Nuttallia obscurata, the purple varnish clam
or the purple mahogany-clam

Photo by Claudia Mills of specimens collected at Eastsound, Orcas Island, San Juan Archipelago, Washington

This information is provided by Claudia Mills of the University of Washington Friday Harbor Laboratories, who has been working on the Nuttallia invasion in Washington State since December 1997. Most of the content of this page is still unpublished, and is provided here as an information service. If you use this information in a project I would appreciate your citing this page as follows: Mills, C.E. 1999-present. Nuttallia obscurata, the purple varnish clam or the purple mahogany-clam. Electronic internet document available at Published by the author, web page established March 1998, last updated (see date at end of page).

This asian clam is a recent migrant into Washington and British Columbia waters. It is readily distinguished from resident clams by most naturalists and experienced clam diggers. The shell is fairly flat (low volume), with a brown coating, the "periostracum," which is typically peeling off a bit, like old varnish. The inside of the shell is usually a uniform purple color (especially when wet), unlike any other clams on Washington beaches. It has already acquired a series of common names, from which I have selected the "purple varnish clam" as the name most worth promoting (I have also been admonished for not adhering to the common name already assigned (but not in common use!) by the American Fisheries Society, the purple mahogany-clam -- but "common names," by definition, are the names people actually use, not those that they are told to use by some group of distant authorities). This clam is presently being called (mostly in writing, as opposed to on the street) the varnish clam, the purple varnish clam, the purple mahogany-clam, the dark mahogany-clam and the dark mahoganyclam; in 2002 it began to be marketed in Washington state as the "savory clam". There is another commercial clam available in Washington State markets (at Costco, for instance) that is already called the mahogany clam, which is not the same as, or even closely related to, this recent arrival to our beaches. (The mahogany clams in Seattle markets are juvenile Arctica islandica, or Atlantic Ocean Quahogs, according to Alan Kohn of the University of Washington.) In order to avoid confusion with the commercial mahogany clam, I favor use of the common name purple varnish clam in reference both to its shiny brown outer coat and the usually purple-colored inside surface, but as is readily evident from the above list of available common names, the use of a scientific name is often the least confusing form of identification. The designation "savory clam" is a marketing decision, that is not particularly descriptive or apt, so I choose at this time not to use it.

Nuttallia obscurata is believed to have arrived in the Strait of Georgia region (near Vancouver, British Columbia) in ballast water ** (a by-product of shipping) in the late 1980s. The earliest references to its presence on the west coast were published in the Vancouver Shell News and the Pacific Northwest Shell Club Dredgings, both late in 1993, reporting shell collections by Robert Forsythe in August 1991 near Vancouver (actually the second newsletter erroneously reports the collections as 1989 - I checked with Mr. Forsyth who confirms that he found it only in 1991). Its spread northward into the Strait of Georgia appears to have been quite rapid and it was already quite thoroughly distributed by the time scientists became aware of it. In a report dated January 1997 at the Washington State website for non-indigenous species Glen Jamieson of the Department of Fisheries and Oceans in Nanaimo, Canada, described its distribution in British Columbia to be from Quadra Island to Saanich Inlet; he further guessed that it may also be distributed throughout the Strait of Georgia and on the southern aspect of Vancouver Island along the Strait of Juan de Fuca. Nuttallia obscurata is also established in Barkley Sound on the outside of Vancouver Island.

As of summer 1998, we knew that Nuttallia obscurata has crossed the Strait of Juan de Fuca into Washington and it had been found on the Olympic Peninsula at least near Neah Bay, Clallum Bay, Piller Point, Port Townsend, Sequim, and Hansville (the last reported in a summer 1998 Pacific Shell Club Dredgings as collected by Keith Daniels), but it appears to be spreading south much more slowly than it moved north. The Puget Sound Expedition found about 20 dead Nuttallia shells on the beach to the left of the Port Ludlow Marina, on September 10, 1998. By summer 1999, shells had also been collected at both Hood Head and Foulweather Bluff, both at the top of Hood Canal; spring 2004 is the first time that I have been aware of its spread further south into Hood Canal, at least as far as Seabeck (thanks, Julie). This species is also now scattered throughout the San Juan Islands to varying degrees from rare to very abundant (up to 700 per square meter) on different beaches (presence of dead shells on about 30 San Juan County beaches documented by August 1999 - C.E. Mills), and has moved down the mainland from the Vancouver area into Washington State from Boundary Bay and Birch Bay at least as far south as Padilla Bay and Fidalgo Bay. We now know it was observed in Washington as early as 1993 in at least 4 locations - Cowlitz Bay on Waldron Island, West Beach on Orcas Island, and in Discovery Bay and Sequim Bay on the Olympic Peninsula and was present in Padilla Bay on the mainland and Spencer Spit on Lopez Island at least by summer 1996.

