Sargassum muticum

Invasion History

First Non-native North American Tidal Record: 1944
First Non-native West Coast Tidal Record: 1944
First Non-native East/Gulf Coast Tidal Record:

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General Invasion History:

Sargassum muticum was first described from Wakayama Prefecture, Japan, by Yendo in 1907. Its native range extends from the East China Sea (China and South Korea) to the coasts of Japan and the north shore of Hokkaido, and the Sakhalin and Kurile Islands, Russia (Critichley 1983b; Eneglen et al. 2015). It is introduced in the Eastern Pacific and Eastern Atlantic and genetic studies indicate that these populations probably originated from the Seto Inland Sea, Japan (Cheang et al. 2010). In North America, S. muticum was first introduced to Vancouver Island, British Columbia, where it was discovered in 1944, probably introduced with Pacific Oysters (Crassostrea gigas) from Japan. By 1977, it had spread north to Ketchikan, Alaska (Scagel et al. 1956; Scagel et al. 1989). The pattern of spread/discovery to the south was rather jumpy, with first records in Coos Bay, Oregon in 1947; Mission Bay, California (CA) in 1959; San Francisco Bay, CA in 1963; and Los Angeles, CA in 1973. By the 2000s it had colonized most of the Pacific coast of Baja California (Scagel et al. 1956; Scagel et al. 1989; Engelen et al. 2015). In Europe, it was first collected in Southeast England in 1971 and rapidly spread north and south, reaching Norway by 1988, Spain by 1985, and Morocco by 2012 (Farnham 1980; Runess 1989; Fernandez et al. 1990; Sabour et al. 2013). In 1980, it was first collected in lagoons on the Mediterranean coast of France and by 1992 it was established in the Lagoon of Venice (Knoepfller-Peguy et al. 1985; Curiel et al. 1998), but its distribution in the Mediterranean Sea remains localized (Engelen et al. 2015; Thibaut et al. 2015). The large size of this seaweed, its rapid spread, and high abundances in many locations, have led to the extensive study of its ecology and impacts (Engelen et al. 2015).

North American Invasion History:

Invasion History on the West Coast:

Sargassum muticum was first collected on the West Coast of North America, at White Rock and Buccaneer Bay, north of Vancouver, British Columbia in 1944, in sites where Pacific Oysters where cultivated. It was initially mistaken for the native seaweed Cytoseira geminata. By 1952, it was found at many sites in the Strait of Georgia and Puget Sound, including the San Juan Islands and the Strait of Juan de Fuca (Scagel 1956). The pattern of discovery along the coast of Oregon and Washington suggests several independent introductions with oyster transplants, with subsequent spread by drifting weed and fouled boats. It was found in Willapa Bay, Washington in 1953; Oceanside Beach (near Tillamook Bay), Oregon, in 1951; and Coos Bay, OR in 1947 (Scagel 1956). Spread to the north was slower, but it was found near Ketchikan, Alaska in 1974-1977 (Scagel et al. 1989; Engelen et al. 2015; University of Alaska Southeast Herbarium 2016), and Haida Gwaii, British Columbia in 1981 (Sloan and Bartier 2004).

The first record of S. muticum in California is a report of its occurrence in Mission Bay, San Diego in 1958, which looks like a sudden jump (Stewart 1991), possibly by ship transport. Other records are consistent with a progressive march to the south: Crescent City Harbor in 1963 (Norton 1981, Cohen 2005); Humboldt Bay in 1965 (Dawson 1965, cited by Boyd et al. 2002); Tomales Bay and San Francisco Bay in 1973 (Cohen and Carlton 1995); Monterey Bay in 1977 (Carlton et al. 1996; Cohen 2005); Santa Barbara in 1977 (Cohen 2005); Los Angeles in 1973; (Norton 1981); and San Diego Bay in 1969 (Cohen 2005). The rapid spread is surprising, since S. muticum requires sheltered waters for establishment (Norton 1981). By 1973, it was already established in Ensenada, Mexico (Norton 1981) and by 1984 it had reached Punta Abreojos, Baja California, Mexico (Espinosa 1990). Overall, in 70 years since its invasion on the West coast, it has colonized ~30 degrees of latitude and a span of 4000 km of coastline (Engelen et al. 2015).

Invasion History Elsewhere in the World:

In the Northeast Atlantic, S. muticum was first found on Portsea Island, Portsmouth, England in 1971, and rapidly spread through the Solent, between the south coast of England and the Isle of Wight (Farnham 1980). It colonized the French side of the English Channel by 1977 (Gruet 1977, cited by Farnham 1980). This seaweed reached the Netherlands by 1980, the German portion of the Wadden Sea by 1983 (Buschbaum et al. 2012), and the western mouth of the Limfjord, Denmark, by 1984 (Staer et al. 2000). Sargassum muticum colonized the Kattegat in Denmark and Sweden, the westernmost portion of the Baltic Sea, in 1993-1996, probably reaching the lower limit of its salinity tolerance (Karlsson and Loo 1999; Thomsen et al. 2007). Moving north, S. muticum spread into Norwegian and Swedish waters of the Skaggerak in 1987-1988 (Rueness 1989; Karsson and Loo 1999) and north up the west coast of Norway, to Bergen in 1989 (Hopkins 202) and Sogn og Fjordane (Bjærke and Fredriksen 2005). To the south, this seaweed reached Roscoff, at the tip of Brittany in 1980 (Belsher and Pommellec 1988), and reached the Basque Province of Spain by 1985 (Fernandez et al. 1990), and Portugal by 1989 (Chainho et al. 2015). In 2012, S. muticum was collected at several sites on the Atlantic coast of Morocco. The southernmost was Oualidia at 32°N (Sabour et al. 2013). In ~40 years, S. muticum has spread over 39 degrees of latitude and a 5600 km span of coastline (Engelen et al. 2015).

