Invasion History

First Non-native North American Tidal Record: 1991
First Non-native West Coast Tidal Record: 1991
First Non-native East/Gulf Coast Tidal Record:

General Invasion History:

The native range of Nuttallia obscurata extends from Guangdong Province, China and the South China Sea, north to Klyuchi Lake, Russia on the northern Sea of Japan (44°48'N) (Roth 1978; Huang 2001; Kolpakov and Kolpakov 2005). On the Pacific coast of Japan, it occurs at least as far north as Mutsu Bay, at the northern tip of Honshu (Roth 1978). This bivalve shows a preference for estuaries and sites near freshwater seeps (Gillespie et al. 1999; Kolpakov and Kolpakov 2005). In 1991, this clam was discovered in Blaine, Washington, at the Washington-British Columbia border. It has extended its range north into the Strait of Georgia and south into Puget Sound, Willapa Bay, and as far as Coos Bay, Oregon (Mills 2002; Dudas and Dower 2006).

North American Invasion History:

Invasion History on the West Coast:

The first record of N. obscurata in North American waters was from Semiahmoo Bay, Blaine, Washington (WA) in 1991, on the US-Canada border (Forsyth 1993, cited by Dudas 2005; USGS Nonindigenous Aquatic Species Program 2007). It is presumed to have been introduced as larvae in ballast water from ships entering the nearby Port of Vancouver. By 1998, this clam had dispersed through much of the southern Strait of Georgia and Strait of Juan de Fuca, including the San Juan Islands (1993), Discovery and Sequim Bays (1993), Padilla Bay (1996), and Clallam and Neah Bays, on the Olympic Peninsula (Mills 1998-2004). In Puget Sound, N. obscurata was first found in 1998 in Port Ludlow at the mouth of the Hood Canal (Cohen et al. 1998; Mills et al. 1998). It is now widespread in the Sound (Eissinger 2009). To the north, it has spread up the Strait of Georgia as far as Smith Sound, on mainland British Columbia, north of Vancouver Island (51° 17.07’ N), and up the entire Pacific coast of Vancouver Island (Dudas and Dower 2006; Gillespie 2007; Gillespie et al. 2007). To the south, N. obscurata was collected in Willapa Bay, WA in 2002 (Mills 1998-2004) and in estuaries along the coast from the mouth of the Columbia River south to Coos Bay, Oregon in 2003 (Mills 1998-2004; Sytsma et al. 2004; USGS Nonindigenous Aquatic Species Program 2007). We do not know if the Varnish Clam has spread further south. One record, from Elkhorn Slough, California (Wasson et al. 2005), was of a single shell and could have been from discarded bait or seafood, or a misidentification of N. nuttalli (Kerstin Wasson, personal communication).


Nuttallia obscurata is a bivalve with an oval shell and an umbo which is located slightly to the anterior of the midpoint of the shell. The anterior and posterior ends of the shell are about equally rounded, and the two valves are about equally deep. The pallial sinus does not extend anteriorly to the beak. The hinge ligament is broad, nearly straight, and slopes at an angle to the shell. The shell is marked with concentric ridges. The exterior of the shell is covered with a dark brown periostracum, which is worn through in places, exposing the white shell. The interior of the shell is pearly in texture and deep purple. Adults are 16-65 mm in length (Roth 1978; Mills 2001; Meacham 2002; Coan et al. 2000; Coan and Valentich-Scott 2007; Eissinger 2009). Larval development was summarized by Dudas et al. (2006), but without illustrations. Larvae settle at about 180-200 µm (Dudas et al. 2006).


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Mollusca
Class:   Bivalvia
Subclass:   Heterodonta
Order:   Veneroida
Superfamily:   Tellinoidea
Family:   Psammobiidae
Genus:   Nuttallia
Species:   obscurata


Nuttallia japonica (Kira, 1953)
Nuttallia solida (Kira, 1953)
Psammobia olivacea (Jay, 1857)
Soletellina obscurata (Reeve, 1857)

Potentially Misidentified Species

Nuttallia nuttallii
Nuttall's Mahogany Clam, Native, Pacific Coast, Bodega Bay to Baja California, adults ~62 to 127 mm, valves unequal, posterior end tapered, interior white to light purple



