Potamocorbula amurensis - Overbite clam / Brackish-water corbula

The original range of the Brackish-water corbula Potamocorbula amurensis includes the temperate northwestern Pacific region, specifically the silt-rich bottoms of estuarine environments in China, Japan, Korea, and East Siberia ^[2-7].

First observation in Belgium
    
In March 2018, hundreds of fresh empty shells and dozens of live specimens of the Brackish-water corbula were found near Lillo ^[2,7], marking the first record of the species on the European continent ^[3].
 

Distribution in Belgium

The distribution map (2018-2019) by Dumoulin and Langeraert (2020) ^[3] shows a rapid spread ('boom phase') of the species across the Lower Sea Scheldt.
 

Distribution in neighbouring countries

In March 2018, several individuals of this species were found near the low-water line near the Dutch village of Bath (Western Scheldt). Subsequently, the species experienced rapid spread in the Western Scheldt ^[3]. In 2019, the species was observed on mudflats 10 km east of Terneuzen (the Netherlands) ^[3,8].

The suspected introduction vector is ship ballast water transport ^[2,7]. Once in a region with favorable environmental conditions and successful reproduction, rapid secondary spread can occur as the planktonic larvae are carried by currents ^[2,9].

The Brackish-water corbula's tolerance for fluctuating temperatures and salinities makes it an opportunistic species. Therefore, it is expected that the Brackish-water corbula may eventually establish itself in other brackish water areas, such as the North Sea Canal, the Ems-Dollard area, and the Wadden Sea ^[8]. Additionally, the sediment characteristics of the Scheldt resemble the sand- and silt-rich environments in its natural range, which could facilitate further spread ^[8].

The species has a planktonic larval stage lasting 17 to 19 days ^[10]. This time span provides ample opportunity for local rapid dispersal of veliger larvae through the water column. Additionally, the Brackish-water corbula has few requirements regarding water temperature and salinity. In its native areas, water temperatures range from 0 to 28°C, and the species is tolerant of salinities between 2 and 33 psu, although a salinity between 5 and 25 psu is required for reproduction ^[2,10].

In San Francisco Bay, the species rapidly spread after its introduction in 1986, and by 1990, it constituted about 95% of the biomass in the benthic zone at many locations, with densities exceeding 16,000 individuals per square meter ^{[2,7,8,11,12]}. The Brackish-water corbula is an efficient filter feeder (bacterioplankton, phytoplankton, microzooplankton) with a high reproductive rate (becoming fertile at a shell length of around 5 mm). This combination allows it to compete with native species for food and potentially hinder the reproduction of zooplankton species (e.g., copepods) by directly preying on the eggs and larvae of local species ^[2,7,12]. As a result, the species can cause large-scale changes throughout the entire trophic chain ^[12,13]. For instance, the Brackish-water corbula has been implicated in the disruption of phytoplankton and zooplankton biomass and has contributed to the collapse of the local pelagic fishery in San Francisco Bay ^[14,15]. Conversely, the presence of the Brackish-water corbula may also promote an increase in species capable of preying on it ^[10,11].

One proposed management measure is to increase stress on eggs, embryos, and pelagic larvae by reducing salinity through increased freshwater inflow ^[9]. However, the required volume and duration of such measures are practically unfeasible. Additionally, the Brackish-water corbula is a particularly opportunistic species ^[16].

The Brackish-water corbula lives buried in both subtidal and intertidal sand or mud substrates ^[11,12], with the short in- and outflow openings (siphons) protruding above the sediment ^[8]. Measurements of 362 right valves at Liefkenshoek yielded an average shell length of 2.38 cm (ranging from 1.6 to 3.1 cm) and a height of 1.73 cm (ranging from 1.2 to 2.3 cm) ^[3]. The shell is robust, oval to triangular in shape. The bivalve has two unequal valves (overbite). The left valve is smaller than the right valve, features a slight keel, and has an internal protruding chondrophore with a corresponding hollow ^[8]. The right valve contains a sturdy, triangular cardinal tooth with a deep notch behind it ^[3]. Both valves are externally white to yellowish with often gray or blue spots. The entire shell is covered with a thick periostracum. The umbo is smooth and directed forward ^[3].

[1]          World Register of Marine Species (WoRMS) (2024). Potamocorbula amurensis (Schrenck, 1862). https://www.marinespecies.org/aphia.php?p=taxdetails&id=397175 (2024-10-18). 

