Overview

Brief Summary

Saxidomus giganteus, or the butter clam, is a bivalve mollusk of the family Veneridae occuring along the Pacific coastline of North America from California to Alaska. Its shell measures on average three inches in length with light grey and white coloration and smooth ridges; inside its flesh is totally white. The butter clam can be found in the intertidal zone, usually buried to a depth of approximately one foot in sandy, gravelly, and mixed-shell substrate. Like most bivalves, it lives a sedentary lifestyle filter-feeding and consuming a diet of phyto-plankton. Unlike most bivalves, it has the ability to sequester paralytic shellfish poisons (PSTs) in its siphon and store them there for as long as two years, deterring any siphon-nipping predator. The butter clam is used for a variety of purposes, ranging from archaeological research to monitoring water pollution.

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Comprehensive Description

Biology/Natural History: This species burrows moderately deep (to 35 cm). It has been extensively commercially harvested, especially for clam chowder. Predators include the seastars Pycnopodia helianthoides and Evasterias troschelii, the moon snail Euspira lewisii, Dungeness crabs Cancer magister, and sea otters. The species is especially vulnerable to paralytic shellfish poisoning (PSP), which accumulates especially in the dark tips of the siphons, so great caution should be used before eating them. The toxin, accumulated aftereating the dinoflagellate Alexandrium catanella, which blooms in sea temperatures over 13C, may persist for months in the tissues. Sea otters and seabirds seem to be able to detect the toxin and avoid tainted clams, though humans cannot. Pacific staghorn sculpins Leptocottus armatus nip off siphon tips, but after feeding on a tainted siphon will avoid them. Gulls regurgitate contaminated clams and avoid them after that. Pea crabs Pinnixa littoralis Pinnixa faba, and Fabia subquadrata may be found in the mantle cavity. The crystalline style contains many large spirochaete bacteria (Cristispira sp). Spawns in the summer. In British Columbia, about half the clams are large enough to spawn by their third year. Larvae settle from the plankton after 4 weeks. This species may live 20 years or more.

Indians formerly used the shells of this clam for money.

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This common intertidal clam has an large, black external hinge ligament (photo) and well-developed concentric ridges but no radial ridges. The shell is only slightly longer than it is high, and the angle formed by the umbones is more than 110 degrees. The periostracum, if present, is not yellow or glossy. The adductor muscle scars are of nearly equal size (photo). It has a true hinge plate with 3 cardinal teeth in each valve (photo), but no chondrophore or byssal threads. The valves have a smooth but not glossy interior with a pallial sinus and a continuous pallial line (photo). The shell gapes slightly at the posterior end. Length to 15 cm. The outside of the shell may be white, or may be stained by iron sulfide in anoxic sediments. The siphons are moderately long (about 4 cm).
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Distribution

National Distribution

United States

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

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Geographical Range: Aleutian Islands and SE Bering Sea, Alaska to San Francisco Bay, CA (rarely seen S of Humboldt Bay)

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The butter clam is only distributed along the Pacific coast of North America. The species can be found as far north as the Aleutian Islands, Alaska (54° 49' 0.0012", -164° 1' 59.9982") and as far south as Monterey Bay, California (36° 52' 41.4906", -121° 56' 50.319") (Keep 1911, 69). The butter clam abounds in Alaska, where it is common enough to provide an excellent food source (Abbott 1954, 417) to natives. However, the state of Washington (U.S.) claims the densest populations of the butter clam, specifically at the Strait of Juan de Fuca (48° 26' 4.632", -124° 18' 55.767") and Puget Sound (47° 49' 14.7828", -122° 26' 33.4638") (Cheney and Mumford 1986, 134) which consequently are the locales of many studies (Gilikin et al., 2005) on the organism.

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Physical Description

Morphology

Larval

Butter clam larvae have a straight hinge until attaining a length of about 160 micrometers (.016 cm or .0063 inches) at which point the anterior end is longer and more pointed than the broadly rounded posterior end. With further development, the umbo (rounded or pointed extremity of shell) broadly rounds out with both anterior (mouth) and posterior (anal) ends. Butter clam larvae are longer than they are tall, and because of this have a squat appearance (Shanks 2001, 145; Breese and Phibbs, 1970).