In contrast with its spread on the West Coasts of Europe and North America, Sargassum muticum's spread in the Mediterranean has been limited, spotty, and punctuated by extinctions. In 1985-1990, S. muticum was found in 14 lagoons on the French Mediterranean Coast, but in surveys in 2012 established populations were found in only four lagoons (Thibaut et al. 2015). In 1992, established populations of the seaweed were found in the Venice Lagoon. Dense populations occur on wharves and breakwaters (Curiel et al. 1998). Sargassum muticum was found in a lagoon on the east coast of Corsica, but was not seen in later surveys (Thibaut et al. 2015). Some floating specimens of this seaweed have been found in the open Mediterranean off Spain, but S. muticum has only become established in lagoons with limited exchange (Engelen et al. 2015).

Culture of the Pacific Oyster (Crassostrea gigas) has been the initial vector for the three major invasions of S. muticum in the Northeast Pacific and Atlantic, and the Mediterranean (Scagel 1956; Farnham 1980; Knoepffler-Peguy et al. 1985). Regional and local dispersal could occur by hull fouling of commercial ships or recreational boats and by the natural dispersal of floating mats of seaweed (Engelen et al. 2015).

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Description

Sargassum muticum usually grows upright in the water column, attached by a solid, conical, spongy holdfast about 50 mm across. The holdfast gives rise to a tapered axis up to 50 mm in height. In young plants, leaves, up to 200 mm long, arise from the main axis. These basal leaves have a well-defined mid-rib. In older plants, the main axis may branch, making the plant bushier. Lateral shoots grow off the main axis in spirals with scale-like triangular leaves at the base, which protect the growing bud. The main axis is perennial, while the lateral shoots die off after the growing season. Primary lateral stems are cord-like, usually growing to 1-3 m, but sometimes 6-10 m. In populations on the West Coast of Ireland, mature plants had 40-100 primary lateral branches (Baer and Stengel 2010), while a population in the Netherlands had 1-16 (Critchley et al. 1987). Secondary lateral stems arise off the primary laterals, branching off in the axil of a leaf. As the lateral stems grow, they become twisted due to water movement. Leaves from the primary and secondary laterals, produced in the winter, are small, less than 200 mm long, thick, and have a weak mid-rib. During the summer, the fronds become fertile and the new leaves are thinner, narrow, and lacking a midrib. They have an uneven outline, resembling holly leaves. The lateral branches bear air-bladders and reproductive receptacles in the axils of the leaves. The air-bladders are spherical to pear-shaped, on stalks, about 3 mm in diameter. The receptacles vary in shape from elliptical to cylindrical to spindle-shaped and are usually 20--30 mm long (sometimes up to 60 mm). The plants are yellow-green to olive-brown. This description is based on: Abbott and Hollenberg 1976; Critchley 1983a; Cohen 2005; and Engelen et al. 2015.

Sargassum muticum is one of several similar species known in Japan, and its taxonomic history is complex. It was initially described as a variety of S. kjellmanianum, which is now regarded as a synonym of S. miyabe (Critchley 1983b; Cheang et al. 2010; Engelen et al. 2015). The genetic diversity of introduced populations in North America and Europe is low and all populations are close to S. muticum populations from western and central Japan (Cheang et al. 2010).

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Taxonomy

Ecology

General:

Sargassum muticum is monoecious and mature plants produce receptacles, which contain both male and female conceptacles for the production of sperm and eggs (Bold and Wynne 1978; Engelen et al. 2015). Receptacles can constitute 24-55% of the plant's biomass, but this reproductive output is fairly typical for fucoid seaweeds (Engelen et al. 2015). Gamete release is synchronized in a semilunar, 14-day cycle, with release peaking at the full and new moons, just after spring tides (Engelen et al. 2008; Engelen et al. 2015), though with some variation with local cues (Monteiro et al. 2009b). Eggs are fertilized in the receptacles, but are retained, attached to the receptacle surface for a few days and are then released as germlings that sink quickly. The propagules usually settle within a few meters of the parents, but can be found up to 1.3 km away. They could potentially disperse over longer distances, since they retain the ability to settle for 49 days (Deysher and Norton 1982). Each receptacle releases ~ 300 propagules and a small plant can release up to 500,000 propagules (Norton and Deysher 1989, cited by Engelen et al. 2015). The fertilized zygotes average about 0.25 mm in diameter (Deysher and Norton 1982). Germlings have comparatively large, basal leaves, with developing lateral axes (Critchley 1983a). Breeding seasons vary geographically, peaking in spring-summer in Northern Europe and Northwestern North America, but with a longer season at southern locations such as Portugal, southern California, and Mexico (Deysher 1984; Aguilar-Rosas and Galindo 1990; Engelen et al. 2015). After releasing their propagules, adult plants lose their fronds and disintegrate, except for the perennial holdfast, which gives rise to new fronds in the next growing season (Critchley 1983a; Engelen et al. 2015). This dormant period is usually in autumn in the northern part of the range, but occurs in summer-fall in southern Portugal, California, and the Venice Lagoon (Deysher 1984; Sfriso and Facca 2013; Engelen et al. 2015).