Nuttallia obscurata is a burrowing bivalve, found in sand, gravel, and mud, often in the upper intertidal and in areas affected by freshwater inflows. In British Columbia, these clams occurred on beaches from the low-water tide line to the lower-high tidal zone (1.1-3.0 m above chart datum), with peak abundances at 1.66 m (Gillespie et al. 1999). A similar pattern was observed in the San Juan Islands, with clams occurring at 1.0 - 2.0 m, and peaking at 1.6. Predation by crabs, primarily Cancer productus, appeared to limit occurrence in low-tide and subtidal zones (Byers 2005). Varnish Clams burrow 7 to 30 cm deep in the substrate (Gillespie et al. 1999). However, N. obscurata does occur in shallow (0.2-1.5 m depth) subtidal habitats in Russian Pacific estuaries. In Klyuchi Lake, Russia, this clam occurred at 8-22 PSU (Kolpakov and Kolpakov 2005), but this clam is common at more marine salinities (30-35 PSU) (Gillespie et al. 1999; Mills et al. 2004; Gillespie et al. 2007).

Two populations of N. obscurata in British Columbia were slightly (46:49) or strongly (56:39) skewed towards males, with 4-5% hermaphrodites (Dudas and Dower 2006). The clams mature at 1 year old (~16-23 mm), and have a life span of at least 5-6 years (Gillespie et al. 1999; Dudas et al. 2006; Dudas and Dower 2006) and possibly as much as 10 years (Kolpakov and Kolpakov 2005). Fecundity of average-sized females (40-45 mm) is ~1 million eggs, but can reach 6 million eggs for clams 60-65 mm in length (Dudas and Dower 2006). Eggs and sperm are broadcast into the water column. The eggs hatch into trochophores, which develop into veligers and pediveligers, settling at 180-200 mm size in 33 days at 15°C and 19 days at 20°C (Dudas and Dower 2006).

Nuttallia obscurata is capable both of filter-feeding on phytoplankton and deposit-feeding on organic material in the sediment (Gillespie et al. 1999; Meacham 2004). In the laboratory, N. obscurata extends its siphons and filter-feeds at night and deposit-feeds in the sediment by day (Meacham 2004), a pattern which could reduce the risk of being preyed upon by visual predators. Predators include crabs (Cancer productus and Metacarcinus magister), diving ducks [Melanitta perspicillata (Surf Scoter) and M. fusca (White-Winged Scoter)], and shorebirds [Black Oystercatchers (Haematopus bachmanni), gulls, and crows] (Gillespie et al. 1999).


Phytoplankton, detritus


Crabs, sea ducks, humans

Trophic Status:

Deposit Feeder



General HabitatUnstructured BottomNone
Salinity RangeMesohaline5-18 PSU
Salinity RangePolyhaline18-30 PSU
Salinity RangeEuhaline30-40 PSU
Tidal RangeSubtidalNone
Tidal RangeLow IntertidalNone
Vertical HabitatEndobenthicNone

Tolerances and Life History Parameters

Minimum Temperature (ºC)0Based on geographical distribution (Kolpakov and Kolpakov 2005)
Minimum Salinity (‰)8Field, Kolpakov and Kolpakov 2005
Maximum Salinity (‰)35Typical marine salinities (Gillespie et al. 1999)
Minimum Reproductive Temperature15Experimental, larval tolerance, Dudas and Dower 2006
Maximum Reproductive Temperature20Experimental, highest tested, larval tolerance (Dudas and Dower 2006(
Minimum Reproductive Salinity15Experimental (Dudas and Dower 2006)
Maximum Reproductive Salinity35Based on field distribution.
Minimum Duration19Larval development, at 20 C (Dudas and Dower 2006)
Maximum Duration33Larval development, at 15 C (Dudas and Dower 2006)
Minimum Length (mm)16Smallest male observed; smallest female was 23 mm (Dudas and Dower 2006).
Maximum Length (mm)68Gillespie et al. 1999
Broad Temperature RangeNoneCold temperate-Warm temperate
Broad Salinity RangeNonePolyhaline-Euhaline

General Impacts

Nuttallia obscurata has spread rapidly in the Pacific Northwest, since its first collection in 1991, and developed high densities in much of its range. Its economic and ecological impacts may still be expanding with its range and population growth (Gillespie et al. 1999; Meacham 2004; Carswell et al. 2006).