In addition to Barkley Sound on the outer coast of Vancouver Island, N. obscurata is now known from the outer coast of both Washington and Oregon. This clam made the front page of the Sports section of The Oregonian (Portland newspaper) on March 22, 2001. In that article, John Johnson of the Oregon Department of Fish and Game, speculated that purple varnish clams were probably in "every coastal estuary" in Oregon; I know positively that this species has been collected as far south as Coos Bay (reported by Richard Emlet). The first N. obscurata on the Washington outer coast was discovered in Willapa Bay in February 2002 (reported to me by Brett Dumbauld). Whether it is spreading north or south (or both) on the outer coast is a question, as is how many separate introductions of N. obscurata might have occurred on our coast.

In the Strait of Georgia and the San Juan Archipelago, Nuttallia obscurata occurs in the high-mid intertidal in sediments ranging from cobbles to muddy sand, above (in tidal height) or near the native littleneck (Protothaca staminea) and earlier-introduced Manila clam (=Venerupis philippinarum = Ruditapes japonica), often 8-10 inches deep. It is a suspension feeder, with quiescent siphons held open just at the sediment surface. In some areas, it seems especially numerous near freshwater runoffs, and often contains a pea crab inside its shell (Harbo,1997; Mills observations). The largest Nuttallia that I know of in Washington was at least 75 mm in maximum dimension (conservative estimate from broken shell), collected from Madrona Point, Eastsound, Orcas Island, by Winslow Moran-Hodge in June 1999. Nuttallia shells in the San Juan Islands are frequently pitted on the interior surface (see poorly illuminated photo above - sorry). Dead shells on beaches are usually broken, indicating predation by birds or crabs, and probably racoons, and are mostly easily noticed by their unusual purple inside color. (The shell interiors became somewhat orange in 3 specimens that I microwaved to remove the animals and small numbers of these clams with orange interiors have been collected from the shore). Click here for another article on Nuttallia in the Strait of Georgia region, written by Phil Lambert, invertebrate curator at the Royal British Columbia Museum.

It should be noted by people interested in eating Nuttallia that we still know relatively little about this species, since it is quite new to North America. I regularly sent in Nuttallia samples for Paralytic Shellfish Poison/Toxin (PSP or PST) testing to the Washington State Shellfish Lab between April 1999 and July 2001. These clams do store the PSP toxin, so collectors should heed Hotline warnings before considering a meal - in Washington telephone 1-800-562-5632. N. obscurata clams were "hot" enough with PSP toxin in early July 2001 to close Griffen Bay, San Juan Island, to shellfish collection.

As of winter 2001-2002, N. obscurata is being sold commercially at least in an Asian market in Richmond, British Columbia, under the name Savory Clam (reported by George Holm in The Dredgings - Pacific Shell Club Newsletter, Nov.-Dec. 2001).

Nuttallia obscurata is native to Japan, Korea, and perhaps China - its distribution is somewhat unclear because of historic taxonomic confusion among closely related species of clams in Asia. It is not valued as a commercial species and apparently does not appear in markets in Japan, although it is eaten locally there. Little seems to have been written about its biology in its native range. It apparently does best in clean water and in Japan is common near freshwater (river) runoffs, as it is in British Columbia (seeps). It has apparently been eliminated by pollution from several regions where it was once common (Y. Hirano, Japan, personal communication to C.E. Mills).

There is one related west coast species, Nuttallia nuttallii (which used to be called Sanguinolaria nuttallii) that occurs from central California to Baja California. This clam is a non-selective suspension feeder that keeps its siphons slightly below the surface of the sand or muddy bays that it occupies (Pohlo, 1972). N. obscurata has now been found as far south as some Oregon coastal beaches - will N. obscurata eventually make its way down into the range of N. nuttallii in central California?

