Littorina littorea

Invasion History

First Non-native North American Tidal Record: 1840
First Non-native West Coast Tidal Record: 1968
First Non-native East/Gulf Coast Tidal Record: 1840

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General Invasion History:

Littorina littorea is native to Europe from the White Sea, Russia to Gibraltar, but absent (except as failed introductions) from the Mediterranean (Bequaert 1943; Reid 1998). The first collection of living animals on the Atlantic Coast of North America was in 1840 at Pictou, Nova Scotia (Willis 1857, cited by Ganong 1887). Although its status has been disputed (Clarke and Erskine 1963; Wares et al. 2002), general historical records and recent molecular data strongly support its introduction to the Atlantic coast of North America sometime in the last 1,000 years, possibly with multiple introductions (Spjeldnaes and Henningsmoen 1963; Blakeslee et al. 2008; Chapman et al. 2008; Brawley et al. 2009). On the west coast of North America, scattered specimens are known from Newport Bay, California to Puget Sound, Washington but establishment of populations has not been documented (Hanna 1966; Carlton 1969; Carlton 1979; Carlton 1992; Chang et al. 2011).

North American Invasion History:

Invasion History on the West Coast:

Scattered occurrences of Littorina littorea have been noticed from California to Washington since 1937. Early occurrences in Puget Sound, Washington in 1937 (Hanna 1966), and Trinidad Bay, California (Carlton 1969) may have resulted from transplants of Eastern Oysters (Crassostrea virginica). Littorina littorea was found in San Francisco Bay in 1968-70 and 1977, but did not become established (Carlton 1992). Another specimen was found in 1995 (Cohen and Carlton 1995), and occasional occurrences have been noted since (Ruiz et al. unpublished data; Chang et al. 2011). In southern California, a single shell was collected in Newport Bay in 1975 (Carlton 1979). A small patch of abundant snails was found in Anaheim Bay in 2002 and was eradicated, apparently successfully (Chang et al. 2011). Several populations of L. littorea in San Francisco Bay were examined, and found to consist entirely of adult snails, with high diversity of genotypes typical of the East Coast, strongly suggesting that these animals had failed to reproduce and had not undergone the 'genetic bottleneck' of reduced diversity typical of newly established invasive populations (Chang et al. 2011). In 2010, populations of L. littorea were found at two sites in Burrard Inlet, Vancouver, British Columbia (354 animals, all adults). These were similar in size to snails purchased from local markets, but had been resident on the shore for some time and had epibionts including algae and barnacles. The periwinkles apparently have not become established, as none were collected after August 2011 (Harley et al. 2013).

Invasion History on the East Coast:

Living specimens of Littorina littorea were first collected on the East Coast at Pictou, Nova Scotia (Willis 1857, cited by Ganong 1887), but they rapidly became one of the most abundant littoral gastropods on Northeastern shores (Verrill 1880; Ganong 1887). As noted above, there has been some uncertainty about the native/introduced status of L. littorea on the East coast of North America. A single fossil specimen from ~40,000 B.P. is known from Nova Scotia (Wagner 1977, cited by Reid 1996). Specimens of L. littorea (dating back to ~1000-1300 B.C.) have been found in Nova Scotia and Newfoundland in Indian and Norse archaeological sites (Clarke 1961; Clarke and Erskine 1963; Spjeldnaes and Henningsmoen 1963). This has been taken to infer the presence of native populations of L. littorea in eastern Canada (Clarke and Erskine 1963) or the introduction of this species by Viking explorers (Spjeldnaes and Henningsmoen 1963). One recent molecular analysis (nuclear and mitochondrial DNA) suggested that populations of Littorina littorea on the Atlantic coast of North America are native (Wares et al. 2002). It has been argued that oceanographic conditions prevented the spread of L. littorea until their rediscovery around 1840 (Clarke and Erskine 1963; Wares et al. 2002). However, the combination of historical, paleontological, and genetic evidence supports introduced status for L. littorea (Chapman et al. 2007).

A more comprehensive genetic analysis by Blakeslee et al. (2008), incorporating molecular analysis with a greater number of sampled haplotypes, parasitological studies, and more sophisticated statistical analysis strongly supports an introduced status for L. littorea in North America and for its common trematode parasite, Cryptocotyle lingua. Other evidence for its introduced status includes: the absence of L. littorea shells in natural deposits, the lack of early collections of a relatively large (and edible) snail, and the improbability of a sudden range expansion, from a small range in Atlantic Canada to ~1,000 km of coastline, in about 50 years (Chapman et al. 2007). The genetic diversity of L. littorea suggests multiple introductions, possibly as early as Viking times, based on estimates using genetic divergence and assumed mutation rates (400 to 1,000 years ago), but with wide confidence intervals. Brawley et al. (2009) examined more closely historical shipping records at Pictou, Nova Scotia, a major port for immigrants from Great Britain and Ireland, and compared genotypes of Pictou and British Isles populations. They estimated divergence times with a lower confidence limit of 192-384 years, roughly comparable with a late 18th-early 19th century introduction suggested by shipping records (Brawley et al. 2009).

From the first collection at Pictou, Nova Scotia in 1840, on the southern Gulf of St. Lawrence, the known range of L. littorea expanded rapidly to the north and south. In 1855, it was collected in Bathurst. New Brunswick, on the Bay of Chaleur (Morse 1880, cited by Bequaert 1943), and in 1882 in Lake Melville, Labrador (USNM 34326, US National Museum of Natural History, 54°N; 60°W, collected by Stearns). This appears to be the current northern limit for this species (Bequaert 1943; Reid 1996). In 1857, it was collected in Halifax, on the Atlantic Coast of Nova Scotia, and in the Bay of Fundy in 1861 (Ganong 1887). By 1873, it occurred at Saco, Maine 'in abundance', and reached Provincetown, Massachusetts by 1870, at which time it was 'very rare'. By 1875, it was abundant there (Bequaert 1943; Dexter; 1961).

