Mercenaria mercenaria

Invasion History

First Non-native North American Tidal Record: 1935
First Non-native West Coast Tidal Record: 1935
First Non-native East/Gulf Coast Tidal Record: 1958

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General Invasion History:

Mercenaria mercenaria is native to the East Coast of North America from southern Florida to the Gulf of St. Lawrence, Canada. North of Massachusetts Bay, it is mostly confined to warmer bays, and is more widely distributed in the southern Gulf of St. Lawrence (Bousfield 1960; Abbott 1974; Gosner 1978; Baker et al. 2008). It is a major commercial shellfish on the East Coast, widely used in chowder, as stuffed quahogs, raw 'cherrystone' clams, etc. (Food and Agriculture Organization 2012). It has been widely transported with oysters, discarded by restaurants and consumers, used as fish bait, and stocked officially and unofficially in Puerto Rico (Juste and Cortes 1990), on the West Coast (Carlton 1979; Carlton 1992, Chew 2001, limited establishment, declining- see discussion below), in Hawaii (Carlton and Eldredge 2009, failed), Japan (Hiwatari et al. 2006, established), China (Chavanich et al. 2010, unknown), and in Atlantic and Mediterranean Europe (Utting and Spencer 1992; Zenetos et al. 2003, established). Mercenaria mercenaria has been introduced for aquaculture in bays on the Gulf Coast of Florida, where it has spread, and hybridizes with the native M. campechiensis (Southern Hard Clam) (Baker et al. 2008; Arnold et al. 2009). A frequent pattern of this species in invaded regions is to form relatively small, established populations in confined estuaries. In many regions, it may be prevented from spawning by low water temperatures, lack of conditions for larval retention, or other factors (Chew 2001). Such confined populations are also vulnerable to pollution and overfishing.

North American Invasion History:

Invasion History on the West Coast:

The earliest definite record of M. mercenaria on the West Coast is of dead shells collected in 1901 (Keep 1901, cited by Carlton 1979). However, some earlier records of 'Mercenaria kennicotti' (Dall 1871) from the Strait of Juan de Fuca (WA), reported from Mendocino County to British Columbia, may represent shells or animals transported from the Atlantic (Carlton 1979). Extensive attempts were made by the Washington Department of Fish and Game to establish this clam in Puget Sound, but only a few were found one year after planting, and there was no evidence of reproduction (Chew 2001). There are sporadic records of one or a few live specimens, probably representing discarded seafood or bait, from San Francisco Bay in 1940 and 1968 (Carlton 1979); Mission Bay, San Diego in 1991 (Herz and Herz 1992); and from Boundary Bay, British Columbia in 1996 (Forsyth and Forsyth 2001). In Humboldt Bay, where clams were planted in 1935, and reported to reproduce every few years, they were collected as late as 1970, but were considered extinct by 1979 (Carlton 1979). From 1965 to 1963, deliberate attempts were made to stock M. mercenaria in 14 California bays, from Humboldt Bay south to Alamitos Bay, Long Beach. It is likely that additional, unauthorized stockings occurred (Carlton 1979). Most plantings failed, some being destroyed by crabs, rays, or over-fishing. In South San Francisco Bay in 1981, at least five extensive plantings of 2,700 to 55,700 clams were made in the vicinity of the San Mateo Bridge, at Foster City and Burlingame, by biologists in the California Department of Fish and Game. No M. mercenaria were found in later surveys of the area (Chew 2001).

The one stocking that has had long-term survival is in Colorado Lagoon, a sub-estuary of Alamitos Bay, Long Beach. This is the only known established population on the West Coast. It is believed to have resulted from an unauthorized stocking in 1951. By the 1980s, the population of M. mercenaria had reached 400 clams (Murphy and Kremer 1985), and was the dominant bivalve in the lagoon. However, by 2004, the population had collapsed. In a 2009-2010 survey, two specimens were found. Causes of the decline are not clear. Overfishing, predation, eutrophication, high salinities, and competition with newly introduced species (particularly Vernerupis philippinarum the Japanese Littleneck and Musculista senhousia the Senhouse Mussel), but none of these could clearly be tied to the population collapse (Burnaford et al. 2011).