Economic Impacts

Fisheries- Recreational fisheries for Purple Varnish Clams have developed from British Columbia to Oregon, and some commercial fishing occurs in British Columbia, mostly as bycatch from the Manila Clam/Japanese Littleneck (Venerupis philippinarum) fishery. Young Japanese Littleneck clams are farmed by directly seeding shores and protecting them from predators by plastic netting. Varnish Clams are taken incidentally during harvesting (Carswell et al. 2006), and are marketed in Canada as 'Savoury Clams'. In US waters, N. obscurata is harvested by recreational clammers. In general, Purple Varnish Clams appear to be well-regarded for their size and flavor (Meacham 2004, numerous websites).

Health- In the Pacific Northwest, red tides and other toxic algae are a major concern for wild and cultured bivalves. The harvested species are monitored by government agencies for toxins causing several types of shellfish poisoning, and sections of shoreline are closed to harvesting when toxins are detected. Nuttallia obscurata may retain toxins at different times of year from other clams, since as a deposit feeder it can ingest dinoflagellate cysts in the sediment, between red tides, and may require testing year-round (Gillespie et al. 1999; Meacham 2004). Because this clam often occurs in estuaries and freshwater seeps, it may be at greater risk than other clams of accumulating toxins such as metals and pesticides from rivers and groundwater (Gillespie et al. 1999; Meacham 2004).

Ecological Impacts

Nuttallia obscurata overlaps to some extent with native and other exotic clams Mya arenaria (Softshell Clam) and Venerupis philippinarum (Japanese Littlenecks, Manila Clams), but because of its deposit-feeding capabilities, resistance to desiccation, and deeper-burrowing habits, it ranges higher up in the intertidal zone than other clams. As a facultative deposit-feeder, it overlaps with other clams, to some extent, in feeding (Gillespie et al. 1999; Byers 2005; Carswell et al. 2006). In the lower intertidal and subtidal zones, the Varnish Clams thin shell makes it more vulnerable to predators than other clams, and so in many regions, it is confined to a narrow band of shore (Gillespie et al. 1999; Byers 2005; Carswell et al. 2006).

Food/Prey- Nuttallia obscurata has developed dense populations in the intertidal zone in many parts of its range and has become a potentially important food source for shorebirds, diving ducks, crabs, and probably other land-based and aquatic predators as well (Gillespie et al. 1999; Lewis et al. 2007; Byers 2005). However, it is not clear whether the exotic clam has altered the abundance or distribution of predators.