Any further reports of the purple varnish clam on Washington or Oregon beaches are welcome. Please send email with this information to

This site is maintained by C. E. Mills and all photographs are copyrighted by the author
** This page was established March 1998; last updated 7 June 2004 **

** Ballast Water - A Primer

The Problem

Ballast water is the primary mechanism moving exotic marine and freshwater organisms around the world today, and has introduced many types of exotic organisms into California, Washington, and elsewhere.

It is against the law to introduce exotic organisms into California waters. Yet every day ships dump exotic species into the San Francisco Bay and Delta and southern California ports, amounting to hundreds of species and millions of organisms each year. Legislation concerning ballast water in Washington is less specific at the moment, and ballast water continues to be dumped in Washington waters, just as it is dumped daily into the waters of all other coastal states.

What is Ballast Water?

At the start of a voyage, a ship may take on large quantities of water to achieve proper buoyancy and trim. Later when it is no longer needed, the waste ballast water is typically dumped on arrival at the destination port, sometimes thousands of miles from its source. Studies show that ballast water carries a daunting variety of living marine or freshwater organisms.

What has Ballast Water Introduced into California?

Ballast water discharges have released Japanese gobies, New Zealand sea slugs, Black Sea jellyfish, dozens of Asian zooplankton species, possibly the Chinese mitten crab, and scores of other organisms into the San Francisco Estuary. Other species have been introduced into Los Angeles/Long Beach and San Diego harbors. The amount of foreign ballast water arriving at these ports will increase sharply with port expansions and with the projected rapid growth in international markets and trade. Many exotics that first arrive at these major shipping ports then spread or are reintroduced to other sites along the coast.

The asian clam Potamocorbula was introduced by ballast water discharges into San Francisco Bay in 1986. Within a year it was the most abundant clam in the northern part of the system, filtering the entire water column twice a day, destroying phytoplankton blooms and disrupting the food webs that support fish in the Estuary. It has now become evident that Potamocorbula also acts as a contaminant vector, concentrating and directing the toxic metal selenium into the diets of fish and birds, which have accumulated enough selenium to cause reproductive defects. (It was selenium poisoning that caused the epidemic of bird deformities in the 1980s that closed Kesterson Wildlife Refuge in California.)

What has Ballast Water Introduced into Washington?

We are just getting started on field-based inventories of the non-indigenous fauna in the marine waters of Washington State. A round-the-Sound field trip in early September 1998 (The Puget Sound Expedition 1998, including more than 10 marine taxonomic specialists and several local generalists) made a rapid assessment for alien fauna and flora at about 25 sites, primarily marinas, to ground-truth literature-based surveys of Washington's non-indigenous marine species. The Expedition team identified 39 nonindigenous species in this brief study. Nuttallia obscurata, the purple varnish clam, is probably the most spectacular ongoing invasion in inland Washington marine waters, yet is little-known even by most marine biologists, and continues to get almost no publicity (especially when compared to the green crab, which has not yet reached the inside marine waters of Washington). It is thought that Nuttallia arrived in the Strait of Georgia/Puget Sound region via ballast water in the late 1980s; in some localized areas, the N. obscurata population is now greater than 500 clams per square meter.

The green crab Carcinus maenas is another recent arrival in Washington, first sited at Willipa Bay in July 1998. It is thought that this crab became established in San Francisco Bay in the late 1980s, arriving either via ballast water or by release of adults inadvertently packed with other invertebrates such as baitworms arriving from the east coast of the U.S. The green crab has moved up the west coast from San Francisco Bay much faster than was predicted based on larval transport in coastal currents and it is very possible that it has been separately introduced recently into both Washington and Oregon via ballast water, in addition to arriving here by spreading northward from central California. I have read in one publication that "legislative solutions have been sought to slow the invasion" of the green crab. Once populations become established, legislation is not not likely to be an effective deterent to new invertebrate residents of Puget Sound or our outer coast shores.

Aside from Ecological Issues, is there Cause for Concern?

