dark mahogany clam. The scientific name is Nuttallia obscurata, although it has in the past been identified as Soletellina obscurata, Psammobia olivacea, Nuttallia solida, and was once thought to be a subgenus of Sanguinolaria (Gillespie et al., 1999). The clam is one of three Nuttallia species native to Japan, Korea and possibly China. There has been little research done on the clam in Asia, and it is not commercially marketed there (Mills, 2000).

The varnish clam is nearly oval shaped, with the posterior end slightly longer and more angular. The shell is relatively flat, with a large external hinge ligament. The clam has a large foot and long unfused siphons, indicative of a burrowing species able to live deep in the substrate. There is some uncertainty as to how large it may grow. The nearest related species on the west coast, Nuttallia nuttalli found in southern and Baja California, reportedly grows to a maximum length of 150mm. The largest live N. obscurata reported locally to date was 68mm (a little over 2.5 inches) total length and weighed 45.1 grams (Gillespie et al., 1999). However, Claudia Mills reports finding a shell at least 75mm in total length at Orcas Island in 1999.

It is difficult to develop an accurate size to age relationship in varnish clams by counting the growth rings. These may be affected by density and tidal elevation as well as by substrate consistency, which can create additional rings due to disturbances in growth. Therefore, local studies have relied on interpretation of length and frequency distributions. Research suggests the clam attains a length of 38 mm in about four years, a rate similar to that of the Manila clam (Gillespie et al., 1999). There are some indications that the clam may grow more rapidly in Puget Sound (Fidalgio Bay) than in the upper regions of the Georgia Straits.

Like the Manila clam (Venerupis philippinarum), varnish clams are broadcast spawners, with a planktonic larvae period. The rapid expansion of populations in the Georgia Strait and along the Oregon Coast suggests a fairly long planktonic period, which may result in broader dispersal. Recruitment surveys in British Columbia indicate reproduction occurs between early spring and summer (Gillespie et al., 1999). The British Columbia surveys, and density surveys done at Argyle Lagoon by Dr. David Duggins (University of Washington Friday Harbor Labs) indicate high settlement rates and/or a very low mortality rate in this region. In the N.W. Pacific the clam reaches maturity at one year of age, and spawns each year (Hushando, as cited by Gillespie, et al., 1999). There is every reason to expect that the varnish clam could become as ubiquitous as the Manila clam in the Pacific Northwest.

Varnish clams are capable of both filter (suspension) feeding and deposit feeding. Laboratory observations indicate that the clam tends to extend its siphons and filter feed at night. When deposit feeding the clam extends its foot and rotates it, collecting deposit materials, which are passed into the oral region when the foot is retracted. Detritus is also picked up and passed into the oral region when the foot is used for locomotion. This dual feeding capability may enable the clam to outcompete other species living in the same intertidal zone. The most commonly encountered clams in the region are the manila clam (also a nonnative species), and native littleneck (Protothaca staminea), butter clam (Saxidomus gigantea), Nuttall cockle (Clinocardium nuttallii), horse clam (Tresus spp.), eastern softshell (Mya arenaria) and macoma species such as the bent-nose (Macoma nasuta) and the polluted clam (Macoma irus).

Habitat , Distribution and Associated Species

The purple varnish clam has rapidly established densities as high as 1,780 clams per square meter in the southern portion of the Georgia Straits (Gillespie et al., 1999), and is expanding into northern beaches. Expansion southward into Washington waters has been somewhat slower, but it has reached densities exceeding 800 per square meter on some beaches in the San Juan Islands (Dr. David Duggins and Dr. Paul Dinnel, personal communication). The clam is found frequently now on beaches in Admiralty Inlet and in Hood Canal as far south as Potlatch State park. It is found in substrates ranging from cobble to muddy sand (Mills, 2000) up to 30 cm. subtidal depth. The majority of surveys indicate the greatest abundance occurs in the upper one third of the intertidal zone, decreasing in the middle and lower intertidal zone. However, on beaches where no other clam species were present it ranged into the lower tidal levels (Gillespie, et al., 2002). The varnish clam is more freshwater tolerant than other species, and the most abundant populations are found in areas where there are significant influxes of fresh water.

