Mulinia lateralis - Dwarf surfclam

The Dwarf surfclam is native to the Northwest Atlantic Ocean, between the Gulf of St. Lawrence and the Gulf of Mexico ^[2,3].

First observation in Belgium

The Dwarf surfclam was first observed in Belgium in January 2019, on the beach at De Haan ^[4]. By September of the same year, adult specimens of this species were already found to be common, including at the Wenduine Bank ^[5].

Distribution in Belgium

The species has since been observed along the entire coast from De Panne to Knokke, with a substantial population in the Port of Zeebrugge. Additionally, in May 2021, juvenile specimens were also found at Vlotkom in Brugge (where the Boudewijn Canal flows into the Ringvaart). It is suggested that these may have originated from the outer harbour of Zeebrugge and dispersed inland as larvae. Four months later (August 2021), the Dwarf surfclam was no longer found at that location ^[6]. In the same year, the Dwarf surfclam was also discovered in Groot Buitenschoor (Western Scheldt) ^[7].

Distribution in neighbouring countries

In 2017 and 2018, populations of the Dwarf surfclam were found in the Voordelta, the Wadden Sea, the Ems Estuary, and the mouth of the Western Scheldt. In the Voordelta, the species was already present in densities of up to nearly 6,000 individuals per square meter, while in the Western Scheldt, the density was 820 individuals per square meter ^[8]. Since 2021, the Dwarf surfclam has also been found in the Thames Estuary ^[9].

The species was likely introduced to Western Europe via transport in ship ballast water ^[4].

The Dwarf surfclam is primarily an estuarine species ^[10,11], with the highest numbers observed in its native range in waters with salinities between 5 and 30 psu ^[12]. However, the species can tolerate salinities up to 80 psu ^[13,14]. Optimal growth conditions (embryo development, larval growth) occur at temperatures between 20 and 25°C ^[15], but depending on the developmental stage, the optimal temperature ranges from 7.5 to 27.5°C ^[16]. This bivalve lives in substrates composed of mud and sand ^[4]. Therefore, rather than salinity and temperature, the texture of the substrate will determine whether the Dwarf surfclam can occur in certain areas in our region. For instance, in the Wadden Sea, the species is only found in areas with a muddy substrate and fine grain sizes ^[17].

The species has a planktonic larval stage lasting 7 to 22 days ^[18], allowing the larvae to further disperse by drifting with prevailing currents. With a short generation time of just 3 months ^[19], the species can reproduce multiple times per year ^[20]. This contributes to its ability to rapidly colonise areas after environmental disturbances (e.g., dredging) ^[8,21]. Additionally, the species is opportunistic, can withstand short periods of anoxia ^[22], and is abundant in locations with poor water quality ^[12].

Based on its rapid generation time ^[19], high fecundity (releasing 0.5 to 4 million eggs per spawning event) ^[15,19,23], high tolerance for physicochemical variables ^[12-16], and efficient filtration of phytoplankton and particles at the sediment-water interface, the Dwarf surfclam has the potential to compete with native species for food and space ^[8]. This could impact local biodiversity and lead to habitat modification ^[24,25]. The presence of predators may help control the adult population. In its native range, this clam, which lives close to the sediment surface, primarily serves as food for crabs and fish ^{[14,20,26,27]}, but is also preyed upon by sea stars ^[28], ducks ^[11,29,30], and waterfowl ^[31,32].

The size of the Dwarf surfclam varies significantly between studies and regions, with lengths ranging from 7.0 to 21.2 mm ^[8,14,15,33]. The shell has a smooth white to cream-colored surface, is slightly convex, and has a triangular outline ^[8]. The species closely resembles the Cut trough shell Spisula subtruncata, but the two can be unmistakably distinguished in several ways, preferably by examining them from the top. (1) The Cut trough shell has an external hinge or ligament, which is absent in the Dwarf surfclam; (2) the umbos of the Dwarf surfclam are slightly further apart and are somewhat rolled toward each other; and (3) the Cut trough shell displays a distinct sculpture of parallel lines on either side of the umbo, a feature missing in the Dwarf surfclam. Additionally, Dwarf surfclam specimens are generally somewhat more convex than young Cut trough shells, though this is a somewhat subjective characteristic ^[4]. For a detailed description, please refer to the literature ^[8].

[1]          World Register of Marine Species (WoRMS) (2024). Mulinia lateralis (Say, 1822). https://www.marinespecies.org/aphia.php?p=taxdetails&id=156870 (2024-10-18). 

[2]          Brunel, P.; Bossé, L.; Lamarche, G. (1998). Catalogue des Invertébrés marins de l’estuaire et du golfe du Saint-Laurent = Catalogue of the marine invertebrates of the Estuary and Gulf of Saint Lawrence. Canadian Special Publication of Fisheries and Aquatic Sciences = Publication Spéciale Canadienne des Sciences Halieutiques et Aquatiques, 126. NRC Research Press: Ottawa. ISBN 0-660-60366-7. 405 pp. [https://www.vliz.be/nl/imis?module=ref&refid=213868] 

[3]          Turgeon, D.D.; Lyons, W.; Mikkelsen, P.; Rosenberg, G.; Moretzsohn, F. (2009). Bivalvia (Mollusca), in: Felder, D.L. et al. Gulf of Mexico Origin, Waters, and Biota: Biodiversity. pp. 711-744 [https://www.vliz.be/en/imis?module=ref&refid=393279] 