Sargassum muticum grows over a wide latitudinal range, from cold-temperate to subtropical conditions, from temperatures below 0°C in Sweden (Karlsson and Loo 1999) to 30°C in the Venice Lagoon (Sfriso and Facca 2013). In the laboratory, germlings survived at salinities as low as 6.8 PSU, but with minimal growth (Hales and Fletcher 1989; Karlson and Loo 1999; Steen 2004; Thomsen et al. 2007). Germlings are more sensitive to temperature and salinity than adult plants and grow successfully between 15 and 25°C and 15-35 PSU (Hales and Fletcher 1989; Steen 2004). Early germlings (2 weeks old) showed increasing growth at 9 to 44 µE m-3s-1 and had decreased growth at 88 µE m-3s-1, while older germlings grew well at 18 to 88 µE m-3s-1, and were more tolerant of high light (Hales and Fletcher 1989). Sargassum muticum is not a good competitor at low light and has a strategy of growing fast to form a canopy in shallow water. It can adjust its pattern of growth and branching according to the density of its neighbors, permitting it to form very dense populations (Engelen et al. 2015).

In its native range in Asia, S. muticum occurs on rocky shores from the lower intertidal to 4 m depth and is not known as an aggressive colonizer of artificial structures (Engelen et al. 2015). It is generally associated with sheltered locations. Plants transplanted to exposed locations suffered increased breakage and lower growth rates (Viejo et al. 1995). In invaded habitats, it occurs on pilings, floats, marinas, in canals, aquaculture cages, marinas, breakwaters, oyster beds, and eelgrass beds (Belsher and Pommellec 1988; den Hartog 1997; Harries 2007; Kraan 2008; Engelen et al. 2015). In Strangford Lough, Northern Ireland, S. muticum has colonized soft-sediment habitats by 'stone-walking' on rock fragments and shells and spreading by water movements (Strong et al. 2006). Adult plants, dislodged by waves or disturbance, or fragments of degenerating plants with fertile receptacles, form floating mats (Norton 1981). Sargassum muticum, like other fucoid seaweeds, produces organic compounds for chemical defense against microbes, epiphytes, and grazers. Phenolic compounds are greatest during the reproductive period, possibly for protection of the receptacles (Plouguerne et al. 2006; Plougerne et al. 2008). In spite of these compounds, S. muticum supports a dense community of epiphytes and epifauna, and is grazed by a variety of invertebrates, with varying degrees of preference (Norton and Benson 1983; Monteiro et al. 2009; Strong et al. 2009; Baer & Stengel 2010; Cacabelos et al. 2010; Gestoso et al. 2010).

Consumers:

snails, amphipods, sea urchins

Competitors:

Native seaweeds, seagrasses

Trophic Status:

Primary Producer

PrimProd

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Habitats

General Habitat	Marinas & Docks	None
General Habitat	Rocky	None
General Habitat	Oyster Reef	None
General Habitat	Grass Bed	None
General Habitat	Canals	None
General Habitat	Unstructured Bottom	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	Epibenthic	None
Vertical Habitat	Littoral	None

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Life History

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Tolerances and Life History Parameters

Minimum Temperature (ºC)	0	Field observations, Sweden (Karlsson and Loo 1999)
Maximum Temperature (ºC)	30	Highest tested, lab experiments (Hales and Fletcher 1989)
Minimum Salinity (‰)	10	Lab experiments- Growth greatly reduced compared to 27-34 PSU (Hales and Fletcher 1989)
Maximum Salinity (‰)	35	Highest tested, lab experiments (Hales and Fletcher 1989)
Minimum Reproductive Temperature	15	Experimental (Kerrison and Le 2016)
Maximum Reproductive Temperature	25	Experimental (Kerrison and Le 2016)
Minimum Reproductive Salinity	20	Minimum for fertilization. Germlings can tolerate brief exposure to 5 PSU, especially in 2nd and 3rd weeks after fertilization (Steen 1984)
Maximum Reproductive Salinity	40	Experimental, germlings in receptacle (Kerrison and Le2016)
Minimum Length (mm)	300	Moyrus, Ireland, stunted plants at exposed site, but 100% fertile (Baer and Stengel 2010)
Maximum Length (mm)	10,000	Engelen et al. 2015
Broad Temperature Range	None	Cold temperate-Warm temperate
Broad Salinity Range	None	Mesohaline-Euhaline

General Impacts

Sargassum muticum is one of many Sargassum species in its native Asian range, and is not especially prominent or aggressive there. In the Northeast Pacific and Northeast Atlantic, however, it has spread rapidly along coastlines and has become a major component of algal communities (Engelen et al. 2015). Economic impacts to beach use, boating, power plants, fisheries, and aquaculture, were reported in the early years of the invasion in England and France. These impacts led to a number of unsuccessful eradication attempts (Critchley et al. 1986; Belsher and Pommellec 1989). Ecological impacts of S. muticum have been extensively studied, particularly along the coasts of northern Europe, northern Spain and Portugal, and the San Juan Islands, in northern Puget Sound, Washington. These include competition for space, light, and nutrients, alterations of habitat and associated epibiota, and effects on grazers (Schaffelke and Hewitt 2007; Engelen et al. 2015).

