Types of west-coast clams
  After a general entry, a few of the more common species are presented in alphabetical order by genus.
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photograph of clams in a bottleMost west-coast clams inhabit sand/sand-mud/sand-gravel areas, and these are the types that we commonly visualise when we think of clams.  Other species, however, burrow into clay (Zirfaea pilsbryi), wood (Teredo navalis, Bankia setacea), rock (Penitella penita and other species of Pholadids), and into shells of abalone and other molluscs (Penitella conradi).  Other unusual habitats include burrows of ghost crabs (Cryptomya californica). 

 

 

Unidentified clams in a bottle. Note the several small
individuals clustering at the neck with their siphons
extended towards the bottle opening

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Mya

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Research study 1
 
photograph of soft-shell clam Mya arenaria
Soft-shell clam Mya arenaria 0.5X

graph comparing size and age of extinct and extant populations of soft-shell clams in Grays Harbor, WashingtonAn interesting account of the fate of soft-shell clams Mya arenaria introduced to areas of Washington State in the 1870s is provided by researchers at the School of Fisheries, University of Washington. Historical records indicate a rapid expansion during the 20yr period after introduction followed by a mass mortality that created the shell deposits seen today (see photograph middle Left).  The researchers describe extensive remains of the original population on intertidal flats of Grays Harbor, and note their large sizes in comparison with extant1 populations (up to 50% larger; see histograms lower Left).  Note in the graph at upper Right that the extinct populations grew faster and lived2 longer than present populations in the same area, suggesting that the original introductions were into prime habitat.  After their extinctions the populations never recovered in this prime habitat, despite a steady inflow of settling larvae.  In explanation, the authors suggest that preferential settlement of Dungeness-crab larvae into the shell deposits, leading to a high density of clam-eating juvenile crabs, may be the chief factor presently limiting clam recruitment. The authors provide fascinating historical accounts taken from government bulletins, early journal reports3, and newspaper articles, describing the original outplantings by ship’s captains and the later mass-mortality.  Palacios et al. 2000 Aquat Conserv: Mar Freshw Ecosyst 10: 279.

NOTE1 the authors sample 3 living populations, only one of which (South Channel) is represented here

NOTE2  shells are aged by counts of growth rings on the chondrophore (the spoon-shaped attachment site for the hinge ligament on the upper portion of the inner left valve).  Counts in this part of the shell are apparently more reliable than counts of external shell lines. Data for 2 extant populations are shown, combined into a single line

NOTE3  one of these reports describes the discovery of Mya arenaria in San Francisco Bay in 1874 and its subsequent spread along the eastern shores of the Bay.  By the time of publication of the 1881 article cited in this note the species had become the “leading clam” in the markets of San Francisco and Oakland.  Presumably, so taken was he by the species’ tastiness and cognizant of its ability to colonise new areas quickly, the author writes:

“…it would be a wise, public spirited act if the captains of our coasting vessels would take the trouble and incur the slight expense attending the planting of this clam at such points as their vessels touch at in the ordinary course of business.” Stearns 1881 The Am Nat 15 (5): 362.

photograph of "extinct" population of soft-shelled clams in Grays Harbor, Washington
Extinct population dating from the late 1870s
histograms comparing extinct and extant populations of soft-shell clams in Grays Harbor, Washington
 
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Nutricola

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Research study 1
 

drawing of clam Nutricola tantillaA detailed description of the morphology of the tiny clam Nutricola tantilla is available from research at the Pacific Marine Station, Dillon Beach, California, from which the following details are taken.  In Tomales Bay, California Nutricola lives in sand/mud areas to a depth of about 2cm.  The siphons are short, fused for part of their length, and the inhalent siphon is protected by tentacles.  The exhalent siphon has a constricting sleeve or valvular membrane which is retractable.  It presumably functions to extend the siphon above the mud/water interface and/or to increase flow velocity of the exhalent stream.  Adult specimens are observed to employ byssus threads (usually a single one per individual) attached to sand grains possibly, as noted by the author, a juvenile feature evolutionarily retained in the adult. These anchors may prevent this shallow-burrowing species from being washed away in tidal movements. Narchi 1970 The Wasmann J Biol 28: 233 and Narchi 1971 Bull Mar Sci 21: 866. 

NOTE  formerly Transennella tantilla.  This species has brood-protection, a topic presented in more detail in the section on REPRODUCTION & DEVELOPMENT

NOTE  the function of this sleeve might represent an interesting research topic, if it hasn’t been investigated already

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Nuttallia

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Research study 1
 

map showing limits of distribution of varnish clams Nuttallia obscurata along the west coast of N.A.photograph of dead shells of varnish clamsThe varnish clam Nuttallia obscurata is a recent invader to the west coast, having been introduced into Vancouver Harbour, British Columbia in 1991, possibly in ballast water.  Since that time it has spread more than 500km north and 900km south to Coos Bay, Oregon. Given the presence of strong southward current directions in summer and the relatively long pelagic larval life, the authors predict rapid range expansion in Nuttallia. Dudas & Dower 2006 Mar Ecol Progr Ser 320: 195.