Invasion History on the Gulf Coast:

Mercenaria mercenaria appears to have been historically absent from the Gulf of Mexico, where the native species is M. campechiensis, based on genetic studies. A hybrid zone exists along the southeastern coast of the US, from Georgia to the tip of Florida (Foighil et al. 1996; Arnold et al. 2009). The Northern Hard Clam was considered superior to the Southern Quahog, because of its longer shelf life. Experimental introductions of M. mercenaria from Connecticut were made in 1958-1960, to several sites on the West Coast of Florida. Sites stocked included Alligator Harbor, Tampa Bay, and Charlotte Harbor (Arnold et al. 2009). Some of these stocked populations have persisted and expanded. In the 1990s, aquaculture of this clam began on a large scale at many sites. In addition, extensive hybridization occurred, even at sites with no history of introduction (Arnold et al. 2009).

Invasion History in Hawaii:

In Hawaii, two M. mercenaria specimens were collected in Kaneohe Bay, Oahu, in 1967, probably from a planting in the 1930s (Carlton and Eldredge 2009). In 1967-1968, the state government planted 1,985 clams in Nomilo Fishpond, Kauai, which apparently survived for several years, but are now extinct in the wild. This species is raised in aquaculture in Hawaii (Carlton and Eldredge 2009).

Invasion History Elsewhere in the World:

The first recorded collection of M. mercenaria in Europe was a single shell found near Liverpool in 1850. Live specimens were found in the Humber estuary by 1864, and were harvested commercially by 1889. Many attempts at deliberate introductions were made in England and France, including unsuccessful stockings in the Arcachon Bay in 1861, off Kent , England in 1869, and near Liverpool in 1883 (Chew 2001). An established population in the Solent Estuary, Southampton, England, was apparently established by 1948 (Ansell 1963) and supported a small fishery into the 1980s. The population may have benefited from the effluent of a power plant. After the plant was shut down, the population declined (Utting and Spencer 1992; Chew 2001), but it still persists (Eno et al. 1997-2012). In France, a successful population was established in the Seudre Estuary, in Charente-Maritime, south of the Brittany peninsula in 1910, and in the Gulf of Morbihan in 1939 (Goulletquer et al. 2002). This clam was cultured in Brittany in oyster ponds, and apparently produced local populations in the estuaries of the Rivers Auray and Bono (Chew 2001). On the Iberian Peninsula, M. mercenaria was found in 1978, and is established near Cape Vidio, Asturias, Spain/Bay of Biscay (1978, Arias and Anadon 2012). It was also found in the Sado Estuary, Portugal, but is not known to be established there (2010, Paula Chainho in ICES Working Group on Introduction and Transfers of Marine Organisms 2012).

In the Mediterranean Sea, M. mercenaria is established in the Thau Lagoon, Sete, France, where it was first collected in 1965 (Zenetos et al. 2003). It is farmed elsewhere in the Mediterranean, but its occurrence in the wild is uncertain, due to misidentifications. Eighteen adult specimens were collected in the Po River delta, probably introduced as a contaminant in plantings of Venererupis philippinarum, but there was no evidence of reproduction (Torolla 2006). A single specimen was found in Marano Lagoon, in the Gulf of Trieste (Torolla 2006; Crocetta 2011).

In Puerto Rico, M. mercenaria were first collected in July 1985 in Laguna Catano and Laguna Torrecilla on the northeast coast of the island. Local populations were found at Fajard, Ceiba, and Playa Santa on the east and south coasts. They are not known from the fossil record, but had probably been established for some years before their 'discovery', since this clam supported local fisheries (Juste and Cortes 1990). The vector of introduction is not clear. Since the populations were far from ports, ballast water is possible, but not strongly implicated. Unofficial introductions are possible.

In Japan, a large exotic clam was discovered to be established in estuarine areas of Tokyo Bay in the 1990s. It was identified as M. mercenaria by molecular methods. This clam occurs in areas of variable salinity and oxygen levels. These clams may have been introduced through aquaculture, ballast water, or as discarded seafood (Hiwatari et al. 2006). Genetic studies suggest that the Tokyo Bay populations may have originated in Florida (Murakami-Sugihara et al. 2012). Mercenaria mercenaria was listed as introduced to Laizhou Bay, China in 1997, but its establishment and impacts are unknown (Chavanich et al. 2010). This clam is reared in aquaculture in China and Taiwan (Food and Agriculture Organization 2012b).