To the south of Cape Cod, in 1875 L. littorea was rare at Woods Hole, Massachusetts, but abundant by 1876 (Ganong 1887). Its spread to the south was rapid: New Haven, Connecticut in 1879 (Bequaert 1943); Newport, Rhode Island in 1880 (Ganong 1887); Staten Island, New York in 1888 (Bequaert 1943); Atlantic City, New Jersey in 1892 (Bequaert 1943). However, the range expansion slowed down sharply, south of Atlantic City. Littorina littorea reached Cape May, New Jersey by 1928 (Bequaert 1943) and Cape Henlopen, Delaware by 1970 (Kraueter 1974). In 1959, Wells (1965) collected L. littorea in the Maryland region, stating it was rare 'attached to the rocks of a jetty at West Ocean City, directly opposite Ocean City Inlet'. At Assateague Island, Maryland-Virginia in 1988-1989 (exact site not given), it was found 'only on rock jetties and wooden groins' (Counts and Bashore 1991). It was reported from Chincoteague, Virginia in 1971 (Kraueter 1974) and later at Wachapreague (date not given) (Vermeij 1982b).

Invasion History Elsewhere in the World:

In the Mediterranean, Littorina littorea has been collected in Italy, in the Lagoon of Faro, Sicily in 1978 and in Livorno, on the Tyrrhenian Sea, but these populations were extinct by 1988 (Johanesson 1988; Zenetos et al. 2004; Crocetta 2012).

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Description

The shell of Littorina littorea is solid, heavy and turban shaped, topped with an onion-shaped spire. It is dextrally coiled, with 6 or 7 whorls, and lacks an umbilicus. The shell surface has shallow sutures, resulting in flat-sided whorls on the spire, sculpted by low, narrow ribs and fine spiral lines. The color varies from uniform cream to brown, black, orange, or red, but usually pale brown with 8-25 narrow black or brown lines. The columella is white and the outer edge of the aperture is brown or with brown lines. The interior of the aperture is brown. The shell commonly reaches about 31 mm in length, but the maximum is 53 mm. Description from: Bequaert 1943; Morris 1975; Gosner 1978; and Reid 1996.

This species lays its eggs in distinctive transparent, planktonic, egg capsules, in the form of flat disks (0.3 - 1.1 mm diameter) (Reid 1998). The planktotrophic veligers are distinguished by a prominent red or black pigmented spot on the anterior corner of each lobe of the velum. The veligers reach about 0.6 mm before settling (Newell and Newell 1997).

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Taxonomy

Ecology

General:

Littorina littorea occurs on rocky coasts, but also on mud, sand and marsh habitats in some parts of its range. Sexes are separate, with males slightly larger than females, and fertilization is internal. This snail reaches a mature size in one year and has a life span of at least 5 years. Aquarium specimens are reported to have lived 10 years or more (Clay 1961). The eggs are laid in water and are contained in gelatinous capsules, in the shape of a shallow dome, surrounded by a flat brim, each containing 1-9 (but usually 1-3) eggs. The spawning season varies with latitude, from summer in the Barents Sea to late winter-spring or early summer in Denmark, England, and the Netherlands (Clay 1961; Reid 1996). In New Brunswick, spawning occurs in April to November (Chase and Thomas 1995a), while in Narragansett Bay, Rhode Island egg capsules and larvae occurred year-round, but were rare in July-August (Fofonoff, unpublished data). Females in New Brunswick released an estimated 50,000 to 200,000 embryos per season. The egg capsules are usually suspended in the plankton and can be abundant in nearshore waters. The eggs take about 6 days to hatch into planktotrophic veligers, which remain in the plankton for 2-4 weeks (Fretter and Graham 1962). The larvae settle with shells about 0.275 mm in length and 0.375-390 mm in diameter (Reid 1996).

Juveniles and adults of the Common Periwinkle inhabit the middle and upper intertidal zone, primarily of rocky shores, but also extending into mudflats and marshes. On rocky shores, their distribution is often associated with rockweeds (Fucaceae) and is affected by wave action and food availability (Clay 1961; Reid 1996). The preference of L. littorea for the mid- and upper intertidal zone is related to the presence of more effective predators in the subtidal zone. Growth of caged juvenile snails in the Gulf of Maine, was greater in the subtidal and lower intertidal, but a subtidal predator, the Jonah Crab (Cancer borealis), was a more effective predator on larger snails than the intertidal Green Crab (Carcinus maenas) (Perez et al. 2009). However, some specimens of L. littorea have been found at maximum depths ranging from 15 m to 60 m, especially in more northern locations (Reid 1996). On soft sediments, in sheltered areas, periwinkles are usually associated with scattered stones, shells, mussel beds, or wood, which provide sites of attachment. The snails leave these refuges to make feeding excursions on the sediment surface (Reid 1996).

As an inhabitant of the temperate intertidal zone, the Common Periwinkle is tolerant of a wide range of temperature and salinity. It can tolerate temperatures as high as 41°C for up to 48 h (Fraenkel 1960), but an average summer temperature of 21°C seems to mark the southern limit for this species (Wells 1965). It can also tolerate short-term exposure to fresh water from rain and runoff, but reported lower limits for long-term survival range from 9.5 to 16 PSU (Todd 1964; Remane and Schleiper 1971; Reid 1996). Todd (1964) found a strong effect of temperature on salinity tolerance, with animals surviving well at 11 PSU at 5°C, but dying at salinities below 16 PSU at 15°C. The lower salinity limit for reproduction was about 15 PSU, in Kiel Bay, Germany, but was around 20 PSU in other European locations (Hayes 1929, cited by Remane and Schlieper 1971).

Littorina littorea is an omnivorous grazer, feeding on a wide range of algae and on small attached organisms, especially opportunistic algae, such as Ulva, Cladophora and Ectocarpus, and early settlement stages (germlings) of more robust seaweeds such as Fucus (Lubchenco 1978; Lubchenco and Menge 1978; Lubchenco 1983; Bertness et al. 1984; Reid 1996; Bertness et al. 2004). On soft sediment, L. littorea grazes benthic microalgae on the sediment surface and also grazes on the leaves and rhizomes of Spartina alterniflora (Smooth Cordgrass), the dominant marsh grass in northeastern salt marshes (Reid 1996; Bertness 1984; Tyrell et al. 2008). This periwinkle is basically a herbivore, but does ingest small animals, such as newly attached barnacles and mussels, and the egg capsules of other snails, such as those of Ilyanassa obsoleta. Whether it digests these organisms is unknown (Brenchley 1982; Brenchley and Carlton 1983; Reid 1996).