Economic Impacts

Early accounts of the spread of Sargassum muticum on the West Coast of North America (Scagel 1956; Norton 1981) make no mention of adverse economic impacts. A later survey, focused on Sargassum horneri and Undaria pinnatifida refers to S.muticum as 'naturalized' and does not refer to economic impacts (Kaplanis et al. 2016). In Southeastern England and Atlantic France, extensive impacts, including fouling of boats, fishing nets, aquaculture equipment, and power plant intakes, was reported, together with large floating mats in nearshore waters and rotting masses piling up on beaches (Critchley et al. 1986; Belsher and Pommellec 1988). In 1974-1975, an eradication was attempted in England with large volunteer groups, hand-picking intertidal plants. The volunteers collected 31 metric tons, but missed germlings and juvenile plants and eradication was found to be impractical. Trawls, cutting machines, and suction machines were tested as alternatives to hand-picking, but considered un-economical. Sargassum muticum was considered unsuitable for industrial and food products (Critchley et al. 1986), although it is eaten in Korea. A recent review of potential economic uses found that use of 'wild' plants in Europe for biofuel, food, or fertilizer was unsafe because of the accumulation of heavy metals and high ash content. However, S. muticum contains fucoxanthins, antioxidants, and other compounds of pharmaceutical interest (Milledge et al. 2106).

Ecological Impacts

Competition- Interactions between S. muticum and native biota, in many different marine communities have been studied, including rocky shores, tidepools, kelp beds, seagrass meadows, and soft-bottom communities (Schaffelke and Hewitt 2007; Engelen et al. 2008). Competitive interactions have been especially noted with the algae of the closely related perennial genera Cytoseira and Halidrys spp. Sargassum’s semi-perennial life-style, dying back to the holdfast, in unfavorable seasons may be a competitive advantage. In Portugal, its dying fronds denied space to C. humlis (Engelen and Santos 2009). In Denmark, S. muticum outgrew Halidrys siliqua, possibly because it did not need to invest in structural strength for winter survival (Wernberg et al. 2000). Sargassum muticum's bushy growth form and buoyant branches permit it to form a canopy and shut out light and occupy space during the most favorable time for growth (Curiel et al. 1998; Britton-Simmons 2004; Harries et al. 2007; Salvaterra et al. 2013). However, S. muticum often seems to require disturbance to invade established communities (de Wreede 1983, cited by Schaffelke and Hewitt 2007; Strong and Dring 2011; Bertocci et al. 2014). Sargassum muticum invaded rocky areas off California and in the San Juan Islands after die-offs or experimental removal of kelps (Ambrose and Nelson 1982; Britton-Simmons and Abbott 2008).

Habitat Change- When S. muticum replaces native seaweeds and seagrasses, it can alter the structure of native habitats. In many cases, S. muticum supports a similar or a more abundant and diverse epiphytic and epifaunal community than the native flora (Viejo 1999; Buschbaum et al. 2006; Engelen et al. 2015). Its ability to colonize bare, soft substrate by 'stone-walking' on shells and stones, and to colonize Eelgrass (Zostera marina) beds greatly increases the structural complexity of these habitats (den Hartog 1997; Strong et al. 2011; DeAmicis et al. 2015). It is used as habitat by fishes, crustaceans, and cuttlefish (Engelen et al. 2015; Stiger-Pouvreau and Thouzeau 2015).

Food/Prey- Herbivores vary considerably in their response to S. muticum. Strong avoidance was noted for sea urchins (Strongylocentrotus droebachiensis) in northern Puget Sound (Britton-Simmons 2004) and Psammechinus miliaris in Denmark (Pedersen et al. 2005). Other grazers showed lesser levels of avoidance, or no preference (Monteiro et al. 2009; Cacabelos et al. 2010; Engelen et al. 2011). Some grazers appear to feed heavily on S. muticum, such as the amphipod Dexamine spinosa in Strangford Lough, Northern Ireland (Strong et al. 2009), and the snail Lacuna vincta in Northern Puget Sound. The latter seems to have developed its preference in the last 30 years (Britton-Simmons et al. 2011). In many of the systems where it has been studied, S. muticum seems to be under light grazing pressure (Norton and Benson 1983; Pedersen et al. 2005; Monteiro et al. 2009; Cacabelos et al. 2010; Engelen et al. 2011; Mabey et al. 2022). However, because of its rapid growth, large biomass, and seasonal die-offs, it is a major contributor to primary production in many intertidal and subtidal systems, and to subtidal detritus and intertidal wrack (Rossi et al. 2009; Olabarria et al. 2010; Vaz-Pinto et al. 2014; Engelen et al. 2015).