Shell

The butter clam is a relatively large bivalve, measuring typically 3 to 5 inches long at maximum, (Ricketts & Calvin 1952, 303) averaging about 3 inches (7.62 cm) length, and attaining an average height of about 3 inches (7.62) as well. Its shell is thick, hardy, and can be measured at a half-inch (1.27 cm) in width (Weymouth 1920, 36). Inscribed are several lateral ribs (or grooves) which are closely-spaced and run the course of the entire shell in horizontal manner, from anterior to posterior margin (Hurst 2003, 10). The ribs are fine, inconspicuous and not as bold as in its cousin Saxidomus nuttalli, (Weymouth 1920, 11) which contributes to a smooth feel to the outside of the shell. The shell is also comparatively circular when compared to the oval shape of Saxidomus nuttalli, and can be described as ovate-subquadrate (Weymouth 1920, 35; Coan et al. 2000, 367). The shell exhibits a protruding ligament (elastic structure connecting shells at hinge), and lacks an anterior lateral tooth, lunule, and escutcheon (Coan et al. 2000, 384). In color, the shell ranges from a light gray or yellowish white (mainly in young specimens) tint to a darker gray (Keep 1911, 79).

Anatomy

The butter clam has white flesh and is devoid of any purple tinge. The mantle (fleshy outer tissue surrounding its organs) edge has four folds with fused lobes which leave a large pedal opening. The siphons are joined and exposed due to their gaping of the siphonate extremity of the shell. The pallius sinus (embayment where siphons retract) is deep (Weymouth 1920, 35). Ctenidia (gills) are large, synaptorhabdic (its filaments are joined only by tissue junctions), plicate, and heterorhabdic (more than one type of filament is present). The labial palps (thin plates bordering mouth) are small and triangular (Coan et al. 2000, 363). The siphon provides a defense for the butter clam against predators that regularly eat siphons. The butter clam enables this defense by sequestering diet-derived algal toxins - highly potent neurotoxins - in its siphon thereby discouraging predation (Kvitek 1991, 369). Because of this, neurotoxins responsible for the toxicity of poisonous shellfish have been dubbed "saxitoxins" in relation to the use of neurotoxins by Saxidomus giganteus (Lane 1968, 410).

The digestive system of the butter clam consists of an alimentary canal with type 5 stomach, numerous ducts to digestive diverticula, a style-sac joined to mid-gut, and intestine passing through the ventricle of the heart (Coan et al. 2000, 363).

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Diagnostic Description

Saxidomus giganteus can be unambiguosly identified by a combination of these characteristics: light to dark grey coloration, size averaging 3 inches (7.62 cm) long and almost same in height with close to a half-inch (1.27 cm) width, and closely-formed ribs which run horizontally from anterior to posterior margins (Hurst 2003, 10). The butter clam can be distinguished from Saxidomus nuttalli by its average smaller size, absence of bold ribs, and overall smoother shell (Weymouth 1920, 11). Compared with Saxidomus purpuratus, Saxidomus giganteus can be distinguished by the absence of purple coloration, as no purple is contained within or on any part of the butter clam (Coan et al. 2000, 384). The coloration of Saxidomus giganteus is even more distinguishable given the fact that it is the only Saxidomus clam to retain a totally white internal flesh, with absolutely no purple tinge (Weymouth 1920, 35).

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Look Alikes

How to Distinguish from Similar Species: Of shells commonly found on the same, protected beaches, the cockle Clinocardium nuttallii has radial ridges and an undulating aperture where the two valves come together. The gaper clam Tresus capax and the softshell clam Mya arenaria have a large chondrophore at the hinge, and Tresus capax also has a large gape at the posterior end. The bentnose clam Macoma nasuta has valves bent to the right on the posterior end. The littleneck clam Protothaca staminea has fine radial ribs, a row of small teeth on the inside of the valves close to their ventral margins, and may also have a mottled pattern of periostracum. Saxidomus nuttalli is very similar but has more prominent raised concentric lines on the shell, plus the shell is stained purple inside. It lives primarily farther south in California.
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Saxidomus giganteus is most confused with Saxidomus nuttalli, with which it shares location, appearance, and even common name (also called butter clam in some instances) (Ricketts & Calvin 1952, 303). Saxidomus giganteus also looks like Saxidomus purpuratus with slight color differentiation, Saxidomus giganteus of pure white flesh inside while Saxidomus purpuratus of purple tinge (Coan et al. 2000, 384). Saxidomus giganteus also has a strong resemblance to Saxidomus squalidalus but can be distinguished by geographic distribution - Saxidomus giganteus occurring along the North American Pacific coast, and Saxidomus squalidalus along South America (Keep 1911, 79).