As a very abundant intertidal organism on the coast of Northeastern North America, L. littorea is an important component of coastal food webs. It is a major food item for crabs, fishes, and shorebirds, and also a major final and intermediate host of numerous parasites. Among frequent predators in East Coast waters are Green Crabs (Carcinus maenas), Asian Shore Crabs (Hemigrapsus sanguineus), Blue Crabs (Callinectes sapidus), Jonah Crabs (Cancer borealis), Dogwinkles (Nucella lapillis), Oyster Drills (Urosalpinx cinereus), Mummichogs (Fundulus heteroclitus), sandpipers (Calidris spp.), gulls (Larus spp.), and crows (Corvus spp.) (Dexter 1947; Peterson 1979; Gerard et al. 1999; Trussell et al. 2004; Ellis et al. 2007; Byers et al. 2008; Perez et al. 2009).

The Common Periwinkle hosts a variety of parasites. Eleven species of digenean trematodes are known from Europe, while only six have been found in Eastern North American populations (Blakeslee and Byers 2008). The most abundant of these, Cryptocotyle lingua, was introduced to North American waters, probably with L. littorea, based on genetic data (Blakeslee et al. 2008). The rediae (first parasitic stage) of C. lingua occur at an average prevalence of 9-10% in North American L. littorea populations, but can be much more abundant locally. Prevalence at New England intertidal sites was strongly influenced by the abundance of Herring Gulls (Larus argentatus) the final host of the parasite (Byers et al. 2008). Heavy infection with these parasites can damage the snail's digestive organs and greatly reduce their grazing and reproductive rates (Blakeslee and Byers 2008). In addition to obvious parasites, the bodies and shells of introduced L. littorina (sampled in Nova Scotia) host a wide variety of endosymbionts (at least 7 phyla), whose exact identity, native status, and impacts on the snail's biology are unknown (Buckland-Nicks et al. 2013).

Food:

benthic algae; detritus; egg capsules

Consumers:

crabs, fishes, shorebirds, humans

Competitors:

Trophic Status:

Herbivore

Herb

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Habitats

General Habitat	Unstructured Bottom	None
General Habitat	Marinas & Docks	None
General Habitat	Rocky	None
General Habitat	Salt-brackish marsh	None
Tidal Range	Low Intertidal	None
Tidal Range	Mid Intertidal	None
Vertical Habitat	Epibenthic	None

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Tolerances and Life History Parameters

Minimum Temperature (ºC)	0.1	Field Data- Denmark (Clay 1961)
Maximum Temperature (ºC)	41	Survival - Lethal temperature from 48 hour experiment. (Fraenkel 1960). There is a relationship between lethal temperature and the level of shore inhabited by snail- Periwinkles from high tide levels have higher lethal temperatures than those from low tide level; also varies seasonally (Clay 1961). Although this snail tolerates very high short-term temperatures, such as might be found in tide pools, an average summer temperature of 21 C seems to mark the southern limit for this species (Wells 1965).
Minimum Salinity (‰)	9.5	Field: Remane and Schleiper 1971; Reid 1996. Todd (1964) found that survival at low salinities varied with temperature, with lower limits at 11 PSU at 5 C, but 16 at 15 C.
Maximum Salinity (‰)	48	Experimental (Todd 1964)
Minimum Reproductive Salinity	15	Field, Kiel Bay, Germany. However, 20 PSU is a more common limit for populations outside the Baltic (Hayes 1929, cited by Remane and Schleiper 1971)
Minimum Duration	20	Planktonic egg (6 days, typical) + Larval Period (Fretter and Graham 1962)
Maximum Duration	34	Planktonic egg (6 days, , typical) + Larval Period (Fretter and Graham 1962)
Minimum Length (mm)	10.6	Minimum size at maturation (Reid 1996)
Maximum Length (mm)	53	Maximum reported size, but a more usual maximum is ~31 mm (Bequaert 1943; Morris 1975; Gosner 1978; Reid 1996)
Broad Temperature Range	None	Cold-Temperate
Broad Salinity Range	None	Mesohaline-Euhaline

General Impacts

Littorina littorea is a common food item in Europe, but is rarely eaten in the US. However, it has had major impacts on food webs, parasite communities, and even on the physical structure of coastlines in Northeastern North America. It is often one of the most abundant grazers in rocky intertidal areas, where it can control the abundance and composition of seaweeds. It is also abundant in marshes, mudflats, and cobble beaches where its grazing can remove vegetation and result in the erosion of sediment, converting soft-bottom shores to rocky habitats (Clay 1961; Bertness 1984; Lubchenco 1986). It is a competitor with native snails and is prey for introduced crabs and many native predators, including carnivorous snails, crabs, fishes, and birds (Dexter 1947; Brenchley and Carlton 1983). It is also a host to many parasites, including digenean trematodes which infect fishes and birds in later stages of their life cycle. One of these parasites, Cryptocotyle lingua, was introduced to North America along with L. littorea (Stunkard 1930; Blakeslee and Byers 2008; Blakeslee et al. 2008).

Economic Impacts

Fisheries: Littorina littorea is a common food item in European cultures, but is rarely part of American cuisine. However, it is frequently eaten by European, Asian, and African immigrants. In the British Isles, 'winkles' are eaten raw or roasted, and the animal extracted from the shell with a bent pin (Clay 1961). Cohen (2012) found L. littorea for sale at 6 of 11 Asian seafood markets, which were visited in San Francisco and Los Angeles. We have no estimate of the economic value of this snail's harvest on North American shores.

Ecological Impacts

Competition: Littorina littorea competes with native shore molluscs for food and living space on Northwest Atlantic shores. Manipulative experiments show that L. littorea can decrease the growth rate of the native L. saxatilis (Rock Periwinkle) and Notoacmea testudinalis (Tortoiseshell Limpet). Field observations and experiments indicate that the Eastern Mudsnail (Ilyanassa obsoleta) emigrates to avoid large concentrations of L. littorea (Brenchley and Carlton 1983). After the Littorina invasion, I. obsoleta abandoned pilings and cobble beaches, and became rare in peat habitats of marshes, habitats where it was formerly abundant. It is now largely restricted to soft-mud habitats not used by L. littorea (Brenchley and Carlton 1983). One of the mechanisms of this competition is interference – L. littorea will climb onto the shells of I. obsoleta and graze algae growing on the shell, causing the mudsnail to twist to dislodge the periwinkle, interfering with the mudsnail's feeding (Brenchley and Carlton 1983).