With the exception of the manila, which is found in the lower reaches of the high intertidal zone, most of the local species are found in the mid to low intertidal zone. The manila, which is a non-native species that was accidentally introduced with Pacific Oyster seed, has become a primary fishery in both British Columbia and Washington. The overlap between manila clams and varnish clams is therefore a matter of concern. To address this concern, British Columbia researchers conducted controlled field experiments with the two species over a period of five months. Growth and survival rates in areas occupied by both species were compared in a variety of situations. The varnish clam had a slight advantage over the manila in the high intertidal zone, possibly because of its bimodal feeding abilities. Growth in length was inhibited 47% in the manila clams, and 32% in the varnish clams. Weight gain was inhibited 34% in the manila and 26 % in the varnish clam. However, in the mid intertidal zone varnish clams did not gain weight at all when manila clams were present, while the impact on the manila clam was about the same as in the high intertidal zone (Gillespie, et al., 2002). These studies are not conclusive, because the varnish clam does not exhibit the same high correlation between growth in length and weight as the manila does, and the varnish clam may put more resources into gonad development at certain times of the year. Lastly, the densities between treatment effects in the study were not consistent - combined densities were greater than when either species was alone. The average density of varnish clams alone was 800 per square meter, while manila clams averaged 400 per square meter, and the combined densities were at 1,200 per square meter. The data would seem to suggest that the varnish clam does establish itself in areas where other species are already present. Further research is needed to determine whether other species, such as the manila, would have difficulty re-establishing populations after being harvested if the varnish clam moves into the area.

About 35% of varnish clams greater than 45mm in length are host to immature native pea crabs (Pinnixia faba). The infestation rate is lower in smaller varnish clams and in other clam species. This has two negative marketing implications. Consumers that see the crabs may find them unappetizing. A greater concern would be that a consumer who is allergic to crustaceans might unknowingly consume them and become ill. British Columbia growers unsuccessfully tried a variety of purging methods to rid the clams of the crabs.The varnish clam (like the cockle, littleneck, horse and manila clams) is susceptible to predation by Lewis' moonsnail (Polinices lewisii). Gulls, crows, and black oystercatches have also come to recognize them as prey (Gillespie, et al., 1999). Dr. Paul Dinnel (Dinnel Marine Research) conducted some tank experiments, and found that dungeness crabs easily pry the shells open and eat varnish clams. Research conducted by James Byers of the University of Washington also indicates that the species suffers high mortality from crab predation when it is unable to find loosely packed substrate where it can readily bury itself. The clam’s ability to live relatively high up in the intertidal zone may limit predation by most species of crab. On the other hand, it is possible that predation by crab and moonsnails may play a role in limiting recruitment in mid and lower intertidal zones. However, in the lower intertidal areas even being able to bury itself did not offer adequate protection from crab predation. It appears that native predators may be excluding Nuttallia obscurata from areas inhabited by some native species. However, there is not enough known about the interaction between the varnish clam and other species to accurately determine the role this interaction plays in mediating community level impacts or the success of invasion.

Harvest and Marketability

Limited test harvests were conducted in British Columbia during the summer of 2001 to determine size distribution and to evaluate harvest efficiency and breakage. The average length of the clams harvested was 41mm, and the average weight 13.7 grams. Standard equipment was used to collect the clams and place them in 5 gallon buckets. The harvested clams were checked for breakage, weighed and measured. The harvested plot was then hand excavated and screened to remove remaining clams, and determine total breakage during harvest and harvest efficiency. Efficiency was estimated to be between 60% and 80%. Total breakage was estimated to be less than 2%, although this can be expected to vary depending upon substrate type. Breakage is higher when large gravel or rocks are present than in fine gravel or sand. Higher breakage and lower efficiency can be expected under normal harvest conditions when harvest may occur at night, time is limited, and the crew may have less experience.

The clams can be purged to acceptable levels of grit accumulated by deposit feeding in twenty-four hours. However, no method has been found to purge the clams of pea crabs. Careful selection of harvest sites and harvesting before the clams get over 40mm in length may be the only way to minimize pea crab infestation. The minimum harvest size for manilas in Washington is 1.5" or 38mm. Targeting smaller varnish clams could lead to incidental harvest of undersize manilas. It is also unknown whether a smaller clam will be as marketable.

Varnish clams in dry storage retain water better than manila clams, and have a longer shelf life. According to Dr. Paul Dinnel, the clams can be dry stored in a refrigerator for up to 30 days with no significant mortality. Clams held in wet storage (sub-tidal or in a tank) increased their weight by almost 9% over a 9-day period (Gillespie, 2002). These factors, coupled with the overall aesthetic attractiveness of the clam contribute to a good potential fishery.

