Xenostrobus securis

Invasion History

First Non-native North American Tidal Record: 2024
First Non-native West Coast Tidal Record: 2024
First Non-native East/Gulf Coast Tidal Record:

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General Invasion History:

Xenostrobus securis is native in Australia in addition to New Zealand, and includes the estuaries of rivers of eastern, southern and Western Australia (Wilson 1967, Powell 1979, Roberts & Wells 1980, Morton & Leung 2015). This mussel had been widely introduced (Sanz-Latorre et al. 2023) through ballast water in China, Japan and Korea (Iwasaki 2006, Iwasaki et al. 2004, Kimura et al. 1999, Shirafuji & Sato 2003, NIES 2025). The first records in Europe are from 1992 along the Italian Adriatic coast (Lazzari & Rinaldi 1994, Sabelli 1993). Now, X. securis can be found widespread along Italian, French and Spanish Mediterranean coasts (Barbieri et al. 2011), Giusti et al. 2008, Gofas & Zenetos 2003), and along the shores of the Spanish Atlantic Ocean and Cantabrian Sea (Adarraga & Martínez 2012, Bañón et al. 2008, Devloo-Delva et al. 2016, Garci et al. 2007, Gestoso et al. 2012, Pejovic et al. 2016). In the Bay of Biscay, its presence was reported in 2012 only at the estuary of Nerbioi, Bizkaia (Adarraga & Martínez 2012). In 2024 it was found in Los Angeles in California, the first known occurrence in North America (CDFW 2024).

North American Invasion History:

Invasion History on the West Coast:

In 2021 the presence of Xenostrobus securis was detected in Long Beach via DNA metabarcoding analysis but no morphological presence of the mussel had been identified at the time. Surveys conducted by California Department Fish and Wildlife in December 2024 to detect golden mussel Limnoperna fortunei also found Xenostrobus securis in Dominguez Channel in Wilmington CA, part of the Los Angeles/Long Beach port complex, and verified by DNA sequencing (CDFW 2024). This is the first report of Xenostrobus securis on a United States coast. Likely vectors for introduction in California are hull fouling and/or ballast water due to their proximity to shipping complexes. Xenostrubus securis can reproduce rapidly, colonize a variety of soft and hard substrates and overtake habitats, making it a highly successful colonizer and ecosystem engineer. It can also displace ecologically and economically important native species as well as become a fouling pest.

Invasion History Elsewhere in the World:

Xenostrobus securis was introduced in Japan in 1972 in Kojima Bay, central Japan (Iwasaki 2024, Iwasaki et al. 2024). It has since then expanded to Seto Inland Sea, Pacific Ocean, Eastern China Sea and the Sea of Japan (Kimura et al. 1999, Iwasaki et al. 2004, Iwasaki & Kinoshita 2004, Iwasaki 2006, Morton & Leung 2015). Its first records in Europe are from 1992 in the Adriatic coast of Italy (Lazzari & Rinaldi 1994, Barbieri et al. 2011). In addition, it has also been found in the Rio de la Plata estuary in Argentina (Pastorino et al. 1993) Republic of Korea (Shirafuji & Sato 2003), Hong Kong (Morton & Leung 2015), Slovenia (De Min & Vio 1997), Mediterranean lagoons of France (Gofas & Zenetos 2003), and estuaries of the Atlantic coast of Spain (Montes et al. 2020). Spread of the species is likely via ballast water discharge from shipping, or accidental introduction with oyster and other shellfish aquaculture (Pascual et al. 2010). Adult mussels biofoul surfaces, attaching with byssal threads, which facilitates potential hitchhiking on boats and other materials that have been submerged in water long enough for veligers to settle and attach.