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Ecology

Habitat

Depth range based on 59 specimens in 1 taxon.
Water temperature and chemistry ranges based on 36 samples.

Environmental ranges
  Depth range (m): -1 - 77
  Temperature range (°C): 8.977 - 11.849
  Nitrate (umol/L): 3.346 - 8.886
  Salinity (PPS): 31.460 - 31.893
  Oxygen (ml/l): 6.227 - 6.649
  Phosphate (umol/l): 0.737 - 1.101
  Silicate (umol/l): 12.005 - 22.448

Graphical representation

Depth range (m): -1 - 77

Temperature range (°C): 8.977 - 11.849

Nitrate (umol/L): 3.346 - 8.886

Salinity (PPS): 31.460 - 31.893

Oxygen (ml/l): 6.227 - 6.649

Phosphate (umol/l): 0.737 - 1.101

Silicate (umol/l): 12.005 - 22.448
 
Note: this information has not been validated. Check this *note*. Your feedback is most welcome.

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Depth Range: Low intertidal to 40 m

Habitat: Sheltered sand, sandy mud, and gravel beaches

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The butter clam ranges from intertidal to shallow water, with an optimal range of +1.5 ft. to -3.5 ft. (+.456 meter to -1.07 meters) (Baxter 1971, 243), but often occurring to depths of 10 ft. (3.048 meters) (Baxter 1971, 243). Its maximum recorded depth is ~177.4 ft. (54 meters) (Cheney and Mumford 1986, 134). Butter clams typically dig to a depth below ground of up to 30cm (Nickerson 1977, ) or about 11.8 inches (Cheney and Mumford1986, 134). They dig deeper down with further aging, as their siphon elongates (Quayle and Bourne 1972, 27). Therefore, mature butter clams are found usually at least 11.8 inches (30cm) down but usually deeper. The preferred substrate of butter clams can be characterized as sandy, or gravelly, with mixed shell remnants (Cheney and Mumford 1986, 134). They are known to disfavor muddy substrate (Baxter 1971, 155).

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Dispersal

Butter clams disperse through their larva, which may be carried through currents to other beaches or locales on the same beach (Fraser & Smith 1928, 275b).

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Trophic Strategy

The butter clam is a heterotroph, littoral, suspension-feeder which consumes phyto- and zoo-plankton (Reid 1976, 573).

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General Ecology

The butter clam's surroundings

The butter clam's annual growth rate sways with salinity, water temperature and sediment type. Although few studies have centered on salinity and butter clam shell growth, both oyster studies and the few dedicated to butter clams indicate a positive correlation between salinity and shell growth (Goong and Chew 2001, 145). There is also a range of water temperatures that is most fitting for butter clams. This is suggested by a study of three beaches in which the beach with greatest temperature fluctuations also spurred the most fluctuations in growth rates of butter clams (Goong and Chew 2001, 145). In general, however, shell growth slows in winter with a decline in water temperature, many times even coming to a halt until spring of the following year (Goong 1999, 14). Goong's study also examined sandy, sandy-with-rock, and gravelly beach types. The beach which promoted fastest shell growth among the three studied offered a sand-rock mix as environment to its butter clams. The nature of the beach has also been hypothesized to influence the shape of the butter clam's shell. In substrates of sand, or fine and smoother gravel, shells appear longer than their breadth. In substrates of coarser gravel, mud, and stone like those of rocky beaches, shells appear equal in length and breadth (Fraser & Smith 1928b, 274).

Strong tidal currents have also been related to more rapid shell growth among butter clams, as opposed to quiet bays which seem to slow their growth (Fraser & Smith 1928b, 274). This is because the continual movement of water ensures a constant renewal of food for filter-feeders, and new sources of plankton (Smith 1928, 290)

The butter clam's predators

The butter clam has numerous predators, such as sea otter, avian, and fish predators which the butter clam's special, poisonous siphon is known to deter (Kvitek & Beitler 1991, 47). One well-known predator is the Giant Pink Sea Star, or Pisaster brevispinus, which is known to sense buried clams in sand and feed on them (Morris et al. 1980, 127). The butter clam is also known to account for a majority of the sea otter's diet in Alaskan locales (Kvitek et al. 1993, 168).