[2]          Kattenwinkel, L. (2019). Brakwatercorbula in de Westerschelde: Potamocorbula amurensis (Schrenck, 1861) een nieuwe exoot. ‘t Heelblaadje 36(2): 20-23 [https://www.vliz.be/nl/catalogus?module=ref&refid=316945] 

[3]          Dumoulin, E.; Langeraert, W. (2020). De Brakwaterkorfschelp Potamocorbula amurensis (Schrenck, 1861) (Bivalvia, Myida, Corbulidae), een nieuwkomer in het Schelde-estuarium; of het begin van een lang verhaal. De Strandvlo 40(4): 113-172 [https://www.vliz.be/nl/catalogus?module=ref&refid=332308]

[4]          Cohen, A.N. (2011). The exotics guide: non-native marine species of the North American Pacific coast. Center for Research on Aquatic Bioinvasions, Richmond, CA, and San Francisco Estuary Institute, Oakland, CA. Revised September 2011. Available from: http://www.exoticsguide.org

[5]          Okutani, T. (Ed.) (2000). Marine mollusks in Japan. University of Tokyo Press: Tokyo. ISBN 4-486-01406-5. 1173 pp. [https://www.vliz.be/nl/imis?module=ref&refid=70562] 

[6]          Zhongyan, Q. (Ed.) (2004). Seashells of China. China Ocean Press: Beijing. ISBN 7-5027-6170-5. 418, 193 plates pp. [https://www.vliz.be/nl/imis?module=ref&refid=70561] 

[7]          Kattenwinkel, A.H. (2019). Eerste waarneming van de Brakwatercorbula Potamocorbula amurensis (Schrenk, 1861) in Europa. Spirula 419: 33-36. [https://www.vliz.be/en/imis?module=ref&refid=393434]  

[8]          Gmelig Meyling, A. (2021). Brakwaterkorfschelp in opmars. Kijk op Exoten 9(3): 2-3 [https://www.vliz.be/nl/catalogus?module=ref&refid=354287]

[9]          Thompson, J.K.; Parchaso, F. (2010). Corbula amurensis Conceptual Model. U.S. Geological Survey: Washington, DC. 39 pp. [https://www.vliz.be/nl/catalogus?module=ref&refid=354288]

[10]       Nicolini, M.H.; Penry, D.L. (2000). Spawning, fertilization, and larval development of Potamocorbula amurensis (Mollusca: Bivalvia) from San Francisco Bay, California1. Pac. Sci. 54(4): 377-388. [https://www.vliz.be/nl/catalogus?module=ref&refid=354290]

[11]       Carlton, J.T.; Thompson, J.K.; Schemel, L.E.; Nichols, F.H. (1990). Remarkable invasion of San Francisco Bay (California, USA) by the Asian clam Potamocorhula amurensis. I. Introduction and dispersal. Mar. Ecol. Prog. Ser. 66: 81-94. [https://www.vliz.be/en/imis?module=ref&refid=381343] 

[12]       Cohen, A.N.; Carlton, J.T. (1995). Nonindigenous aquatic species in a United States estuary: a case study of the biological invasions of the San Francisco Bay and delta. NOAA: USA. 251 pp. [https://www.vliz.be/nl/imis?module=ref&refid=117462] 

[13]       Greene, V.E.; Sullivan, L.J.; Thompson, J.K.; Kimmerer, W.J. (2011). Grazing impact of the invasive clam Corbula amurensis on the microplankton assemblage of the northern San Francisco Estuary. Mar. Ecol. Prog. Ser. 431: 183-193. https://dx.doi.org/10.3354/meps09099 [https://www.vliz.be/nl/catalogus?module=ref&refid=354289]

[14]       Bax, N.; Williamson, A.; Aguero, M.; Gonzalez, E.R.; Geeves, W. (2003). Marine invasive alien species: a threat to global biodiversity. Mar. Policy 27(4): 313-323. [https://www.vliz.be/nl/imis?module=ref&refid=207966] 

[15]       Sommer, T.; Armor, C.; Baxter, R.; Breuer, R.; Brown, L.; Chotkowski, M.; Culberson, S.; Feyrer, F.; Gingras, M.; Herbold, B.; Kimmerer, W.; Mueller-Solger, A.; Nobriga, M.; Souza, K. (2007). The collapse of pelagic fishes in the Upper San Francisco estuary. Fisheries 32(6): 270-277. [https://www.vliz.be/en/imis?module=ref&refid=381344] 

[16]       Dumoulin, E. (2024). Persoonlijke mededeling.

VLIZ Alien Species Consortium (2024). Potamocorbula amurensis – Overbite clam / Brackish-water corbula. Introduced alien species of the Belgian part of the North Sea and adjacent estuaries anno 2024. Flanders Marine Institute (VLIZ). 5 pp.