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Description

Mercenaria mercenaria is a large bivalve with a broadly triangular-oval shell and a prominent beak, shifted anteriorly. The shell's height is about 5/6 length. The valves are slightly inflated. The interior is white, with a prominent purple stain. The inner margin is toothed, and the pallial sinus is prominent. The exterior is dull white to gray. Young shells have raised, concentric ridges, while older shells are smoother, but still with distinct growth lines. The usual maximum size is ~100 mm, but sometimes M. mercenaria grow to 150 mm (Abbott 1974; Gosner 1978; Coan et al. 2000). Veliger larvae of M. mercenaria are described and illustrated by Chanley and Andrews (1971).

A form found in the Gulf of Mexico (M. m. texana Dall 1902), with differing shell sculpture, has been regarded as a subpsecies of M. mercenaria (Abbott 1974; Rosenberg 2012). However, genetic analysis supports a status as a subspecies of M. campechiensis (Foighil et al. 1996) and some authorities regarded it as a full species (M. texana; Appeltans et al. 2012).

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Taxonomy

Ecology

General:

Mercenaria mercenaria is a large bivalve which inhabits sandy to mud-sand bottoms, from the mid-intertidal to about 18 m depth (Gosner 1978; Eversole 1987). They can be abundant on bare seabed, but also occur in seagrass and oyster beds. They grow at temperatures of 10-30°C, but do best at 18-25°C and become dormant below 5°C (Eversole 1987; Food and Agriculture Organization 2012b). Adult clams tolerate and feed at salinities as low as 12 PSU (Castagna and Chanley 1973), but are rare below 15-17 PSU (Gosner 1978; Eversole 1987). Mercenaria mercenaria is a protandrous hermaphrodite, in which juveniles develop male gonadal characteristics first, and at sexual maturity either develop fully as males, or change into females. Maturity occurs at one to three years of age, depending on food and temperature, and at about 33 mm length (although length at maturity for males is smaller than females; Eversole 1987). In the northern part of its native range (Connecticut-New York), there is one spawning season per year in May-October peaking in July August, but further south (North Carolina-Florida), there is a prolonged spawning season, with peaks in spring and fall. Spawning occurs at 20-27°C (Eversole 1987).

Average fecundity is about 25 million eggs, although only about 6 million eggs are released when individuals are induced to spawn. The eggs and sperm are released through the exhaling siphon. Fertilized eggs develop into trochophore larvae within 10 hours, and develop their first shell (called 'D-shaped', or 'straight-hinged) at about 24 hours. The larvae swim and feed on phytoplankton, using a ciliated velum. At about 6-20 days, and 170-230 μm, the larvae develop a ciliated foot and begin to investigate substrates for settlement (Chanley and Andrew 1971; Eversole 1987). At the end of this pediveliger stage, the velum is lost and the larvae settle, moving by crawling, and attaching to grains of sediment using byssal threads. As the clams grow, they burrow deeper, the siphons elongate, and the byssus glands atrophy. Settlement tends to be gregarious, but dense settlements of small clams are very vulnerable to predators. Adult clams tend to burrow about 1-2 cm deep, and have little horizontal movement, but can dig deeper and rapidly when disturbed. Maximum longevity is about 33-36 years, but average size and age is greatly affected by predation and fisheries (Eversole 1987).