The butter clam's food

The butter clam filter-feeds mainly on diatoms and zooplankton which occur in renewing water sources where it resides. The commonest genera of diatoms to be found in the digestive tract of the butter clam include Chaetoceros, Nitzschia, Thalassiosira, Asterionella, Coscinodiscus, Pleurosigma, Biddulphia, and Diatom (Smith 1928, 288). Zooplankton can be seen in the digestive tract, including many forms of ciliates and dinoflagellates. The most common genus of ciliate eaten by the butter clam is Tintinnidae (Smith 1928, 288). The most common genus of dinoflagellates eaten by the butter clam is Peridinium (Smith 1928, 289). Also prevalent are heliozoon and radiolarian protozoans, and some species of Rotifers (Smith 1928, 289). Occasionally, the butter clam will feed on gastropod veligers, bivalve veligers, barnacle larvae, ostracods, cladocera, and round worms (Smith 1928, 290).

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Life History and Behavior

Behavior

Once settled, the butter clam is primarily sedentary throughout its life (Hurst 2003, 11) and limits its movement mostly to vertical migration, and much less commonly horizontal (Quayle and Bourne 1972, 27). After larval metamorphosis the butter clam will stay relatively in the same position even in suboptimal conditions (Dethier 2006, 5). Because its siphon grows with maturity, older clams are capable of digging deeper in substrates, on average at least 11.8 inches (30 cm) down (Cheney and Mumford 1984, 134). Furthermore, butter clams are known to expose their siphons above the sediment surface more under dark conditions (i.e., night) than light conditions (i.e. day.) (Kvitek 1991, 372). Although toxic and non-toxic butter clams expose their siphons with the same frequency, toxic butter clams are known to extend their siphons much higher, as theirs are less eaten by deterred predators (Kvitek 1991, 373). This allows for toxic butter clams to reach higher water levels with more nutrients for filter-feeding.

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Life Cycle

Butter clam larvae develop into bivalve veligers (larval type with ciliated locomotion) in as little as two weeks. Within four weeks, while still less than 3/16 of an inch (.476 cm) long, they will settle in gravel/sand (Ricketts and Calvin 1952, 303). The sequence of events (Dudas & Dower 2006, 202; Bourne 1971) has been described as: 48 hours after birth - early veliger (D-stage) length of 142 micrometers (.0142 cm or .005591 inches); 16 days after birth - veliger length of 225 micrometers (.0224 cm or .008819 inches); 22-30 days after birth - settlement length of 311 micrometers (.0311 cm or .01224 inches).

The larvae have a straight hinge until attaining a length of approximately 160 micrometers (.016 cm or .0063 inches), and their metamorphosis occurs at a length of approximately 230 micrometers (.023 cm or .00906 inches) (Shanks 2001, 145; Breese and Phibbs, 1970).

Although able to reach a length of 5 inches (12.7 cm), a butter clam of such measurement would imply an age of at least 9.5 years. Typically, growth follows that 2-3 year old butter clams hover around ~1.5 inches (3.8cm) while 6 year old butter clams attain around ~ 2.5 inches (6.3cm) (Goong and Chew 2001, 146). Regularity in butter clam growth allows age to be estimated via measurements of shell lines, which indicate regular increments of growth (Hallmann et al. 2009, 2354).

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Life Expectancy

The butter clam can live for up to 20 years. (Hurst 2003, 14)

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Reproduction

A butter clam is commonly ~ 1.5 inches (3.8cm) in shell length when it reaches maturity (Cheney 1986, 135). The age fluctuates and it can take between 1 and 9 years for the clams to become sexually mature (Cheney 1986, Table 1). The average age at maturation is 3 years (Ricketts and Calvin 1952, 303). After maturity, shell growth decreases exponentially; this results in very minor growth between maturity and death (Hurst 2003, 12).