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Regional Impacts

MED-VII	None		Ecological Impact	Competition
In the Venice Lagoon, S. muticum was found to compete with the native red, brown, and green macroalgae by forming a canopy and shutting out light (Curiel et al. 1998).
NEA-II	None		Ecological Impact	Competition
Sargassum muticum has been reported to displace native algae in the Limfjord, Denmark, (Stæhr et al. 2000, cited by Schaffelke and Hewitt 2007). Its advantage over the native, perennial Halidrys siliqua may be due to its semi-perennial lifestyle, not having to invest in a sturdier structure for winter survival, and so is capable of more rapid growth in spring and summer (Wernberg et al. 2000). On the Isle of Cumbrae, Scotland, S. muticum was found to displace the native Dictyota dichotoma, probably through competition for light and substrate (Harries et al. 2007). In Strangford Lough, Northern Ireland, competition with the native Saccharina latissima was not seen. Instead, growth of S. muticum was slower in single-species plots, due to intraspecific competition. Disturbance and removal of the native seaweed was considered responsible for the expansion of S. muticum (Strong and Dring 2011). In the German Wadden Sea, competition with the native algae Polysiphonia nigrescens, Antithamnion plumula and Elachista fucicola, and also with settling Pacific Oysters (Crassostrea gigas) was seen. Reduced oyster settlement could affect epibenthic communities by reducing by limiting the expansion of hard substrate (Lang and Buschbaum 2010).
NEP-III	Alaskan panhandle to N. of Puget Sound		Ecological Impact	Competition
The invasion of Sargassum muticum resulted in displacement of native algae in the San Juan Islands, northern Puget Sound. A removal experiment resulted in recovery of native kelps (Britton-Simmons 2004). Modeling and experiments indicated that S. muticum invasions required a combination of disturbance and high propagule pressure (Britton-Simmons et al. 2008). In British Columbia, S. muticum rapidly colonized cleared areas, followed by decreased recruitment of native seaweeds (de Wreede 1983, cited by Schaffelke and Hewitt 2007). At low levels of abundance, S. muticum had few impacts, but at high levels, S. muticum excludes natives through competition for light (primarily) and space, resulting in reduced diversity and productivity. The impact increased with S. muticum density in a non-linear fashion (White and Shurin 2011).
NEP-III	Alaskan panhandle to N. of Puget Sound		Ecological Impact	Food/Prey
There were fewer Green Sea Urchins (Strongylocentrotus droebachiensis) at invaded sites, apparently because they found S. muticum unpalatable (Britton-Simmons 2004). However, the snail Lacuna vincta was 2-9X more abundant on S. muticum than on native algae. This preference seems to have been acquired in the last 30 years (Britton-Simon et al. 2011).
P292	_CDA_P292 (San Juan Islands)		Ecological Impact	Competition
The invasion of Sargassum muticum resulted in displacement of native algae, in the San Juan Islands, northern Puget Sound. A removal experiment resulted in recovery of native kelps (Britton-Simmons 2004). Modeling and experiments indicated that S. muticum invasions required a combination of disturbance and high propagule pressure (Britton-Simmons et al. 2008).
P292	_CDA_P292 (San Juan Islands)		Ecological Impact	Food/Prey
Sargassum muticum was grazed by a high abundance, but low diversity of grazers, compared to native seaweeds. Common grazers were the amphipods Peramphithoe mea, Aoroides columbiae, Caprella laeviscula and Ischyocerus anguipes, and the snail Lacuna variegata. Much of the grazing was on periphytic diatoms, and the tissue consumption mostly occurs during the period of slow growth, before the annual dieback, and does not affect the seasonal abundance or dominance of the plant (Norton and Benson 1983). There were fewer Green Sea Urchins (Strongylocentrotus droebachiensis) at invaded sites, apparently because they found S. muticum unpalatable (Britton-Simmons 2004). However, the snail Lacuna vincta was 2-9X more abundant on S. muticum than on native algae. This preference seems to have been acquired in the last 30 years (Britton-Simon et al. 2011).
NEA-V	None		Ecological Impact	Competition
In a lower intertidal region on the Bay of Biscay, northern Spain had negative impacts of the native red alga Gelidium spinosum, probably due to competition for light (Sanchez et al. 2005). Addition of nutrients in tide pools favored rapid growth and dominance of S. muticum, but colonization was resisted in plots with a dense canopy of the native Bifurcaria bifurcata (Sanchez and Fernandez 2005). Experimental removal of S. muticum found only limited impacts on total numbers of algal species, somewhat reducing the abundance of filamentous and foliose algae (Olabarrio et al. 2009b), or having no detectable impact on other algae (Sanchez and Fernandez 2005). High abundance of S. muticum in tide pools, in northern Portugal, was correlated with decreased abundance of native algae (Viejo et al. 1997). Modeling, based on field observations suggested that the most important feature favoring S. muticum over the native Cytoseira humilis was the persistence of non-fertile fronds of S. muticum, after reproduction, denying the space to Cytoseira humilis (Engelen and Santos 2009). Experiments with the effects of nutrient inputs found that S muticum had a complex response, and was favored by high inputs, with low variability, but not by low, highly variable inputs (Incera et al. 2009). In another set of experiments S. muticum became very abundant in tide pools with high nutrient input and mechanical disturbance (scraping with a chisel) (Bertocci et al. 2014). Nutrient fertilization of tidepools promoted the establishment and functional impacts (increased productivity and respiration) by Grateloupia turuturu and Sargassum muticum (Vieira et al. 2017).