The clams were featured in an “Iron Chef” demonstration at a food show in Vancouver B.C., with good response from persons attending. There was also a trial market, through one processor, in restaurants in the Vancouver and Victoria area. The response was favorable, in spite of fluctuations in product availability. The clams were sold to the restaurants at a price equal to higher than that of manila clams.

However, when a trial wild harvest was conducted, processors were reluctant to develop a market without assurance of a consistent supply. Prices offered were less than 60% of the price for manila clams (Gillespie, 2002). Clearly, the economic potential has not been fully evaluated, and a number of management questions remain unresolved.

Theoretically, size limits may help limit crab infestation, and contribute to an ongoing harvest over the years. However there is inadequate information available to accurately determine an appropriate size limit, or how effective setting a limit may be. Setting a limit similar to that set for manila harvest may be a method that would allow combined harvests. However, it remains unknown what the impact will be on manila clams in areas where both species are harvested together. In overlapping areas it may be possible to harvest the manila without disturbing the deeper varnish clams. If this occurs, will the varnish clams flourish and create a negative impact on future manila harvests? The proposed small-scale limited fishery proposed should provide answers to these and other management questions.

Human Health Concerns

The fact that the purple varnish clam is a bi-modal feeder raises human health concerns. Biotoxin events currently tend to be related to seasonal algal bloom activity. However, Alexandrium catenella is known to form an encysted resting stage that settles to the bottom during cooler winter months. These deepwater areas may serve as seed beds that occasional upwelling or storm activity may disturb and carry back up into the photic zone to create offshore plankton blooms. It is thought that such a seed bed may exist beyond the mouth of the Strait of Juan de Fuca.

A study of the toxicity in resting cysts of the red-tide dinoflagellate Gonyaulax escavata from deep water sediments off the coast of Maine (Dale, et al 1978) found the cysts to be approximately ten times more toxic than the motile stages. There is some concern that cysts could be ingested by shellfish, particularly by deposit feeders. This would explain reports of PSP that occur when there are little or no motile dinoflagellates observed in the water, and times when toxification levels far in excess of maximum published rates have been observed.

Although Claudia Mills has collected varnish clams from San Juan and Samish Bay at PSP monitoring sites in recent years, there is very little data available. In 1998 varnish clams were collected for testing in an area in B.C. where a PSP bloom had occurred, and sea mussels (Mytilus trossulus) used for monitoring peaked at 960 micrograms per 100 grams of weight. Although the clams never exceeded the action level of 80 micrograms of PSP toxin per 100grams, there were indications of higher uptake. The clams were tested twice, once when sea mussels measured at 870Fg/100g and once when the sea mussels measured 210Fg/100g. Varnish clams tested at 77Fg/100g and 62Fg/100g respectively. This was a markedly higher uptake than that of manila clams, which tested at 44Fg/100g the first time and less than 42Fg/100g the second time (Northwest Fisheries Science Center, 1998). Tim Determan, from the Department of Health Office of Food Safety and Shellfish Inspection, felt that the facts taken together suggest that varnish clams have the potential to contain toxins year around, and that more vigilance by DOH biotoxin monitoring staff may be necessary.

The fact that the most abundant populations of the clam are found in areas where there are significant influxes of fresh water may create other human health issues. At certain times of the year these sites may include stormwater runoff. In developed areas stormwater frequently contains fertilizers, pesticides, oil and grease, and heavy metals such as copper zinc and lead, as well as pet waste. Even relatively undeveloped areas may have considerable animal waste washed into them by runoff. These waste products contain bacteria such as fecal coliform and fecal streptococcus, and may contain viruses as well. Bacterial contamination has resulted in shellfish contamination and closed beaches in many areas of the state. Further study is necessary to determine whether varnish clams, due to their propensity to settle in runoff areas and their bimodal feeding habits, may ingest unsafe levels of bacteria from the sediments before the contamination is high enough to impact other shellfish and overall water quality.


Interviews with local shellfish growers indicate that they are not concerned about the varnish clam becoming an aquatic nuisance species. Member states of the Western Regional Panel have taken a stance that the development of a fishery is not an acceptable management tool for aquatic nuisance species. However, the clam has already become well established in British Columbia and throughout Puget Sound, and it is unlikely that any sort of control effort could ever eradicate it.

Will a fishery be considered in Washington?