Herbivory: Littorina littorea is now the most abundant large invertebrate herbivore on the Northwest Atlantic Coast from the Gulf of St. Lawrence to New Jersey. Addition and removal experiments suggest a strong impact on ephemeral green algae (Ulva spp.) in the intertidal, but it also grazes germlings of larger canopy-forming algae, such as Chondrus crispus and Fucus spp. (Lubchenco 1978; Lubchenco and Menge 1978; Lubchenko 1980; Lubchenco 1983; Petraitis 1983; Bertness 1984; Lubchenco 1986). Impacts of L. littorea were greatest at sheltered sites, since L. littorea was rare or absent at sites with heavy wave action (Lubchenco 1978). Littorina littorea also grazes the leaves and rhizomes of the marsh grass Spartina alterniflora, reducing its growth and reproduction on a cobble beach in Rhode Island (Bertness 1984). In a marsh environment in Maine, grazing of L. littorea on S. alterniflora affected the grass' growth only in stressful habitats (poor drainage, long tidal flooding) but did not affect biomass in more optimal habitats. Grazing by Littorina littorea may control the recruitment of introduced Codium fragile ssp. fragile in the upper intertidal, where this alga is also stressed by ultraviolet, freezing, and desiccation. Grazing has less effect in the lower intertidal, where Codium suffers less damage and grows at a faster rate (Scheibling et al. 2008).

Habitat Change: Grazing by Littorina littorea in the rocky intertidal helps to benefit canopy-forming seaweeds such as Chondrus crispus and Fucus spp. by removing ephemeral algal competitors. The canopy-forming seaweeds provide shelter for many species of intertidal invertebrates (Lubchenco and Menge 1978; Lubchenco 1980; Lubchenco 1983; Petratis 1983; Bertness 1984). Grazing by L. littorea had different effects on the Rock Barnacle Semibalanus balanoides (in Long Island sound NY) depending on snail density. Moderate densities favored barnacle settlement by removing algae, but high densities of snails removed large numbers of barnacle cyprids (Petraitis 1983).

Removal experiments on a cobble beach in Mount Hope Bay, Rhode Island indicate that herbivory by Littorina littorea has major effects on habitat structure by removing algae and Spartina alterniflora (Smooth Cordgrass), preventing the accumulation of sediment and removing habitat for soft-bottom fauna (Bertness 1984). Bertness suggests that the Littorina invasion may have resulted in the reduction of marsh habitat and other soft-bottom habitats (Bertness 1984). In marsh environments in the Wells National Estuarine Research Reserve, Maine removal experiments showed that in stressed sites (long inundation, poor drainage), L. littorea reduced the growth of S. alterniflora, but had no effect at a benign site (Tyrrell et al. 2008). Grazing by this introduced snail could reduce the biomass of Spartina at stressed sites, increasing rates of erosion, particularly as a response to sea-level rise (Tyrrell et al. 2008). The extent and magnitude of L. littorea impacts on Spartina may vary among habitats.

Predation: Littorina littorea is a major predator on the egg capsules of I. obsoleta (Eastern Mudsnail) in Barnstable Harbor, Massachusetts (Brenchley 1982). The effect of this predation on the abundance of I. obsoleta is not clear.

Food/Prey: The high biomass of L. littorea at many sites may be benefiting two invading crab species, the Green Crab (Carcinus maenas) and Asian Shore Crab (Hemigrapsus sanguineus), as Littorina littorea is a frequent food item for both species (H. s., Gerard et al. 1999; C. m., Vermeij 1982; Ropes 1989). A wide range of native predators feed on the Common Periwinkle, including predatory snails, crabs, fishes, shorebirds, gulls, and crows (Dexter 1947; Peterson 1979; Ellis et al. 2007).

Parasite/Predator Vector: Littorina littorea has served as a probable vector for one parasite introduced from the coast of Europe, the digenean trematode, Cryptocotyle lingua. This parasite infects Littorina spp. from eggs scattered in the feces of birds and mammals and then develops into the next stage, rediae, which consume some of the snail's tissues, before developing into swimming cercariae, which leave the snail and infect the tissues of fishes, as metacercariae. When the fishes are eaten by birds or mammals, the trematodes mature and shed eggs (Stunkard 1930; Sindermann and Farrin 1962). Genetic analyses of L. littorea and C. lingua from many sites on both sides of the Atlantic, indicates that both the snail and trematode have undergone a similar reduction in genetic diversity and were introduced at roughly the same time (Blakeslee and Byers 2008; Blakeslee et al. 2008). The early stages of the trematode also occur in the native periwinkles L. obtusata and L. saxatilis (Blakeslee and Byers 2008; Blakeslee et al. 2008). In fishes, the cercariae can cause unpleasant pigment spots in fish flesh, while heavy infections can kill young Atlantic Herring (Clupea harengus) (Sindermann and Farrin 1962). In birds ('terns', probably Common Terns, Sterna sterna) and dogs fed infected fish, localized intestinal damage occurred, followed by the development of immunity to infection (Willey and Stunkard 1942). The introduction of L. littorea thus appears to have introduced a parasite that has infected three trophic levels of the Northwest Atlantic food web.