NEA-IV	None		Ecological Impact	Competition
Sargassum muticum became the most abundant species in the intertidal of the French Atlantic coast, and was associated with a decrease in a native kelp (Laminaria digitata) (Belsher and Pommelec 1988; Cosson 1999, cited by Schaffelke and Hewitt 2007; Stiger-Pouvreau and Thouzeau 2015). It was also reported to colonize and displace Eelgrass (Zostera marina), in areas where the Eelgrass has declined, due to disturbance (Den Hartog 1997).
NEP-VI	Pt. Conception to Southern Baja California		Ecological Impact	Competition
Sargassum muticum colonized Giant Kelp (Macrocystis pyrifera) beds on Bird Rock, off Catalina Island, after a die-off, possibly caused by high water temperatures during El Nino. Shading by Sargassum muticum probably inhibited recolonization by the kelp. After experimental removal, the kelp recolonized the cleared areas (Ambrose and Nelson 1982). Several years later, Giant Kelp did recolonize Bird Rock (Foster & Schiel 1992, cited by Engelen et al. 2015). Removal experiments in tidepools at Little Corona del Mar in Newport Beach in southern California, showed little effect level, pool temperature, seaweed biomass and community composition, or faunal composition. Recovery of S. muticum populations was rapid (Smith 2016).
NEA-V	None		Ecological Impact	Food/Prey
Experiments with a range of grazing animals, the snails Littorina littorea, L. obtusata, Gibbula, spp., and Peringia ulvae, the sea-slug Aplysia punctata, the amphipod Gammarus insensibilis, and the isopod Stenosoma nadejda, generally preferred native algae to S. muticum, while the sea urchin Paracentrotus lividus showed no preference. Preferences were variable, but experiments did suggest that S. muticum was not under high pressure from grazers (Monteiro et al. 2009; Cacabelos et al. 2010; Engelen et al. 2011). Sargassum muticum, washed up on beaches, was a major food source for the amphipod Talitrus saltator and, to a less extent, for the isopod Tylos europaeus (Rossi et al. 2009; Olabarria et al. 2010). The S. muticum wrack had higher nutrient content than that of a native alga (S. muticum), but there were not consistent differences in invertebrates using the two types of wracks (Rodil et al. 2008). Overall, the invasion of S. muticum has increased the biomass, light-use efficiency, primary production, and respiration of tide pool systems in Portugal. However, this effect disappears during the seasonal die-off of this seaweed (Vaz-Pinto et al. 2014).
NEA-III	None		Ecological Impact	Competition
In experiments using assemblages of native algae and S. muticum reared in containers in the intertidal of Lough Hyne, Ireland, S. muticum had a negative impacts on the biomass of Fucus vesiculosus and, to a lesser extent, on Cladostephus spongiosus. In the Salcombe River estuary, England, S. muticum was able to attach to soft substrate within Eelgrass (Zostera marina) beds, because of the increased stability and decreased water movement (Tweedley et al. 2008).
NEA-III	None		Ecological Impact	Habitat Change
In experiments in Lough Hyne, Ireland, benthic animal diversity and species richness was higher in assemblages of native algae than those containing S. muticum, probably because S. muticum contains less habitat cover (Salvaterra et al. 2013). On the West Coast of Ireland, S. muticum supports dense growths of the filamentous brown alga Pylaiella littoralis, especially in sheltered sites, where the epiphyte growth, inhibited photosynthesis, growth, and caused mortality. Growth of other epiphytes and survival of S. muticum was better in more exposed sites (Baer and Stengel 2014). In the Salcombe-Knightsbridge Estuary (English Channel), Devon, England, invasion by S. muticum resulted in shorter blade length of Eelgrass (Zostera marina), and altered epibiota communities. Although S. muticum provides a more structurally complex habitat, and supports larger abundances of some taxa, its seasonal die-offs may limit the establishment of populations (DeAmicis et al. 2015).
NEA-IV	None		Ecological Impact	Habitat Change
In turbid waters, Sargassum muticum replaces kelps, but provides habitat for fishes, crustaceans, and cuttlefish (Stiger-Pouvreau and Thouzeau 2015; Roux et al. 2021). Sargassum muticum does not directly compete with Eelgrass (Zostera marina, but when Eelgrass beds near Roscoff, France, are destroyed by natural shifts in the sediment, S.muticum quickly colonizes the empty spaces (den Hartog 1997).
NEA-IV	None		Economic Impact	Fisheries
Sargassum muticum can interfere with shellfishing and shellfish aquaculture, by covering the bottom, fouling shells, and equipment (Belsher and Pommelec1988); Stiger-Pouvreau and Thouzeau 2015)
NEA-V	None		Ecological Impact	Habitat Change
In turbid waters, Sargassum muticum replaces kelps, but provides habitat for fishes, crustaceans, and cuttlefish (Stiger-Pouvreau and Thouzeau 2015). In a study of epifaunal invertebrates, in intertidal communities in Galicia, northern Spain, results showed that S. muticum supported levels of abundance and diversity comparable to those of two native seaweeds (Gestoso et al. 2012). At several locations on the coast of Portugal, the epifauna of S. muticum differs from that of Cystoseira humilis in composition or abundance, but not in any consistent way (Viejo et al. 1999; Engelen et al. 2013). It is used by fishes, crustaceans, and cuttlefish (Stiger-Pouvreau and Thouzeau 2015).
NEA-V	None		Economic Impact	Fisheries