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Description

The genus Xenostrobus is smaller than most other mytilids. It is characterized by having almost no sculpture, or weak radial striae at most, and a ridge extending posteriorly from the umbo. The periostracum never bears setae, the hinge completely lacks teeth, the inner mantle fold forms separate, tubular, extensible inhalant and exhalant siphons, and the recurrent loop of the midgut lies on the right-hand side of the stomach (instead of the left as in other mytilids) (Wilson 1967). Xenostrobus securis, a brackish bivalve mollusk of the Mytilidae family, has equivalve and heteromyarian subcylindrical shells. Periostracum is smooth and shining, dorsal margin usually straight, ventral margin straight or slightly arcuate (distinctly arcuate in older specimens) with posterior end evenly rounded. Umbones is nearly terminal, smooth sculpture and internal margin smooth. Xenostrobus securis secretes extremely thin byssus threads (2-3 times thinner compared to Mytilus galloprovincialis) counterbalanced by a huge amount of byssus threads that they use to attach to hard and soft substrates (Babarro & Lassudrie 2011). It has a brown shell and internally is usually purple above and white below the umbonal keel. The average shell length is 20–30 (mm), height/length ratio=0.59, width/length=0.35, even if the maximum of a shell length observed was 47 mm (Russo 2001).

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Taxonomy

Ecology

General:

Xenostrobus securis is a temperate species with a broad euryhaline range; adults can survive in salinities between ~2  to over 40 PSU (Wilson 1968) and the larvae between ~ 15.5 – 31 PSU although at salinities above 31 PSU larval viability declines (Wilson 1969). It is found in many intertidal and subtidal habitats, and can colonize rocks, sandy or muddy bottoms, and artificial structures (Sousa et al. 2009). It is highly tolerant to temperature changes in a range from 14 °C to 30 °C (Astudillo et al. 2017), and is also able to withstand short periods of extreme temperatures up to 42 °C (Olabarria et al. 2016). Salinity appears to be an important regulating factor as it tends to recruit and survive best in polyhaline waters in its native and non-native range (Cottingham et al. 2023) although adult mussels are able to close and cease feeding for as long as 4 months when exposed to consistent salinities below 3.6 PSU (Wilson 1968, Wilson 1969), and are able to survive hyper saline conditions (55 psu) for at least 5 days (Wilson 1968).  Xenostrobus securis inhabits depths of 1 to 20 meters (Pascual et al. 2010). It is a filter feeder, and it tends to live forming dense aggregations (density was reported till 67,000 individuals/m² in introduced environments) which is why it is considered an engineer species (Garci et al. 2007, Gestoso et al. 2012, Pascual et al. 2010). Its lack of strong preference for settling on different substrates indicates high plasticity in its life-history traits, and it is able to induce defensive phenotypic mechanisms in the presence of predators (stronger attachment, thicker shells and heavier adductor muscle; Babarro et al. 2016) improving overall fitness. Although a successful invader in other areas due to its physiological plasticity, this mussel is under threat from the effects of climate change in its native habitat, as estuary marinization and water temperature increase around the world, but it can also represent a solution for managers dealing with this species in it introduced habitats (Cottingham et al. 2023).

Xenostrobus securis can be used as an effective bioindicator as it very closely reflects the concentrations of Cd, Cr, Cu, Pb and Zn in its environment but also the effects of anthropogenic catchment disturbance and nutrient enrichment in estuaries through nitrogen and carbon stable Isotopes ((Moore and Suthers 2005) Moore & Suthers 2005, Markich & Jeffree 2019). Although it accrued less parasites species than other European native mussel like Mytilus galloprovincialis, the pygmy mussel is a carrier of the parasitic protozoan Marteilia refringens (Pascual et al. 2010), which if co-located with other shellfish, can potentially infect and cause lethal and sublethal effects in commercially important species such as the European flat oyster Ostrea edulis and the mussels Mytilus edulis and M. galloprovincialis (Pascual et al. 2010, EURL 2025).