 

A not uncommon sight on beaches in British
Columbia, this one at Savary Island: windrows
of dead varnish clams Nuttallia obscurata

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Panopea

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Research study 1
 

photograph of a collection of geoduc clams Panope generosa being measured for fisheries studyThe largest species of clam on this coast and largest burrowing clam in the world, is the geoduc Panopea abrupta.  It is also known as “Methuselah clam” because it is long-lived (maximum recorded age of about 160yr). Orensanz et al. 2004 Can J Fish Aquat Sci 61: 1355.

NOTE  the biggest bivalves in the world are non-burrowing giant clams Tridacna spp., the largest of which measure over a meter in length and weigh several hundred kg

NOTE  pr. “gooey-duck”, derived from a Salish First Nations word.  Geoducs Panopea abrupta are indigenous to the Pacific west coast, although other Panopea species exist throughout the world




Geoducs being harvested off the coast of British Columbia.
Harvesting involves bottom SCUBA- or hookah-divers using
water jets to clear sediment away from the clams, which are
then gathered, placed in baskets, and hoisted to the surface

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Research study 2
 

A recent communication from researchers at the University of Washington recommends that the scientific name for the geoduc Panopea generosa be resurrected to relieve it from the synonomy of Panopea abrupta.  Both names were assigned during the US Exploring Expedition of 1838-42, abrupta to a a fossil found along the banks of the Columbia River and generosa to a living specimen collected in Puget Sound.  The authors provide convincing reasons for their recommendation.  Vadopalas et al.  2010 Malacologia 52 (1): 169.

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Penitella

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Research study 1
 

photograph showing packing nature of burrows of rock-boring piddocks Penitella penitaRock-boring piddocks Penitella penita are important agents of erosion in some areas of the west coast.  For example, in certain sandstone-rock areas at Coos Bay, Oregon, physical factors alone remove surface rock at a relatively slow rate of 0.5mm per year, while in rock heavily infested with piddocks the erosion rate is estimated to be 12mm per year.  Five species of Pholadidae are found on the west coast, but 90% of total numbers are represented by P. penitaEvans 1968 Ecology 49: 156. 

NOTE  the empty burrows of P. penita are colonised by many types of organisms, including sea anemones, polychaetes, sipunculids, crustaceans, snails, limpets, and tunicates.  If they are filled with sediments they become homes for bivalves, polychaetes, and other burrowing invertebrates. Evans 1967 Veliger 10: 148

Sandstone rock bearing burrows of Penitella penita, one
with shell remnants of its former occupant. At this
stage of near-maximum infestation, the rock becomes
brittle and is susceptible to fracture by wave and rock impact

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Tresus

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Research study 1
 

Two of the largest west-coast clams are Tresus capax and T. nuttallii, both deep-burrowing forms and both prized by clam-harvesters, with preference going to the latter for its supposed photograph of fat gaper horse clam Tresus capax, courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washingtongreater tastiness.  The 2 can be distinguished by shell shape, and by the lighter shell colour and presence of more photograph of Pacific horse clam Tresus nuttallii, courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washingtonperiostracum in T. nuttallii than in T. capax.  In San Juan Islands, WA, T. nuttallii has heavier and harder siphonal plates than T. capax, and often has large barnacles growing on them.  Fewer barnacles on the siphonal plates of T. capax is thought by the authors to owe to greater sloughing off of the surfaces of the plates. Swan & Finucane 1952 Nautilus 66: 19. Photographs courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.

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Research study 2
 

sketch of foot and mantle skirt in a fat-gaper clam Tresus capax behind which parasitic crabs Pinnixia hideAnother difference between the 2 Tresus spp. in San Juan Islands and Puget Sound, Washington includes the presence of a visceral skirt in T. capax but not in T. nuttallii.  The visceral skirt is an extension of the inner palp lamella which drapes partially over the top of the foot on either side of the body, and forms a hiding place for commensal crabs Pinnixa faba and P. littoralis.  In the absence of a visceral skirt in T. nuttallii, the crabs are also absence, although no research has been done to test whether this is cause-and-effect.  The crabs apparently feed on food strings moving along the free edge photograph of crab Pinnixa sp., a parasite of bivalvesof the skirt, although it is not clear whether these are actual food tracts or rejection tracts.  Pearce 1965 Veliger 7: 166.