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Regional Impacts

NA-ET1	Gulf of St. Lawrence to Bay of Fundy		Ecological Impact	Herbivory
Grazing by Littorina littorea may control the recruitment of Codium fragile ssp. fragile in the upper intertidal, where this alga is also stressed by ultraviolet, freezing, and dessication. Grazing by L. littorina has less effect in the lower intertidal, where Codium suffers less damage and grows at a faster rate (Scheibling et al. 2008)
NA-ET3	Cape Cod to Cape Hatteras		Ecological Impact	Herbivory
Removal experiments in Mount Hope Bay RI indicate that herbivory by Littorina littorea removes leafy algae such as Ulva spp. on a cobble beach, resulting in the growth of an algal canopy (Bertness 1984). In Port Jefferson Harbor NY, Long Island Sound, grazing by L. littorea also reduced abundance of Ulva, though dense patches were grazed only at high densities, and the alga managed to persist in irregularities in the rocks (Petraitis 1983). In Mount Hope Bay, grazing by L. littorea also reduced the growth and reproduction of the marsh grass Spartina alterniflora (Smooth Cordgrass) by consuming rhizomes of the plant (Bertness 1984).
NA-ET3	Cape Cod to Cape Hatteras		Ecological Impact	Habitat Change
Removal experiments in Mount Hope Bay, RI indicate that herbivory by Littorina littorea has major effects on habitat structure, by removing algae and Spartina, preventing the accumulation of sediment, and removing habitat for soft-bottom fauna (Bertness 1984). Bertness suggests that the Littorina invasion may have resulted in the reduction of marsh habitat and other soft-bottom habitats (Bertness 1984). Grazing by L. littorea had different effects on the Rock Barnacle Semibalanus balanoides (in Long Island Sound, NY) depending on snail density. Moderate densities favored barnacle settlement by removing algae, but high densities of snails removed large numbers of barnacle cyprids (Petraitis 1983).
NA-ET2	Bay of Fundy to Cape Cod		Ecological Impact	Herbivory
Littorina littorea is the most abundant herbivorous intertidal invertebrate on rocky shores in the Gulf of Maine (Bertness et al. 2002; Bertnes et al. 2004). Addition and removal experiments indicated that L. littorea did not graze significantly on the dominant alga, Chondrus crispus, when it was already established, but did affect algal succession by grazing on emphemeral algae (e.g. Ulva, Enteromorpha spp. which compete with Chondrus, damping variation in Chondrus abundance (Lubchenco 1978; Lubchenco and Menge 1978). The periwinkles graze the germlings of the larger brown seaweed Fucus spp., but also benefit Fucus by removing competitive ephemerals and epibiotic algae (Lubchenko 1980; Lubchenco 1983; Lubchenco 1986) Impacts of L. littorea were greatest at sheltered sites (Canoe Beach Cove, Nahant, MA and Grindstone Neck ME), since L. littorea was rare or absent at sites with heavy wave action (Lubchenco and Menge 1978). In the tidal Damariscotta River estuary, heavy grazing by L. littorea limited growth of most algal species,in areas with slow current flow, leading to the domination of Ascophyllum nodosum (Bertness et al. 2002). In marsh environments in the Wells National Estuarine Research Reserve, ME, removal experiments showed that in a stressed site (long inundation, poor drainage), L. littorea reduced the growth of the marsh grass Spartina alterniflora (Smooth Cordgrass), but had no effect at a benign site (Tyrrell et al. 2008).
NA-ET2	Bay of Fundy to Cape Cod		Ecological Impact	Habitat Change
Grazing by Littorina littorea helps to benefit canopy-forming seaweeds such as Chondrus crispus spp. and Fucus spp. by removing ephemeral alga competitors. The canopy forming seaweeds provide shelter for many species of intertidal invertebrates (Lubchenco and Menge 1978; Lubchenco 1980; Lubchenco 1983). In marsh environments in the Wells National Estuarine Research Reserve, ME, removal experiments showed that in a stressed site (long inundation, poor drainage), Littorina reduced the growth of the marsh grass Spartina alterniflora (Smooth Cordgrass), but had no effect at a benign site (Tyrrell et al. 2008). Grazing by this introduced snail could reduce the biomass of Spartina at stressed sites, increasing rates of erosion, particularly as a response to sea-level rise (Tyrrell et al. 2008)
N170	Massachusetts Bay		Ecological Impact	Herbivory
Littorina littorea is the most abundant herbivorous intertidal invertebrate on rocky shores in the Gulf of Maine. Manipulative experiments indicated that L. littorea did not graze significantly on the dominant alga, Chondrus crispus, when it was already established, but did affect algal succession by grazing on emphemeral algae (e.g. Ulva, Enteromorpha spp. which compete with Chondrus, damping variation in Chondrus abundance; Lubchenco 1978; Lubchenco and Menge 1978). The periwinkles graze the germlings of the larger brown seaweed Fucus spp., but also benefit Fucus by removing competitive ephemerals and epibiotic algae (Lubchenko 1980; Lubchenco 1983; Lubchenco 1986).  Impacts of L. littorea were greatest at a sheltered sites of Canoe Beach Cove, Nahant, MA, since L. littorea was rare or absent at sites with heavy wave action (Lubchenco and Menge 1978).
N036	_CDA_N036 (Maine Coastal)		Ecological Impact	Herbivory
Littorina littorea is the most abundant herbivorous intertidal invertebrate on rocky shores in the Gulf of Maine. Manipulative experiments indicated that L. littorea did not graze significantly on the dominant alga, Chondrus crispus, when it was already established, but did affect algal succession by grazing on emphemeral algae (e.g. Ulva spp. which compete with Chondrus, damping variation in Chondrus abundance; Lubchenco 1978; Lubchenco and Menge 1978). The periwinkles graze the germlings of the larger brown seaweed Fucus spp., but also benefit Fucus by removing competitive ephemerals and epibiotic algae (Lubchenko 1980; Lubchenco 1983; Lubchenco 1986). Impacts of L. littorea were greatest at sheltered sites, since L. littorea was rare or absent at sites with heavy wave action (Lubchenco 1978). Moderate abundances and impacts of L. littorea were found at Grindstone Neck (near Schoodic Point), Maine (Lubchenco and Menge 1978; Lubchenco 1980; Lubchenco 1983).
N050	Penobscot Bay		Ecological Impact	Herbivory
Littorina littorea is the most abundant herbivorous intertidal invertebrate on rocky shores in the Gulf of Maine. Removal experiments indicated that L. littorea did not graze significantly on the dominant alga, Chondrus crispus, when it was already established, but did affect algal succession by grazing on emphemeral algae which compete with Chondrus, damping variation in Chondrus abundance. Impacts of L. littorea were greatest at sheltered sites such as Little Brewster Island, ME, since L. littorea was rare or absent at sites with heavy wave action (Lubchenco 1978).