Impacts on industry. The European zebra mussel was introduced in ballast water into the Great Lakes in the 1980s and became a massive nuisance, causing billions of dollars of damage by clogging the water systems for cities, factories and power plants; fouling boat hulls and maritime structures and sinking navigational buoys; and accumulating on recreational beaches, fouling them with sharp-edged mussel shells and rotting mussel flesh. Ballast water discharges into the Delta are one of two main mechanisms by which zebra mussels may eventually be introduced into California. Recent studies show that major portions of the California water system- including the state and federal water project aqueducts and the Colorado River aqueduct- are environmentally and chemically suitable for zebra mussel colonization.

Impacts on commercial fisheries. A voracious Atlantic comb jelly (Mnemiopsis leidyi) has become a principal consumer of zooplankton in the Black Sea and was apparently an important factor in the demise of the region's anchovy fishery for all six adjacent countries. A North Pacific sea star (Asterias amurensis) has devastated shellfish in Tasmania. Both were introduced in ballast water. Toxic red tides introduced by ballast water have closed clam and mussel farms and fisheries. A recent red tide outbreak in New Zealand was so severe that people breathing the sea air became ill. Both the purple varnish clam (Nuttallia obscurata) and the green crab (Carcinus maenas) are new to Oregon and Washington State marine waters. It is estimated that $44 million in annual fisheries revenues in these two states are now vulnerable to the destructive effects of these two new invaders (Science News, June 13 1998, p. 373). The actual economic effect of their arrival will not be evident for several years, perhaps a decade or more.

Impacts on human health. Beside red tides, we now know that ballast water can transport cholera around the world. During the 1991 South American cholera epidemic, the bacterium that causes cholera was discovered in oysters and fish in Mobile Bay, Alabama, and subsequently found in the ballast water in a third of the ships arriving from South America. Cholera bacteria were found in the ballast water in 15 of 15 ships arriving from the North Sea or the Mediterranean Sea and tested in the Chesapeake Bay in 1998 (Tonya Rawlings et al., Smithsonian Environmental Research Center).


The immediate need: ballast exchange. Ships coming from foreign ports can be required to exchange their ballast water over deep ocean water whenever it is safe to do so - this is referred to as mid-ocean or high-seas ballast exchange. While this is not a complete solution, it could substantially reduce the number of exotic organisms being released, at virtually no cost to the shipping industry. Regulations requiring mid-ocean exchange of foreign ballast have been adopted in the Great Lakes, in Prince William Sound and at the Port of Vancouver (but ships heading for the Port of Vancouver can be seen from San Juan Island to be exchanging ballast water into the inland marine waters of Washington!). Regulations have been in effect at some of these sites for several years, with no reported impacts on shipping and costs so low that no one has even bothered to calculate them.

Additional approaches needed. Because mid-ocean ballast exchange is not 100% effective, may not be safe under some weather conditions, may not be feasible for some vessels, and isn't an available option for coastwise shipping, other approaches need to be pursued. For example, it may be possible to offload ballast water from ships to onshore storage tanks, and then either reuse it for ballast, or treat it in existing wastewater treatment plants. Oil tankers that carry ballast water in cargo holds are already required to offload and treat their ballast water. Wastewater plants in the San Francisco Bay area routinely and cheaply disinfect much larger quantities of water, and the plants run below capacity during much of the year. Efforts should be made to adapt such off-the-shelf treatment technologies and facilities to ballast water treatment.

Long term solutions. In the longer term, new technologies may be developed that can be retrofitted to ships, or incorporated into ship design as fleets are replaced, as was done for double-hulled design for oil tankers. However, this will take decades. Many approaches have been suggested, and some pilot projects are underway.

We Have a Choice

In the Great Lakes, ballast water regulations were adopted after the zebra mussel invaded and devastated the region's water systems. If ballast water regulations had been in place earlier, the zebra mussel would never (might not?) have arrived.

Our options are clear. We can wait until after we've been hit by an economically catastrophic invasion to adopt ballast water regulations; or we can adopt them now, and prevent it.

This primer on ballast water was originally written by Andrew Cohen, San Francisco Estuary Institute, Richmond, CA (email: and was modified here by Claudia Mills.

This site is maintained by C. E. Mills and all photographs are copyrighted by the author
** This page was established March 1998; last updated 17 February 2002 **

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