NOTE  the author considers these crabs to be commensal, but by the damage they can cause by crawling about among the ctenidia and by their potential food-robbing behaviour, they are more correctly termed parasites

NOTE  the author remarks on Pinnixa spp. being present in T. nuttallii in more southerly populations, such as Bodega Bay, California

 

Crab Pinnixa sp. within the mantle
cavity of a bivalve 2.5X

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Venerupis

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Research study 1
 

Manila clams Venerupis philippinarum and varnish clams Nuttallia obscurata are “recent” introductions to the west coast from Asia. In British Columbia these introductions date from the 1930s and 1980-90s, respectively.  Both species have been highly successful, and cultured V. philippinarum represent the bulk of clam sales along the coast.  Triploidy, or the addition of another complement of chromosomes, can be readily induced in V. philippinarum in other bivalves such as mussels and oysters, and in other molluscs.  Triploid bivalves have reduced gametogenic activity and, as a result, grow faster than diploids.  There is a large literature published on triploidy in relation to aquaculture, and the subject will not be considered further here.

photograph of Manila clam Venerupis philippinarum
Manila clam Venerupis philippinarum 1X
photograph of varnish clam Nuttallia obscurata
Varnish clam Nuttallia obscurata 0.5X
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Research study 2
 

photograph of pea crab Pinnixa faba courtesy Dave Cowles, Walla Walla UniversityA study by researchers at the Pacific Biological Station, Nanaimo compares symbiont load in native and introduced clams in sympatric populations at 2 sites on Vancouver Island, British Columbia (separated by less than 50km).  Species examined are native littlenecks Protothaca staminea, and introduced Manila clams Venerupis philippinarum (from the 1930s) and varnish clams Nuttallia obscurata (1980-1990s).  Results show, interestingly, that each bivalve species hosts its own mostly unique assemblage of symbionts and this pattern is reasonably consistent for both study sites.  Most common in varnish clams are Nematopsis-like spores (4% infestation), pea crabs Pinnixa faba (24%), copepods Mytilicola (64%), and occasionally turbellarian flatworms in the kidney tubules (8%).  Most common symbionts in native littlenecks are rickettsia/chlamydia in the gills and digestive glands (88%), fine-matrix inclusion bodies (12%), and copepods (4%).  Manila clams host rickettsia/Chlamydia (36%), ciliate protista in the mantle cavity, digestive gland, and other organs (20%), and trematodes (8%).  Only a few host-specific symbionts are identified, mostly types of protista, and these usually occur at comparatively low levels of infestation.  Some of these are thought by the authors to  have been introduced to British Columbia along with their hosts.  The high incidence of pea crabs in varnish clams is consistent with values previously reported for this bivalve species, but the reason for the preference is unknown.  Marshall et al. 2003 J Shellf Res 22 (1): 185. Photograph courtesy Dave Cowles, Walla Walla University www.wallawalla.edu.

NOTE  the authors use the terms parasites, commensals, and symbionts in their study but, as these terms are not clearly separable, they are all termed symbionts here

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  Other large clams of commercial or sports-fishing interest on this coast:
 
 
photograph of butter clam Saxidomus giganteus
Butter clams Saxidomus gigantea are common from Alaska to N. California. An ecological homologue, the California butter clam S. nuttallii, is indigenous to California 0.6X
photograph of native littleneck clam Protothaca staminea
The native littleneck Protothaca staminea is cultured commercially with the Manila clam Venerupis philippin- arum. The latter is ecologically dominant 1X
photograph of razor lam Siliqua patula
Razor clams Siliqua patula are capable of fast-burrowing, aided by the smooth, proteinaceous periostracum that forms the outer shell surface 0.4X
photograph of horse clam Tresus capax
Horse or gaper clams are often the source of the large squirtings visible on mud flats during low water. The specimen featured is the fat gaper Tresus capax 0.2X
photograph of Pacific gaper clam Tresus nuttallii
The Pacific gaper Tresus nuttallii has a longer shell than the fat gaper T. capax. Both are harvested, but nuttallii is the more commercially important 0.3X

Cockles Clinocardium nuttallii in an aquarium tank. Cockles live close to the sand-water interface and have correspondingly short siphons . 0.2X
 
NOTE a study of allozyme polymorphisms in 3 populations of P. staminea in Puget Sound, Washington by a consortium of researchers shows high diversity of genetic makeup among the 3 populations.  The authors associate the differences with hydrological conditions of slow-moving water masses and poor circulation in each of the 3 locations, thus restricting planktonic veliger larvae to their basin of origin and leading to genetic differentiation.  Parker et al. 2003 J Shellf Res 22 (3): 681.
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