Regional Impacts

NEP-IIIAlaskan panhandle to N. of Puget SoundEconomic ImpactFisheries
The fisheries potential of N. obscurata in British Columbia was examined by Gillespie et al. (1999). They are harvested by recreational clammers, and taken commercially in harvests of Venerupis philippinarum (Japanese Littlenecks, Manila Clams) (Carswell et al. 2006). Websites indicate that Varnish Clams are often sold as 'Savory Clams' and are well-regarded for their flavor. In Washington State, this clam was opened to recreational fisheries in 2004 (Meacham 2004).
NEP-IIIAlaskan panhandle to N. of Puget SoundEcological ImpactFood/Prey
In Bayne Sound, Vancouver Island, British Columbia, Purple Varnish Clams (Nuttallia obscurata) and Japanese Littleneck Clams (Venerupis philipinarum) were the primary food items of the wintering diving duck species Melanitta perspicillata (Surf Scoter) and M. fusca (White-Winged Scoter) and appeared to be preferred over the native Macoma spp. (Lewis et al. 2007). It is not clear whether the exotic clams have altered the distribution or abundance of these migratory ducks. At low tide, Varnish Clams are eaten by shorebirds, including Black Oystercatchers (Haematopus bachmanni), Glaucous-Winged Gulls (Larus glaucescens) and Northwestern Crows (Corvus caurinus) (Gillespie et al. 1999). Experiments indicate that N. obscurata is very vulnerable to predation by crabs (Cancer productus, Red Rock Crabs and Metacarcinus magister, Dungeness Crabs) (Byers 2005; Dudas et al. 2006), but its not clear whether the clam invasion has had any effect on crab populations.
NEP-IVPuget Sound to Northern CaliforniaEconomic ImpactFisheries
Local websites indicate that this clam in harvested recreationally in Oregon Bays: Siletz Bay, (, Netarts Bay (, and Nehalem Bay (
NEP-IIIAlaskan panhandle to N. of Puget SoundEconomic ImpactHealth
As with other native and introduced bivalves, red-tide blooms of toxic dinoflagellates can cause Varnish Clams to accumulate toxins that can cause Paralytic Shellfish Poisoning (PSP). Nuttallia obscurata is a facultative deposit feeder, and is likely to encounter dinoflagellate cysts in sediments. Since it often occurs in estuarine habitats, it has the potential to accumulate other toxins from polluted sediments, including heavy metals and fecal bacteria (Gillespie et al. 1999; Meacham 2004).
P250Nehalem RiverEconomic ImpactFisheries
Local websites indicate that this clam is harvested recreationally in Nehalem Bay, Oregon (
P230Netarts BayEconomic ImpactFisheries
Local websites indicate that this clam is harvested recreationally in Netarts Bay, Oregon (
P220Siletz BayEconomic ImpactFisheries
Local websites indicate that this clam is harvested recreationally in Siletz Bay, Oregon (
NEP-IIIAlaskan panhandle to N. of Puget SoundEcological ImpactHabitat Change
In experiments on Denman Island, in the Strait of Georgia, British Columbia, high densities of varnish clams increased ammonium concentrations, and changed the particle composition by increasing the content of fine sand and silt. The increaed ammonium and silt in the sediments could contribute to phyplankton blooms and turbidity (Chan and Bendell 2013).

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
NWP-4a None 0 Native Estab
NEP-III Alaskan panhandle to N. of Puget Sound 1991 Def Estab
NEP-IV Puget Sound to Northern California 2002 Def Estab
NWP-3b None 0 Native Estab
P170 Coos Bay 2003 Def Estab
P297 _CDA_P297 (Strait of Georgia) 1991 Def Estab
P293 _CDA_P293 (Strait of Georgia) 1996 Def Estab
P292 _CDA_P292 (San Juan Islands) 1993 Def Estab
P288 _CDA_P288 (Dungeness-Elwha) 1993 Def Estab
P286 _CDA_P286 (Crescent-Hoko) 1998 Def Estab
P290 Puget Sound 1998 Def Estab
P270 Willapa Bay 2002 Def Estab
P250 Nehalem River 1999 Def Estab
P230 Netarts Bay 2003 Def Estab
P226 _CDA_P226 (Wilson-Trusk-Nestuccu) 2003 Def Estab
P210 Yaquina Bay 2003 Def Estab
P200 Alsea River 2003 Def Estab
NEP-V Northern California to Mid Channel Islands 2001 None Unk
P080 Monterey Bay 2005 Def Unk
NWP-3a None 0 Native Estab
NWP-2 None 0 Native Estab
P260 Columbia River 2003 Def Estab
NWP-4b None 0 Native Estab
P220 Siletz Bay 2008 Def Estab
NWP-5 None 0 Def Estab

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude


Baldwin, Andy; Leason, Diane (2016) Potential Ecological impacts of Emerald Ash Borer on Maryland's Eastern Shore, In: None(Eds.) None. , <missing place>. Pp. <missing location>

Burfeind, Dana D.; Pitt, Kylie A.; Connolly, Rod M.; Byers, James E. (2012) Performance of non-native species within marine reserves, Biological Invasions published online: <missing location>

Byers, James E. (2002) Physical habitat attribute mediates biotic resistence to non-indigenous species invasion, Oecologia 130: 146-156

Byers, James E. (2005) Marine reserves enhance abundance but not competitive impacts of a harvested nonindigenous species., Ecology 86(2): 487-500

Capa, María; Murray, Anna (2016) Combined morphological and molecular data unveils relationships of Pseudobranchiomma (Sabellidae, Annelida) and reveals higher diversity of this intriguing group of fan worms in Australia, including potentially introduced species, ZooKeys 622: 1-36

Carswell, Barron; Cheesman, Sean; Anderson, Josh (2006) The use of spatial analysis for environmental assessment of shellfish aquaculture in Baynes Sound, Vancouver Island, British Columbia, Canada, Aquaculture 253: 408-414