Sargassum muticum can interfere with shellfishing and shellfish aquaculture, by covering the bottom, fouling shells, and equipment (Stiger-Pouvreau and Thouzeau 2015).
NEA-IV	None		Economic Impact	Shipping/Boating
Sargassum muticum entangled propellers and hampered navigation in Saint-Malo, Saint-Guénolé, and the Gulf of Morbihan (1982-1986, Belsher and Pommelec1988)
B-I	None		Ecological Impact	Habitat Change
Sargassum muticum supports an epiphyte community of 82 species in the Oslofjord, Norway, more than that of two major native structural plants, Fucus serratus and Zostera marina (Bjærke and Fredriksen 2005).
NEA-II	None		Economic Impact	Shipping/Boating
Dense beds of Sargassum muticum were reported to interfere with the movement of small boats and to clog their intake pipes (Critchley et al. 1986).
NEA-II	None		Economic Impact	Industry
Sargassum muticum was reported to clog the intakes of power plants in England (Critchley et al. 1986).
NEA-II	None		Economic Impact	Fisheries
Sargassum muticum is reported to foul fishing lines and nets, and has also interfered with oyster culture and harvesting in England and France (Critchley et al. 1986).
NEA-II	None		Economic Impact	Aesthetic
Large amounts of Sargassum muticum washing up on beaches, were an unpleasant feature on recreational beaches. Dense beds of Sargassum, were reported to interfere with swimming and recreational sailing. Sargassum muticum was also considered a threat to native marine biota and their habitats. A removal program was studied and organized by a coalition of local, regional, and national environmental agencies, in southern England (Critchley et al. 1986). In 1974-1975, a large campaign of hand-picking by volunteers was conducted, with about ~800 collecting trips and 31 metric tons collected. However, hand-collecting overlooked germlings and small plants. Mechanical removal, herbicides, and release of natural herbivores all proved to be ineffective. Specialized cutting machines, trawls, and suction devices were developed and tested, and found to be more effective, but would require continual annual use (Critchley et al. 1986).
NEP-III	Alaskan panhandle to N. of Puget Sound		Ecological Impact	Habitat Change
In the San Juan Islands, Washington, Sargassum muticum supported a total of 107 epifaunal taxa, and on average supported 20 species per plant, compared to 10 species per plant on the native kelp Laminaria saccharina. Epifaunal diversity increased in area invaded by S. muticum (Giver 1999).
P292	_CDA_P292 (San Juan Islands)		Ecological Impact	Habitat Change
In the San Juan Islands, Washington, Sargassum muticum supported a total of 107 epifaunal taxa, and on average supported 20 species per plant, compared to 10 species per plant on the native kelp Laminaria saccharina. Epifaunal diversity increased in area invaded by S. muticum (Giver 1999).
NEA-II	None		Ecological Impact	Habitat Change
On the Isle of Cumbrae, Scotland, canopies of Sargassum muticum support a higher abundance, but lower diversity of epifauna than the native Dictyota dichotoma, probably due to a more complex structure (Harries et al. 2007). In Strangford Lough, Northern Ireland, S. muticum has colonized large areas of bare substrate by 'stone-walking', attached to stones and shells, and moving by water motion. Invertebrate communities were altered under the canopies, and were dominated by smaller, more opportunistic species than the bare substrate (Strong et al. 2011). Sargassum muticum in this estuary was more heavily colonized by epiphytes and herbivorous amphipods than the natives, and appeared not to benefit from 'invader release' (Strong et al. 2009). In the Limfjorden, Denmark, S. muticum supports a similar epifaunal community in species composition to the native Halidrys siliqua, but supports a much higher density of fauna, especially amphipods, because of the greater size and complexity of S. muticum's thallus (Wernberg et al. 2005).
B-II	None		Economic Impact	Industry
Sargassum muticum clogged intakes of the Ringhals nuclear power plant in the province of Halland (Josefsson and Jansson 2009).
B-II	None		Economic Impact	Fisheries
In Sweden, catches of eel (Anguilla anguilla) may have been negatively influenced in some areas (Koster archipelago) through interference with fishing gear (Karlsson et al. 1995).
NEA-II	None		Ecological Impact	Food/Prey
Sargassum muticum had faster rates of growth and decomposition than the native brown seaweed Halidrys siliquosa in the Limfjord, Denmark, resulting in higher productivity, and faster turnover of organic matter. Sargassum muticum was preferred to H. siliquosa by the major grazer, urchin Psammechinus miliaris, but grazing losses of S. muticum were small, compared to those due to decomposition (Pedersen et al. 2005; Pedersen et al. 2016). In feeding experiments in Germany, using the snail Littorina littorea, the urchin Psammechinus miliaris, and the isopod Idotea baltica, the native Fucus vesiculosus was preferred to S. muticum from Germany, but S. muticum and S. horneri, collected in Japan, were both more strongly avoided, suggesting that European populations have reduced chemical defenses (Schwartz et al. 2016). Flat Periwinkles (Littorina obtusata and L. fabalis) from English Channel sites first colonized 6-40 years ago (1970s), fed as readily on S. muticum as on native Ascophyllum nodusum, in comparison to snails from later invaded areas. This difference is suggestive of behavioral or evulutionary adaptation (Kurr and Davies 2018). In Strangford Lough, Northern Ireland, S. muticum was more densely inhabited by the amphipod Dexamine spinosa than native algae (Saccharina latissima, H. siliquosa, Fucus serratus) and more heavily grazed by the amphipod (Strong et al. 2009). In the Wadden Sea, Germany, increased abundance of the native Snake Pipefish (Entelurus aequoreus) was promoted by dense growths of S. muticum, which also supported high densities of harpacticoid copepods, food for the pipefish (Polte and Buschbaum 2008).