Food:

phytoplankton and zooplankton

Consumers:

Fishes, Decapod Thalamita danae, Gastropod Nucella lapillus, Copepod Centropages typicus

Competitors:

Other mussels as Arcuatula senhusia

Trophic Status:

Suspension feeder

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Habitats

General Habitat	Oyster Reef	None
General Habitat	Unstructured Bottom	None
General Habitat	Vessel Hull	None
General Habitat	Canals	None
General Habitat	Rocky	None
General Habitat	Marinas & Docks	None
Salinity Range	Euhaline	30-40 PSU
Salinity Range	Polyhaline	18-30 PSU
Salinity Range	Mesohaline	5-18 PSU
Tidal Range	Subtidal	None
Tidal Range	Mid Intertidal	None
Tidal Range	Low Intertidal	None

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Life History

Xenostrobus securis has a short lifespan but high reproductive capacity with a lifespan of 1–2 years (Kimura & Sekiguchi 2009, Lau et al. 2018, Cottingham et al. 2023). The eggs of this small mussel are successfully fertilized in temperate waters in salinities between 15.5 and 31 ppt (Wilson 1969). It has a very long spawning period that can last up to 10 months (Montes et al. 2020). The mussel reproduces sexually around 15–17mm in size and has rapid gametogenesis (sperm and egg development). Sexual maturity is reached at a smaller size in introduced regions respect to its native range (15.57 mm versus 18 mm). Xenostrobus securis can buildup mature gametes in the fall for an extended spawning period, with the potential for multiple spawning cycles without pause. In non-native locations, breeding season becomes almost continuous, lasting 10 months, in contrast to the 7 to 8 months observed in its native range (Montes et al. 2020). It has planktotrophic larvae that survive almost one month in the water column (Wilson 1969). Planktonic larvae of X. securis settled on the bottom 15 days after fertilization at 25 and 30 °C (Kimura & Sekiguchi 1996). Adults and juveniles inhabit the mouths of rivers from the intertidal to a depth of 10 m (Adarraga & Martínez 2012), at salinities varying from 2 to over 36 psu (Wilson 1968), while embryos and larvae have more limitations, tolerating a narrower range of salinity, between 14 and 31 psu (Wilson 1968), which may regulate recruitment timing and distribution of the mussel in native and non-native reaches (Cottingham 2023). Natural spawning was not observed in its native habitat at salinities below 16 psu (Wilson 1969). Recruitment periods occurred between July and September in introduced European countries (Montes et al. 2021) in contrast to its native range where the recruitment is a bit longer, about 7 months (Wilson 1969).

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Tolerances and Life History Parameters

Minimum Temperature (ºC)	5	(Astudillo et al. 2017)
Maximum Temperature (ºC)	42	Short periods (Olabarria et al. 2016)
Minimum Salinity (‰)	1	(Wilson 1968)
Maximum Salinity (‰)	55	Adults only for short periods, larvae are less tolerant above 31 psu (Wilson 1968)
Minimum Length (mm)	20	Zenetos et al. 2003
Maximum Length (mm)	30	Zenetos et al. 2003
Broad Temperature Range	14	14-32 ºC Temperate
Broad Salinity Range	1	1-36 ‰ adults, more restricted for larvae at 14-31%

General Impacts

This species causes habitat alteration and competition and negatively impacts native species where the mussel is introduced, covering soft sediments and smothering the infauna like Arcuatula senhousia (Lazzari & Rinaldi 1994, Gestoso et al. 2012) or competing with Fistulobalanus kondakovi for space in Japan (Kawasaki et al. 2016, Iwasaki et al. 2024). The growth of Xenostrobus securis in Hong Kong has resulted in the mortality of the cultured oyster, Magallana hongkongensis (Lam & Morton 2003), resulting in an estimated loss of approximately $304 thousands to the local oyster production industry (Lau et al. 2018). The species spread may be controlled by seasonal dispersal barriers associated with topographic features (Montes et al. 2021). Nevertheless, this invader showed positive interaction enhancing the physiological performance of other native species like Mytilus galloprovincialis (Gestoso et al. 2014, Olabarria et al. 2016). Overall, it is considered a pest species where introduced, as it can compete with native shellfish for resources and space, and carry parasites that cause disease in other commercially important bivalves like the parasitic protozoan Marteilia refringens (Pascual et al. 2010).

 

 

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