N170	Massachusetts Bay		Ecological Impact	Habitat Change
Grazing by Littorina littorea helps to benefit canopy-forming seaweeds such as Chondrus crispus spp. and Fucus spp. by removing ephemeral alga competitors. The canopy forming seaweeds provide shelter for many species of intertidal invertebrates. Canoe Beach Cove, Nahant, MA was one experimental site where these impacts were observed (Lubchenco and Menge 1978; Lubchenco 1980; Lubchenco 1983).
N036	_CDA_N036 (Maine Coastal)		Ecological Impact	Habitat Change
Grazing by Littorina littorea helps to benefit canopy-forming seaweeds such as Chondrus crispus spp. and Fucus spp. by removing ephemeral alga competitors. The canopy forming seaweeds provide shelter for many species of intertidal invertebrates (Lubchenco and Menge 1978; Lubchenco 1980; Lubchenco 1983). Moderate abundances and impacts of L. littorea were found at Grindstone Neck (near Schoodic Point), Maine (Lubchenco and Menge 1978; Lubchenco 1980; Lubchenco 1983).
M020	Narragansett Bay		Ecological Impact	Herbivory
Removal experiments in Mount Hope Bay, RI, indicate that herbivory by Littorina littorea removes leafy algae such as Ulva spp. on a cobble beach, resulting in the growth of an algal canopy. Grazing by L. littorea also reduced the growth and reproduction of the marsh grass Spartina alterniflora (Smooth Cordgrass) by consuming rhizomes of the plant (Bertness 1984).
M020	Narragansett Bay		Ecological Impact	Habitat Change
Removal experiments in Mount Hope Bay, RI, indicate that herbivory by Littorina littorea has major effects on habitat structure, by removing algae and Spartina, preventing the accumulation of sediment, and removing habitat available for soft-bottom fauna (Bertness 1984). Bertness suggests that the Littorina invasion may have resulted in the reduction of marsh habitat and other soft-sediment habitats (Bertness 1984).
N120	Wells Bay		Ecological Impact	Herbivory
In marsh environments in the Wells National Estuarine Research Reserve, ME, removal experiments showed that in a stressed site (long inundation, poor drainage), Littorina reduced the growth of the marsh grass Spartina alterniflora (Smooth Cordgrass), but had no effect at a benign site (Tyrrell et al. 2008).
N120	Wells Bay		Ecological Impact	Habitat Change
In marsh environments in the Wells National Estuarine Research Reserve, ME, removal experiments showed that in a stressed site (long inundation, poor drainage), Littorina reduced the growth of the marsh grass Spartina alterniflora (Smooth Cordgrass), but had no effect at a benign site (Tyrrell et al. 2008). Grazing by this introduced snail could reduce the biomass of Spartina at stressed sites, increasing rates of erosion, particularly as a response to sea-level rise (Tyrrell et al. 2008)
NA-ET2	Bay of Fundy to Cape Cod		Ecological Impact	Predation
Littorina littorea is a major predator on the egg capsules of the snail Ilyanassa obsoleta (Eastern Mudsnail) in Barnstable Harbor MA (Brenchley 1984). The effect of this predation on abundance of I. obsoleta is not clear.
N180	Cape Cod Bay		Ecological Impact	Predation
Littorina littorea is a major predator on the egg capsules of the snail Ilyanassa obsoleta (Eastern Mudsnail) in Barnstable Harbor, MA (Brenchley 1984). The effect of this predation on the abundance of I. obsoleta is not clear.
M010	Buzzards Bay		Ecological Impact	Competition
In the Woods Hole area, from 1900 to 1930, Littorina littorea was observed to gradually exclude Ilyanassa obsoleta from cobble shores and wood pilings, and increasingly invading marshes, largely confining I. obsoleta to mudflats (Dimon 1905; Clench 1930, cited by Brenchley and Carlton 1983). However, experimental studies of the mechanisms of competition were not conducted in this region, to our knowledge. In experimental cage manipulations at Nobska Point and Gansett Beach, Woods Hole, MA L. littorea depressed the growth of L. saxatilis (Rough Periwinkle) in the lower intertidal zone, probably due to food competition. However, this effect was not seen in the upper intertidal zone, where L. saxatilis is more abundant (Yamada and Mansour 1987).
NA-ET3	Cape Cod to Cape Hatteras		Ecological Impact	Competition
In the Woods Hole area, from 1900 to 1930, Littorina littorea was observed to gradually exclude Ilyanass obsoleta (Eastern Mudsnail), from cobble shores and wood pilings, and increasingly invading marshes, largely confining I. obsoleta to mudflats (Dimon 1905; Clench 1930, cited by Brenchley and Carlton 1983). However, experimental studies of the mechanisms of competition were not conducted in this region, to our knowledge. In Mount Hope Bay RI, I. obsoleta was observed to emigrate from sites where large numbers of L. littorea had been added (Bertness, unpublished, cited by Brenchley and Carlton 1983). In experimental cage manipulations near Woods Hole MA, L. littorea depressed the growth of L. saxatilis (Rough Periwinkle), probably due to food competition. However, this effect was not seen in the upper interidal zone, where L. saxatilis is more abundant (Yamada and Mansour 1987).
NA-ET2	Bay of Fundy to Cape Cod		Ecological Impact	Competition
In Barnstable Harbor MA, Ilyanassa obsoleta (Eastern Mudsnail) leave areas of mudflat habitat when large numbers of L. littorea are added. This avoidance behavior may be due in part to the tendency of L. littorea to climb onto the shells of I. obsoleta and graze on the shell surface (Brenchley and Carlton 1982). Removal experiments (on Swans Island ME) indicated that growth, weight gain and survival of the native limpet Notoacmea testudinalis was reduced in the presence of L. littorea (Petraitis 1989).
N180	Cape Cod Bay		Ecological Impact	Competition
In Barnstable Harbor MA, Ilyanassa obsoleta (Eastern Mudsnail) leave areas of mudflat habitat when large numbers of L. littorea are added. This avoidance behavior may be due in part to the tendency of L. littorea to climb onto the shells of I. obsoleta and graze on the shell surface (Brenchley and Carlton 1982).
M020	Narragansett Bay		Ecological Impact	Competition
In Mount Hope Bay, RI, I. obsoleta was observed to emigrate from sites where large numbers of L. littorea had been added (Bertness, unpublished, cited by Brenchley and Carlton 1983).
N045	_CDA_N045 (Maine Coastal)		Ecological Impact	Competition