Food:

Phytoplankton

Trophic Status:

Suspension Feeder

SusFed

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Habitats

General Habitat	Unstructured Bottom	None
General Habitat	Oyster Reef	None
General Habitat	Salt-brackish marsh	None
Salinity Range	Mesohaline	5-18 PSU
Salinity Range	Polyhaline	18-30 PSU
Salinity Range	Euhaline	30-40 PSU
Tidal Range	Subtidal	None
Tidal Range	Low Intertidal	None
Vertical Habitat	Endobenthic	None

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Tolerances and Life History Parameters

Minimum Temperature (ºC)	0	Based on geographical range (Abbott 1974).
Maximum Temperature (ºC)	33	Loosanoff and Davis 1963, cited by Eversole 1987
Minimum Salinity (‰)	12.5	Experimental (Castagna and Chanley 1973)
Maximum Salinity (‰)	46	Belding (1931), cited by Castagna and Chanley (1973)
Minimum Reproductive Temperature	16	Eversole 1987
Maximum Reproductive Temperature	30	Eversole 1987
Minimum Reproductive Salinity	20	Castagna and Chanley 1973
Maximum Reproductive Salinity	35	Turner and George (1955) cited by Castagna and Chanley (1973)
Minimum Duration	6	Age to pediveliger, settling stage (Eversole 1987, probably at or near 30 C, maximum reproductive temperature)
Maximum Duration	20	Age to pediveliger, settling stage (Eversole 1987, probably at or near 16 C), minimum reproductive temperature)
Minimum Length (mm)	33	Approximate size at sexual maturity (Eversole 1987)
Maximum Length (mm)	150	The usual maximum size is ~100 mm, but sometimes Hard Clams grow to 150 mm (Abbott 1974; Gosner 1978).
Broad Temperature Range	None	Cold temperate-Subtropical
Broad Salinity Range	None	Polyhaline-Euhaline

General Impacts

Economic Impacts

Fisheries- Mercenaria mercenaria is a highly regarded shellfish species, extensively fished and reared in its native waters. A special feature is that it is more valuable at smaller 'littleneck' (50-65 mm) and 'cherrystone' sizes (66-79 mm), which are often eaten raw, rather than as larger 'chowder' clams (>80 mm), which are processed or used for chowder (Eversole 1987). Fishery methods include the use of rakes in the intertidal zone, the use of long bull rakes from boats, and mechanical 'escalator' dredges (Eversole 1987). 'Seed' clams are often rare in the field due to predation, so hatchery techniques were developed, starting in the 1940s, to augment natural beds. These techniques were adapted for aquaculture and are now used to raise M. mercenaria to commercial size, in its native range from Canada to Florida, but also in the Gulf of Mexico, and recently in China (Food and Agriculture Organization 2012b).

Introduced populations have often been confined to relatively small estuarine areas, such as the Colorado Lagoon in California, lagoons in Puerto Rico, the Solent in England, and others, probably due to the need for higher temperatures for spawning, limited water exchange for the retention of larvae, and other factors (Ansell 1963; Utting and Spencer 1992; Chew 2001). These restricted populations are vulnerable to pollution, predation, and overfishing.

Ecological Impacts

In many invaded areas, the restriction of M. mercenaria to small, confined estuaries has probably limited its impact. However, dense populations of these clams could have substantial impacts by filtering phytoplankton and excreting nutrients, as was seen in the Colorado Lagoon, California (Murphy and Kremer 1985).

In the Gulf of Mexico, widespread plantings and aquaculture of M. mercenaria has resulted in hybridization with the native M. campechiensis. In five estuaries on the Gulf Coast of Florida, near early planting and present aquaculture sites, hybrids comprised 13-45% of the Mercenaria sampled. Widespread introductions and hybridization could result in the decline of purebred M. campechiensis. However, hybrids are more vulnerable than the parent species to neoplasia, which could offset the genetic introgression. Competition between purebred adults of the two species may be limited, due to different habitat preferences. Mercenaria mercenaria prefers oyster reefs, while M. campechiensis is found mostly in the more widespread sand and seagrass habitats (Arnold et al. 2009).