Chan, Kayi; Bendell, L. I. (2013) Potential effects of an invasive bivalve, Nuttallia obscurata, on select sediment attributes within the intertidal region of coastal British Columbia, Journal of Experimental Marine Biology and Ecology 444: 66-72

Coan, Eugene V.; Valentich-Scott, Paul (2007) The Light and Smith Manual: Intertidal Invertebrates from Central California to Oregon, University of California Press, Berkeley CA. Pp. 807-859

Coan, Eugene V.; Valentich-Scott, Paul; Bernard, Frank R. (2000) <missing title>, Santa Barbara Museum of Natural history, Santa Barbara CA. Pp. <missing location>

Cohen, Andrew N., Carlton, James T. (1998) Accelerating invasion rate in a highly invaded estuary., Science 279: 555-558

Cohen, Andrew; and 16 authors. (1998) <missing title>, Washington State Department of Natural Resources, Olympia, Washington. Pp. 1-37

Collado-Vides, L. (2002) Morphological plasticity of Caulerpa prolifera (Caulerpales, Chlorophyta) in relation to growth form in a coral reef lagoon, Botanica Marina 45: 123-129

Dudas, Sara Elizabeth (2005) <missing title>, Ph.D. Dissertation, University of Victoria, Victoria BC, Canada. Pp. <missing location>

Dudas, Sarah E.; Dower, John F. (2006) Reproductive ecology and dispersal potential of varnish clam Nuttallia obscurata, a recent invader in the Northeast Pacific Ocean., Marine Ecology Progress Series 320: 190-205

Dudas, Sarah E.; McGaw, Iain J. ; Dower, John F. (2006) Selective crab predation on native and introduced bivalves in British Columbia., Journal of Experimental Marine Biology and Ecology 325: 8-17

Eissinger, Ann (2009) <missing title>, Nahkeeta Northwest Wildlife Services, Bow WA. Pp. 1-26

Gillespie, G. E.; Parker, M.; Merrilees, W. (1999) Distribution, abundance, and fisheries potential of the exotic varnish clam Nuttallia obscurata in British Columbia., Canadian Stock Assessment Directorate Research Document 99/3: 1-40

Gillespie, Graham E. (2007) Distribution of non-indigenous intertidal species on the Pacific Coast of Canada, Nippon Suisan Gakkaishi 73(6): 1133-1137

2007 Distribution of nonindigenous intertidal species on the Pacific Coast of Canada.

Huang, Zongguo (Ed.) (2001) <missing title>, Krieger, Malabar, FL. Pp. <missing location>

Kolpakov, E. V.; Kolpakov, N. V. (2005) Population size-age structure and growth of the subtropical bivalve Nuttallia obscurata in Primorye waters at the north boundary of its geographic range, Russian Journal of Marine Biology 31(3): 164-167

Lewis, Tyler L.; Esler, Daniel; Boyd, W. Sean (2007) Effects of predation by sea ducks on clam abundance in soft-bottom intertidal habitats, Marine Ecology Progress Series 329: 131-144

2002-2004 Aquatic Nuisance Species: fisheries potential of the Purple Varnish Clam, <i>Nuttallia obscurata</i>. Web Page: Washington Department of Fish and Wildlife

1998-2004 <i>Nuttallia obscurata</i>, the Purple Varnish Clam or the Purple Mahogany-Clam.

Riggs, Sharon R. (2011) <missing title>, Padilla Bay NERR, Padilla Bay WA. Pp. 5

Roth, Barry (1978) On the identification of three Japanese species of Nuttallia,/i> (Mollusca: Bivalvia), Japanese Journal of Malacology 37(4): 223-229

Sytsma, Mark D.; Cordell, Jeffrey R.; Chapman, John W.; Draheim, Robyn, C. (2004) <missing title>, Center for Lakes and Reservoirs, Portland State University, Portland OR. Pp. <missing location>

2003-2015 Nonindigenous Aquatic Species Database. Gainesville, FL.

Wasson, Kerstin; Fenn, Katherine; Pearse, John S. (2005) Habitat differences in marine invasions of central California, Biological Invasions 7: 935-946