Once mature, increased water temperatures spur sexual reproduction among butter clams (Hurst 2003, 11). Observations in Puget Sound record butter clams spawning from late spring to late summer, with most sexual success (peak) occurring when water temperatures exceed 59 degrees Farenheit or 15 degrees Celsius (Cheney and Mumford 1986, 135; Quayle and Bourne 1972, 28; Bourne 1971, 1). Observations in Oregon and British Columbia follow similar suit: the butter clam reproduces from as early as February to July (Quayle and Bourne 1972, 27). Butter clams are dioecious (meaning individuals can only produce one type of gamete) (Fraser & Smith 1928a, 249).

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Evolution and Systematics

Evolution

Fossils of the butter clam have been found and studied extensively along the Pacific Coast. Some known fossil localities and ages (approximate) have been: Amaknak Is., AK (53°53'52", 166°32'00") - 3,410 years old (+/- 65 years); Bainbridge Is., WA (47°35'40", 122°31'10") - 2,250 years old (+/- 80 years); San Juan Is., WA (48°32'15", 123°00'15") - 12,350 years old (+/- 330 years); Hope Is., WA (48°24'00", 122°34'30") - 12,400 years old (+/- 190 years); Whidbey Is., WA (48°07'30", 133°36'00") - 36,200 years old (+/- 2,600 years); Cape Blanco, OR (42°50'15", 124°33'45") - 40,000 years old (+/- 10,000 years); Bandon, OR (43°06'50", 124°26'10") - 80,000 years to 85,000 years old; Bay Center, WA (46°37'45", 123°57'25") - 120,000 years old (+/- 40,000 years); Lynn Point, WA (46°31'20", 123°54'45") - 190,000 years old (+/- 40,000 years); Willapa Bay, WA (46°32'22",123°59'20") - 120,000 years old (+/- 40,000 years) and 190,000 years old (+/- 40,000 years old); (Kvenvolden et al. 1979, 1505; Kvenvolden et al. 1980, 323).

The butter clam's genus comprises only three extant species: Saxidomus purpuratus, Saxidomus nuttalli, and Saxidomus giganteus. The butter clam, in characteristics, is more similar to Saxidomus nuttalli: both are often confused, inhabit the same localities, have the same distribution, and have similar sized and colored shells. Morphologically, they are distinguished only by a smoother shell (Weymouth 1920, 11) and absence of any internal coloration (Coan et al. 2000, 384). These similarities suggest that Saxidomus nuttalli and Saxidomus giganteus are close relatives. Saxidomus purpuratus has a distinct distribution along the coast of East Asian countries Japan, China, and Korea (Kim et al. 2006, 23) and displays purple internal coloration, possibly implying a more distant relationship to Saxidomus giganteus and Saxidomus nutalli.

The butter clam has evolved a unique ability to store paralytic shellfish toxins (PSTs) in the epithelium of its siphon while most other clams detoxify themselves of PSTs in a few weeks. Also, the butter clam's neurons have a high resistance to saxitoxin, one lethal component of PSTs. Because of this ability to sequester PSTs in its siphon, the butter clam is able to deter many siphon-nipping predators who would be poisoned (Kvitek 1991, 369). Predators (like Leptocotus armatu) who nip siphons create a naturally selective force of butter clams in avoiding those with PST-sequestering siphons while preying on those without the mutation (Kvitek 1991, 370). The butter clam of today, with this special siphon, arises from this selective force in evolution.

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Molecular Biology and Genetics

Molecular Biology

Statistics of barcoding coverage: Saxidomus gigantea

Barcode of Life Data Systems (BOLDS) Stats
Public Records: 0
Specimens with Barcodes: 9
Species With Barcodes: 1
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By consuming dinoflagellate algae of the genus Alexandrium, which produces paralytic shellfish poisoning (PST) toxins, the butter clam may store these toxins. Although present in 21 molecular forms, these PST toxins have been collectivelly termed "saxitoxin" as they were first extracted and studied from Saxidomus giganteus (Lane 1968, 410). All saxitoxins are neurotoxins that block movement of sodium through nerve cell membranes, presenting a great threat to animals and humans who consume butter clams and other shellfish that may contain them. The butter clam exacerbates this threat of poisoning with its unique ability to store saxitoxins in its siphon for up to 2 years (Kvitek 1991, 47).