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Regional Impacts

G050	Charlotte Harbor		Ecological Impact	Hybridization
Mercenaria mercenaria X campechiensis hybrids comprised about 13-18% of clams collected in Charlotte Harbor (Arnold et al. 2009).
G070	Tampa Bay		Ecological Impact	Hybridization
Mercenaria mercenaria X campechiensis hybrids comprised about 19-30% of clams collected in Tampa Bay (Arnold et al. 2009).
G080	Suwannee River		Ecological Impact	Hybridization
Mercenaria mercenaria X campechiensis hybrids comprised about 18% of clams collected in a Cedar Key aquaculture site (Arnold et al. 2009).
G090	Apalachee Bay		Ecological Impact	Hybridization
Mercenaria mercenaria X campechiensis hybrids comprised about 45% of clams collected in Alligator Harbor (Arnold et al. 2009).
CAR-I	Northern Yucatan, Gulf of Mexico, Florida Straits, to Middle Eastern Florida		Ecological Impact	Hybridization
Hybrids between Mercenaria mercenaria and M. campechiensis comprised 13-30% of the clams collected. Hybrids were present even at sites where purebred M. mercenaria were absent, suggesting that introgression was primarily affecting the genotypes of N. campechiensis populations. However, this introgression may be offset by the increased prevalence of neoplasia in hybrids (Arnold et al. 2009).
CAR-IV	None		Economic Impact	Fisheries
Mercenaria mercenaria supported small local fisheries at several locations in Puerto Rico (Juste and Cortes 1990).
NEA-IV	None		Economic Impact	Fisheries
Fisheries for M. mercenaria became established in the Gulf of Morbihan and the Marennes-Oleron estuary by the 1960s (Chew 2001).
NEA-II	None		Economic Impact	Fisheries
Local harvests of M. mercenaria in the Humber estuary started by 1889 (Chew 2001), but we have no reports on the persistence of the population or the fishery. In the Solent, a fishery persisted from the 1950s to the 1980s (Utting and Spencer 1992; Chew 2001).
P050	San Pedro Bay		Ecological Impact	Herbivory
The large biomass of M. mercenaria in the Colorado Lagoon appears to have had a substantial impact on phytoplankton populations in 1980, although Murphy and Kremer (1985) did not measure grazing directly. However, their metabolic measurements indicate that M. mercenaria was responsible for more than 50% of the oxygen consumed and ammonium released by the benthic community (Murphy and Kremer 1985). However, those impacts are now minimal, because of the near-extinction of the population (Burnaford et al. 2011).
CAR-I	Northern Yucatan, Gulf of Mexico, Florida Straits, to Middle Eastern Florida		Economic Impact	Fisheries
Mercenaria mercenaria is now being reared in aquaculture operations in many locations on the Gulf Coast of Florida. This clam is considered superior to the native M. campechiensis due to its longer shelf life.  In these operations, hatchery-reared seed clams are planted in intertidal sands and grown in mesh bags to keep out predators (Arnold et al. 2009).
NEP-VI	Pt. Conception to Southern Baja California		Ecological Impact	Herbivory
The large biomass of M. mercenaria in the Colorado Lagoon appears to have had a substantial impact on phytoplankton populations in 1980, although Murphy and Kremer (1985) did not measure grazing directly. However, their metabolic measurements indicate that M. mercenaria was responsible for more than 50% of the oxygen consumption and ammonium release of the benthic community (Murphy and Kremer 1985). However, those impacts are now minimal, because of the near-extinction of the population (Burnaford et al. 2011).
CA	California		Ecological Impact	Herbivory
The large biomass of M. mercenaria in the Colorado Lagoon appears to have had a substantial impact on phytoplankton populations in 1980, although Murphy and Kremer (1985) did not measure grazing directly. However, their metabolic measurements indicate that M. mercenaria was responsible for more than 50% of the oxygen consumed and ammonium released by the benthic community (Murphy and Kremer 1985). However, those impacts are now minimal, because of the near-extinction of the population (Burnaford et al. 2011).
FL	Florida		Ecological Impact	Hybridization
Mercenaria mercenaria X campechiensis hybrids comprised about 13-18% of clams collected in Charlotte Harbor (Arnold et al. 2009)., Mercenaria mercenaria X campechiensis hybrids comprised about 19-30% of clams collected in Tampa Bay (Arnold et al. 2009)., Mercenaria mercenaria X campechiensis hybrids comprised about 18% of clams collected in a Cedar Key aquaculture site (Arnold et al. 2009)., Mercenaria mercenaria X campechiensis hybrids comprised about 45% of clams collected in Alligator Harbor (Arnold et al. 2009).

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