Removal experiments (on Swans Island, ME) indicated that growth, weight gain and survival of the native limpet Notoacmea testudinalis was reduced in the presence of L. littorea (Petraitis 1989).
M040	Long Island Sound		Ecological Impact	Herbivory
In Port Jefferson Harbor, NY, Long Island Sound, grazing by L. littorea also reduced abundance of Ulva, though dense patches were grazed only at high densities and the alga managed to persist in irregularities in the rocks (Petraitis 1983).
M040	Long Island Sound		Ecological Impact	Habitat Change
Grazing by L. littorea had different effects on the Rock Barnacle Semibalanus balanoides (in Long Island Sound, NY) depending on snail density. Moderate densities favored barnacle settlement by removing algae, but high densities of snails removed large numbers of barnacle cyprids (Petraitis 1983).
NA-ET3	Cape Cod to Cape Hatteras		Ecological Impact	Food/Prey
The high biomasses of L. littorea may have benefited two invading crab species, the Green Crab (Carcinus maenas and Asian Shore Crab (Hemigrapsus sanguineus). Littorina littorea is a frequent food item for both species (H. s., Gerard et al.1999; C. m., Vermeij 1982; Ropes 1989).
NA-ET2	Bay of Fundy to Cape Cod		Ecological Impact	Food/Prey
The high biomasses of L. littorea may have benefited two invading crab species, the Green Crab (Carcinus maenas) and Asian Shore Crab (Hemigrapsus sanguineus). Littorina littorea was a frequent food item for both species (H. s., Gerard et al.1999; C. m., Rops 1989). On the Isles of Shoals ME, L. littorea were a major prey for Jonah Crabs (Cancer borealis), Green Crabs, and Dogwhelks (Nucella lapillus) (Ellis et al. 2007).
N070	Damariscotta River		Ecological Impact	Herbivory
In the tidal Damariscotta River estuary, heavy grazing by L. littorea limited growth of most algal species, in areas with slow current flow, leading to the domination of Ascophyllum nodosum (Bertness et al. 2002). On exposed Pemaquid Point, exclusion of L. littorea by cages from cleared areas of rocky shore led to much more rapid regrowth of algae than from control areas (Bertness et al. 2004).
N135	_CDA_N135 (Piscataqua-Salmon Falls)		Ecological Impact	Food/Prey
On the Isles of Shoals, ME, L. littorea were a major prey for Jonah Crabs (Cancer borealis), Green Crabs (Caricnus maenas), and Dogwhelks (Nucella lapillus) (Ellis et al. 2007).
NA-S3	None		Ecological Impact	Parasite/Predator Vector
The introduction of Littorina littorea to the Northwest Atlantic is believed to have also introduced the digenean trematode Cryptocotyle lingua. Common Periwinkles collected at 6 sites in the Gulf of St. Lawrence, were infected with C. lingua (Blakeslee and Byers 2008). The rediae (first parasitic stages) have infected not only L. littorea, but also native populations of L. saxatilis (Rough Periwinkle) and L. obtusata (Smooth Periwinkle) (Blakeslee and Byers 2008; Blakeslee et al. 2008). The trematodes cause extensive damage to the host's digestive and reproductive systems (Wood et al. 2009). The rediae of C. lingua metamorphose into cercariae, which have a swimming tail and infect fishes (Stunkard 1930; Sindermann et al. 1962). When infected fish are eaten by birds or mammals, the metacercariae grow in the final host's digestive tract and and reproduce (Stunkard 1930; Sindermann et al. 1962). Thus, the introduction of the Common Periwinkle has added a new parasite to 3 trophic levels.
NA-ET1	Gulf of St. Lawrence to Bay of Fundy		Ecological Impact	Parasite/Predator Vector
The introduction of Littorina littorea to the Northwest Atlantic is believed to have also introduced the digenean trematode Cryptocotyle lingua. Common Periwinkles, collected at 4 sites on the Atlantic coast of Maritime Canada, were infected with C. lingua (Blakeslee and Byers 2008). The rediae (first parasitic stages) have infected not only L. littorea, but also native populations of L. saxatilis (Rough Periwinkle) and L. obtusata (Smooth Periwinkle) (Blakeslee and Byers 2008; Blakeslee et al. 2008). The trematodes cause extensive damage to the host's digestive and reproductive systems (Wood et al. 2009). The rediae of C. lingua metamorphose into cercariae, which have a swimming tail and infect fishes (Stunkard 1930; Sindermann et al. 1962). When infected fish are eaten by birds or mammals, the metacercariae grow in the final host's digestive tract and and reproduce (Stunkard 1930; Sindermann et al. 1962). Thus, the introduction of the Common Periwinkle has added a new parasite to 3 trophic levels.
NA-ET2	Bay of Fundy to Cape Cod		Ecological Impact	Parasite/Predator Vector
The introduction of Littorina littorea to the Northwest Atlantic is believed to have also introduced the digenean trematode Cryptocotyle lingua. Common Periwinkles, collected at 11 sites in the Gulf of Maine, were infected with C. lingua (Pohley et al. 1976; Blakeslee and Byers 2008). The rediae (first parasitic stages) have infected not only L. littorea, but also native populations of L. saxatilis (Rough Periwinkle) and L. obtusata (Smooth Periwinkle) (Pohley 1976; Blakeslee and Byers 2008; Blakeslee et al. 2008). The trematodes cause extensive damage to the host's digestive and reproductive systems (Wood et al. 2009). The rediae of C. lingua metamorphose into cercariae, which have a swimming tail and infect fishes (Stunkard 1930; Sindermann et al. 1962). When infected fish are eaten by birds or mammals, the metacercariae grow in the final host's digestive tract and and reproduce (Stunkard 1930; Sindermann et al. 1962). Thus, the introduction of the Common Periwinkle has added a new parasite to 3 trophic levels.
NA-ET3	Cape Cod to Cape Hatteras		Ecological Impact	Parasite/Predator Vector
The introduction of Littorina littorea to the Northwest Atlantic is believed to have also introduced the digenean trematode Cryptocotyle lingua. Common Periwinkles, collected at 7 sites from Massachusetts, were infected with C. lingua (Stunkard 1930; Pohley 1976; Blakeslee and Byers 2008). The rediae (first parasitic stages) have infected not only L. littorea, but also native populations of L. saxatilis (Rough Periwinkle) and L. obtusata (Smooth Periwinkle) (Pohley 1976; Blakeslee and Byers 2008; Blakeslee et al. 2008). The trematodes cause extensive damage to the host's digestive and reproductive systems (Wood et al. 2009). The rediae of C. lingua metamorphose into cercariae, which have a swimming tail and infect fishes (Stunkard 1930; Sindermann et al. 1962). When infected fish are eaten by birds or mammals, the metacercariae grow in the final host's digestive tract and reproduce (Stunkard 1930; Sindermann et al. 1962). Thus, the introduction of the Common Periwinkle has added a new parasite to 3 trophic levels.