Saxitoxin has the molecular make-up of: [IUPAC] [(3aS,4R,10aS)-2,6-diamino-10,10-dihydroxy-3a,4,8,9-tetrahydro-3H-pyrrolo[1,2-c]purin-4-yl]methyl carbamate; [MF] C10H17N7O4; [MW] 299.286480 g/mol (found at http://www.ncbi.nlm.nih.gov/pccompound?term=Saxidomus%20giganteus)

The amino acid composition of butter clams is studied for geochemistry dating and for comparing radiocarbon dates. Aspartic acid has been found as the most abundant amino acid in the butter clam, and threonine the least abundant (Kvenvolden et al. 1980, 321).

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Conservation

Conservation Status

NatureServe Conservation Status

Rounded Global Status Rank: G5 - Secure

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National NatureServe Conservation Status

United States

Rounded National Status Rank: N5 - Secure

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Butter clams commonly inhabit popular beaches for human visitation and are the focus of some collection. Increased human activity lends itself to a decrease in biological integrity, followed by decreased species diversity and negative impact on growth and health of taxa in an ecosystem (Karr 1981). If a beach is highly affected by large human groups, the butter clam may suffer disruptions in growth (Goong and Chew 2001, 146).

Recreational and commercial overharvesting can be a threat in disturbing populations, as the butter clam is a staple of many seafood dishes (Dethier 2006, 5). However, many times detrimental human activity comes in the form of land use, shoreline modifications, sewage discharge, industrial discharge, storm water discharge, or aiding invasive species (Dethier 2006, 5). Each such activity can seriously disrupt the conservation of the butter clam, which requires specific levels of salinity, water temperature, water level, tidal range, etc. By aiding invasive species, increased abundance of predators such as eels may deplete otherwise stable butter clam populations.

It is hypothesized that natural activities and changes in environment can be reversed by humans to help conserve the butter clam. For example, alterations to the butter clam's habitat from sediment loads of rivers or streams, or sediment supply from hardened bluffs, can be reverted by humans, aiding the conservation of butter clam populations (Dethier 2006, 5). This, however, may be discouraged as environmental alterations - while helping the butter clam - may create imbalances and be detrimental to other species.

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Relevance to Humans and Ecosystems

Benefits

Used for food and currency

The butter clam is most known for its role in cuisine, contributing to recipes of clam chowder. However, it can also be prepared by itself and as such it constitutes a valuable food source for natives and those willing to dig for the clam (Abbott 1954, 417). The shell of the butter clam has been used as a type of currency dating back to "wampums" of native tribes (Woodward 1913, 142). The shell is also sometimes sold, and it was offered in Portland as the "Oregon Saxidome" circa 1911 (Keep 1911, 70).

Used for science

The butter clam's shell offers a historical record, and its analysis has led to the examination of past relationships of human populations with the butter clam as well as paleoclimate sea surface temperatures. Amino acids from its shell are studied seeking a geochemistry dating solution to compare with radiocarbon dating (Kvevnvolden 1980, 321). By inspecting clam growth-stage profiles, past methods of human resource management and harvest can be hypothesized to have arisen up to 7000 years ago (Cannon & Burchell 2009, 1050). Similar investigations suggest past natures of resource management in archeological sites, such as that of Qwu?gwes (Hurst 2003, 1). Shells also have been assessed for their reliability in reconstructing paleoclimate sea surface temperatures by way of their isotopic composition (Gilikin et al., 2005, 1). The butter clam can be studied in this way to deduce chemical impacts on the environment. By studying how the butter clam interacts with saxitoxins, this poison's nature and its effects in our food and in the ecosystem can be better defined (Kvitek 1991, 369; Kvitek & Beitler 1991, 47). The butter clam has also been studied for its possible role as a biomonitor of heavy metal pollution in marine waters (Boening 1999, 459).

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Wikipedia

Saxidomus gigantea

Saxidomus gigantea, common name the butter clam, is a large edible saltwater clam, a marine bivalve mollusk in the family Veneridae, the venus clams.[1]

Numerous valves of this species have been found in the shell middens on Sidney Island in British Columbia, Canada.[2]

References

  1. ^ "Saxidomus giganteus The Butter Clam". 
  2. ^ The Great Blue Heron by Robert W. Butler [1]


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