Dense populations in Washington waters are presently confined to a relatively small area. However they are being found more and more frequently on beaches in Admiralty Inlet, North Puget Sound, and in Hood Canal. There may be populations building along our coastal beaches as well, a live varnish clam was found in Willapa Bay recently. More than one researcher has suggested that developing a fishery now while the distribution is limited might fish it out, or at least minimize further spread. As of May first 2004 the species is classified as shellfish under WAC 220.12.020 and may be harvested as all or part of the 40-clam per day recreational limit. It is unlikely, for a number of reasons, that WDFW will create a commercial fishery for the varnish clam at any time in the near future. The director would have to recommend to the Fish and Game Commission that limits be set, and a cooperative management plan for the fishery would have to be developed with the Tribes. There is also the concern that if a fishery were developed, the public may be inclined to spread the organism for purposes of enhancing the fishery. However, the clam usually occurs so high up in the intertidal zone that the average individual clamming for other recreational species is not likely to find them without publicity and public education efforts.

Considering the fact that the mussel has spread throughout Puget Sound far more quickly than anticipated , the question of a commercial fishery is one that may need to be addressed at some time. If or when it is, Washington’s system for developing an experimental or emerging commercial fishery differs somewhat from Canada’s.

The director of the Department of Fish and Wildlife may declare an emerging commercial fishery by rule. He then appoints a five-person advisory board representative of the affected fishery to review available information and make recommendations on rules relating to the number and qualifications of the participants eligible for permits under RCW 77.70.160. Participants provide the agency with harvest information. This method provides scientific information necessary to manage an emerging fishery while preserving and protecting the resource. After five years of operating an experimental fishery, the director must report on the fishery and make recommendations regarding its future operation to the appropriate legislative committees.

Conversely, the director may make it a trial fishery and not limit the number of participants. In this case he may classify the species by rule as a food fish or shellfish. He may then issue trial fishery permits to holders of commercial fishing licenses. Applications for trial commercial fishery permits specify the area to be fished, and the methods to be used. A trial commercial fishery may be re-designated as an emerging or expanding commercial fishery, or it may be closed at any time for conservation reasons (WAC 220-88-040).


I thank Graham E. Gillespie, Fisheries and Oceans Canada, for providing the data from the second phase of fishery development research, which has yet to be published, and for granting permission to cite both research documents. I also wish to thank Valerie Anderson (Research Apprentice, University of Washington School of Marine Affairs), Claudia Mills (Friday Harbor Labs, University of Washington), Tim Determan (Office of Food Safety and Shellfish Protection, Washington State Department of Health), Dr. Paul Dinnel (Dinnel Marine Research), and Dr. David Duggins (Friday Harbor Labs, University of Washington) for sharing information regarding varnish clam distribution and ecology in Washington.


Byers, James, 2001. Physical Habitat Property Mediates Biotic Resistance to Nonindigenous Species Invasion. Oral presentation at the International Conference on marine Bioinvasions, April, 2001.

DFO, 1999, Varnish clams. DFO Science Stock Status Report C6-13 (1999).

Gillespie, G.E., B. Rusch, S.J. Gormican, R.Marshall, and D. Munroe, 2002. Further Investigations of the Fisheries Potential of the Exotic Varnish Clam (Nuttallia obscurata) in British Columbia. Pacific Scientific Advice Review Committee Working Paper I2001-09. (Unpublished).

Gillespie, G.E., M. Parker, and W.Merilees, 1999. Distribution, Abundance, Biology and Fisheries Potential of the Exotic Varnish Clam (Nuttallia obscurata) in British Columbia. Canadian Stock Assessment Secretariat Research Document 99/193.

Mills, C.E., 1999-2000. Nuttallia obscurata, the purple varnish clam or the purple mahogany-clam. Electronic internet document available at: Published by the author, web page established March 1988, last updated 23 August, 2000.

NOAA - Northwest Fisheries Science Center, 1998. Test Results Indicate Varnish Clams are Safe. West Coast Marine Biotoxins & Harmful Algal Blooms Newsletter. Electronic internet document available at:

Puget Sound Gillnetters Association, 2001. New clam appears on Oregon coast.

Sea Grant, 2001. MIST Aquatic Non-native Species Update.

Prepared for the Aquatic Nuisance Species Committee
by Pamala Meacham, Washington Department of Fish and Wildlife

For more information on fish management issues,
please contact WDFW Fish Management Program.