MA	Massachusetts		Ecological Impact	Competition
In the Woods Hole area, from 1900 to 1930, Littorina littorea was observed to gradually exclude Ilyanassa obsoleta from cobble shores and wood pilings, and increasingly invading marshes, largely confining I. obsoleta to mudflats (Dimon 1905; Clench 1930, cited by Brenchley and Carlton 1983). However, experimental studies of the mechanisms of competition were not conducted in this region, to our knowledge. In experimental cage manipulations at Nobska Point and Gansett Beach, Woods Hole, MA L. littorea depressed the growth of L. saxatilis (Rough Periwinkle) in the lower intertidal zone, probably due to food competition. However, this effect was not seen in the upper intertidal zone, where L. saxatilis is more abundant (Yamada and Mansour 1987)., In Barnstable Harbor MA, Ilyanassa obsoleta (Eastern Mudsnail) leave areas of mudflat habitat when large numbers of L. littorea are added. This avoidance behavior may be due in part to the tendency of L. littorea to climb onto the shells of I. obsoleta and graze on the shell surface (Brenchley and Carlton 1982).
MA	Massachusetts		Ecological Impact	Habitat Change
Grazing by Littorina littorea helps to benefit canopy-forming seaweeds such as Chondrus crispus spp. and Fucus spp. by removing ephemeral alga competitors. The canopy forming seaweeds provide shelter for many species of intertidal invertebrates. Canoe Beach Cove, Nahant, MA was one experimental site where these impacts were observed (Lubchenco and Menge 1978; Lubchenco 1980; Lubchenco 1983).
MA	Massachusetts		Ecological Impact	Herbivory
Littorina littorea is the most abundant herbivorous intertidal invertebrate on rocky shores in the Gulf of Maine. Manipulative experiments indicated that L. littorea did not graze significantly on the dominant alga, Chondrus crispus, when it was already established, but did affect algal succession by grazing on emphemeral algae (e.g. Ulva, Enteromorpha spp. which compete with Chondrus, damping variation in Chondrus abundance; Lubchenco 1978; Lubchenco and Menge 1978). The periwinkles graze the germlings of the larger brown seaweed Fucus spp., but also benefit Fucus by removing competitive ephemerals and epibiotic algae (Lubchenko 1980; Lubchenco 1983; Lubchenco 1986).  Impacts of L. littorea were greatest at a sheltered sites of Canoe Beach Cove, Nahant, MA, since L. littorea was rare or absent at sites with heavy wave action (Lubchenco and Menge 1978).
MA	Massachusetts		Ecological Impact	Predation
Littorina littorea is a major predator on the egg capsules of the snail Ilyanassa obsoleta (Eastern Mudsnail) in Barnstable Harbor, MA (Brenchley 1984). The effect of this predation on the abundance of I. obsoleta is not clear.
ME	Maine		Ecological Impact	Competition
Removal experiments (on Swans Island, ME) indicated that growth, weight gain and survival of the native limpet Notoacmea testudinalis was reduced in the presence of L. littorea (Petraitis 1989).
ME	Maine		Ecological Impact	Habitat Change
Grazing by Littorina littorea helps to benefit canopy-forming seaweeds such as Chondrus crispus spp. and Fucus spp. by removing ephemeral alga competitors. The canopy forming seaweeds provide shelter for many species of intertidal invertebrates (Lubchenco and Menge 1978; Lubchenco 1980; Lubchenco 1983). Moderate abundances and impacts of L. littorea were found at Grindstone Neck (near Schoodic Point), Maine (Lubchenco and Menge 1978; Lubchenco 1980; Lubchenco 1983)., In marsh environments in the Wells National Estuarine Research Reserve, ME, removal experiments showed that in a stressed site (long inundation, poor drainage), Littorina reduced the growth of the marsh grass Spartina alterniflora (Smooth Cordgrass), but had no effect at a benign site (Tyrrell et al. 2008). Grazing by this introduced snail could reduce the biomass of Spartina at stressed sites, increasing rates of erosion, particularly as a response to sea-level rise (Tyrrell et al. 2008)
ME	Maine		Ecological Impact	Herbivory
Littorina littorea is the most abundant herbivorous intertidal invertebrate on rocky shores in the Gulf of Maine. Removal experiments indicated that L. littorea did not graze significantly on the dominant alga, Chondrus crispus, when it was already established, but did affect algal succession by grazing on emphemeral algae which compete with Chondrus, damping variation in Chondrus abundance. Impacts of L. littorea were greatest at sheltered sites such as Little Brewster Island, ME, since L. littorea was rare or absent at sites with heavy wave action (Lubchenco 1978)., Littorina littorea is the most abundant herbivorous intertidal invertebrate on rocky shores in the Gulf of Maine. Manipulative experiments indicated that L. littorea did not graze significantly on the dominant alga, Chondrus crispus, when it was already established, but did affect algal succession by grazing on emphemeral algae (e.g. Ulva spp. which compete with Chondrus, damping variation in Chondrus abundance; Lubchenco 1978; Lubchenco and Menge 1978). The periwinkles graze the germlings of the larger brown seaweed Fucus spp., but also benefit Fucus by removing competitive ephemerals and epibiotic algae (Lubchenko 1980; Lubchenco 1983; Lubchenco 1986). Impacts of L. littorea were greatest at sheltered sites, since L. littorea was rare or absent at sites with heavy wave action (Lubchenco 1978). Moderate abundances and impacts of L. littorea were found at Grindstone Neck (near Schoodic Point), Maine (Lubchenco and Menge 1978; Lubchenco 1980; Lubchenco 1983)., In the tidal Damariscotta River estuary, heavy grazing by L. littorea limited growth of most algal species, in areas with slow current flow, leading to the domination of Ascophyllum nodosum (Bertness et al. 2002). On exposed Pemaquid Point, exclusion of L. littorea by cages from cleared areas of rocky shore led to much more rapid regrowth of algae than from control areas (Bertness et al. 2004)., In marsh environments in the Wells National Estuarine Research Reserve, ME, removal experiments showed that in a stressed site (long inundation, poor drainage), Littorina reduced the growth of the marsh grass Spartina alterniflora (Smooth Cordgrass), but had no effect at a benign site (Tyrrell et al. 2008).
NH	New Hampshire		Ecological Impact	Food/Prey
On the Isles of Shoals, ME, L. littorea were a major prey for Jonah Crabs (Cancer borealis), Green Crabs (Caricnus maenas), and Dogwhelks (Nucella lapillus) (Ellis et al. 2007).

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