P058	_CDA_P058 (San Pedro Channel Islands)		Ecological Impact	Competition
Sargassum muticum colonized Giant Kelp (Macrocystis pyrifera) beds on Bird Rock, off Catalina Island, after a die-off, possibly caused by high water temperatures during El Nino. Shading by Sargassum muticum probably inhibited recolonization by the kelp. After experimental removal, the kelp recolonized the cleared areas (Ambrose and Nelson 1982). Several years later, Giant Kelp did recolonize Bird Rock (Foster & Schiel 1992, cited by Engelen et al. 2015).
B-I	None		Ecological Impact	Competition
In single-species experiments with Sargassum muticum and 5 native seaweed species, S. muticum grew fastest at the high temperature, 17ºC compared to the natives, but had much slower growth at 7°C. Growth of S. muticum did not differ greatly between high and low nutrient levels (Steen and Rueness 2004).
P040	Newport Bay		Ecological Impact	Competition
Removal experiments in tidepools at Little Corona del Mar in Newport Beach in southern California, showed little effect level, pool temperature, seaweed biomass and community composition, or faunal composition. Recovery of S. muticum populations was rapid (Smith 2016).
NEA-III	None		Ecological Impact	Herbivory
Flat Periwinkles (Littorina obtusata and L. fabalis) from English Channel sites first colonized 6-40 years ago (1970s), fed as readily on S. muticum as on native Ascophyllum nodusum, in comparison to snails from later invaded areas. This could represent behavoral or evoultionary adaptation (Kurr and Davies 2018).
P090	San Francisco Bay		Ecological Impact	Habitat Change
In field cage experiments, juvenile Chinook Salmon (Oncorhynchus tshawystcha) were reared in several habitats, bare sand, Eelgrass (Zostera marina, Sargassum muticum and mixed Eelgrass-Sargassum. Growth was best in the mixed habitat, suggesting that habitat variety is important (Hughes et al. 2020).
NEP-V	Northern California to Mid Channel Islands		Ecological Impact	Habitat Change
In field cage experiments, juvenile Chinook Salmon (Oncorhynchus tshawystcha) were reared in several habitats, bare sand, Eelgrass (Zostera marina, Sargassum muticum and mixed Eelgrass-Sargassum. Growth was best in the mixed habitat, suggesting that habitat variety is important (Hughes et al. 2020).
WA	Washington		Ecological Impact	Competition
The invasion of Sargassum muticum resulted in displacement of native algae, in the San Juan Islands, northern Puget Sound. A removal experiment resulted in recovery of native kelps (Britton-Simmons 2004). Modeling and experiments indicated that S. muticum invasions required a combination of disturbance and high propagule pressure (Britton-Simmons et al. 2008).
WA	Washington		Ecological Impact	Food/Prey
Sargassum muticum was grazed by a high abundance, but low diversity of grazers, compared to native seaweeds. Common grazers were the amphipods Peramphithoe mea, Aoroides columbiae, Caprella laeviscula and Ischyocerus anguipes, and the snail Lacuna variegata. Much of the grazing was on periphytic diatoms, and the tissue consumption mostly occurs during the period of slow growth, before the annual dieback, and does not affect the seasonal abundance or dominance of the plant (Norton and Benson 1983). There were fewer Green Sea Urchins (Strongylocentrotus droebachiensis) at invaded sites, apparently because they found S. muticum unpalatable (Britton-Simmons 2004). However, the snail Lacuna vincta was 2-9X more abundant on S. muticum than on native algae. This preference seems to have been acquired in the last 30 years (Britton-Simon et al. 2011).
WA	Washington		Ecological Impact	Habitat Change
In the San Juan Islands, Washington, Sargassum muticum supported a total of 107 epifaunal taxa, and on average supported 20 species per plant, compared to 10 species per plant on the native kelp Laminaria saccharina. Epifaunal diversity increased in area invaded by S. muticum (Giver 1999).
CA	California		Ecological Impact	Competition
Sargassum muticum colonized Giant Kelp (Macrocystis pyrifera) beds on Bird Rock, off Catalina Island, after a die-off, possibly caused by high water temperatures during El Nino. Shading by Sargassum muticum probably inhibited recolonization by the kelp. After experimental removal, the kelp recolonized the cleared areas (Ambrose and Nelson 1982). Several years later, Giant Kelp did recolonize Bird Rock (Foster & Schiel 1992, cited by Engelen et al. 2015)., Removal experiments in tidepools at Little Corona del Mar in Newport Beach in southern California, showed little effect level, pool temperature, seaweed biomass and community composition, or faunal composition. Recovery of S. muticum populations was rapid (Smith 2016).
CA	California		Ecological Impact	Habitat Change
In field cage experiments, juvenile Chinook Salmon (Oncorhynchus tshawystcha) were reared in several habitats, bare sand, Eelgrass (Zostera marina, Sargassum muticum and mixed Eelgrass-Sargassum. Growth was best in the mixed habitat, suggesting that habitat variety is important (Hughes et al. 2020)., In field cage experiments, juvenile Chinook Salmon (Oncorhynchus tshawystcha) were reared in several habitats, bare sand, Eelgrass (Zostera marina, Sargassum muticum and mixed Eelgrass-Sargassum. Growth was best in the mixed habitat, suggesting that habitat variety is important (Hughes et al. 2020).

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