[4]          Kerckhof, F. (2019). Mulinia lateralis (Say, 1822) de kleine Amerikaanse strandschelp nu ook in België. De Strandvlo 39(1): 4-9 [https://www.vliz.be/en/imis?module=ref&refid=310921] 

[5]          waarnemingen.be. Mulinia lateralis (Say, 1822). https://waarnemingen.be/observation/178961802/ (2024-04-02)

[6]          Bauwens, F. (2021). Enkele nieuwe (week)diersoorten in de Brugse wateren. De Strandvlo 41(3-4): 90-95 [https://www.vliz.be/en/imis?module=ref&refid=348753] 

[7]          Ameye, T. (2021). 14 Augustus 2021 SWG-excursie naar het Groot Buitenschoor. De Strandvlo 41(2): 60-64 [https://www.vliz.be/en/imis?module=ref&refid=345450]

[8]          Craeymeersch, J.A.; Faasse, M.A.; Gheerardyn, H.; Troost, K.; Nijland, R.; Engelberts, A.; Perdon, K.J.; van den Ende, D.; van Zwol, J. (2019). First records of the dwarf surf clam Mulinia lateralis (Say, 1822) in Europe. Marine Biodiversity Records 12: 5. [https://www.vliz.be/nl/catalogus?module=ref&refid=306716]

[9]          Davison, P. Persoonlijke mededeling.

[10]        Walker, R.L.; Tenore, K.R. (1984). Growth and production of the dwarf surf clam Mulinia lateralis (Say 1822) in a Georgia Estuary. Gulf and Caribbean Research 7(4): 357-363. [https://www.vliz.be/en/imis?module=ref&refid=393319] 

[11]        Montagna, P.A.; Kalke, R.D. (1995). Ecology of infaunal Mollusca in South Texas estuaries. Am. Malacol. Bull. 11(2): 163-175. [https://www.vliz.be/en/imis?module=ref&refid=393320] 

[12]        McKeon, C.S.; Tunberg, B.G.; Johnston, C.A.; Barshis, D.J. (2015). Ecological drivers and habitat associations of estuarine bivalves. PeerJ 3: e1348. [https://www.vliz.be/en/imis?module=ref&refid=393321] 

[13]        Parker, R.H. (1975). The study of benthic communities: A model and a review. Elsevier Oceanography Series, 9. Elsevier Scientific Publishing Company: Amsterdam/London/New York. ISBN 0-444-41203-4; e-ISBN 978-0-444-41203-4. X, 279 pp. [https://www.vliz.be/nl/imis?module=ref&refid=38161] 

[14]        Montagna, P.A.; Stockwell, D.; Kalke, R.D. (1993). Dwarf surfclam Mulinia lateralis (say, 1822) populations and feeding during the Texas Brown tide event. J. Shellfish Res. 12(2): 433-442 [https://www.vliz.be/en/imis?module=ref&refid=393322] 

[15]        Calabrese, A. (1969). Mulinia lateralis: molluscan fruit fly? Proceedings of the National Shellfisheries Association 59: 65-66 [https://www.vliz.be/en/imis?module=ref&refid=37697] 

[16]        Calabrese, A.; Rhodes, E.W. (1974). Culture of Mulinia lateralis and Crepidula fornicata embryos and larvae for studies of pollution effects. Thalass. Jugosl. 10(1-2): 89-102 [https://www.vliz.be/en/imis?module=ref&refid=121302] 

[17]        Klunder, L.; Lavaleye, M.; Kleine Schaars, L.; Dekker, R.; Holthuijsen, S.; van der Veer, H. (2019). Distribution of the dwarf surf clam Mulinia lateralis (Say, 1822) in the Wadden Sea after first introduction. Bioinvasions Records 8(4): 818-827. [https://www.vliz.be/en/imis?module=ref&refid=319590]

[18]        Mann, R.; Campos, B.M.; Luckenbach, M.W. (1991). Swimming rate and responses of larvae of three mactrid bivalves to salinity discontinuities. Mar. Ecol. Prog. Ser. 68(3): 257-269. [https://www.vliz.be/en/imis?module=ref&refid=393323]  

[19]        Guo, X.; Allen, S.K. (1994). Sex determination and polyploid gigantism in the dwarf surfclam (Mulinia lateralis Say). Genet. 138(4): 1199-1206. [https://www.vliz.be/en/imis?module=ref&refid=393324] 

[20]        Calabrese, A. (1970). Reproductive cycle of the Coot Clam, Mulinia lateralis (Say), in Long Island Sound. Veliger 12(3): 265-269, plates 37-38 [https://www.vliz.be/nl/imis?module=ref&refid=133058] 

[21]        Flint, R.W.; Younk, J.A. (1983). Estuarine benthos: Long-Term community structure variations, Corpus Christi Bay, Texas. Estuaries 6(2): 126-141. [https://www.vliz.be/en/imis?module=ref&refid=393325] 

[22]        Shumway, S.E.; Scott, T.M.; Shick, J.M. (1983). The effects of anoxia and hydrogen sulphide on survival, activity and metabolic rate in the coot clam, Mulinia lateralis (Say). J. Exp. Mar. Biol. Ecol. 71(2): 135-146. [https://www.vliz.be/en/imis?module=ref&refid=393326] 

[23]        Lu, J.K.; Chen, T.T.; Allen, S.K.; Matsubara, T.; Burns, J.C. (1996). Production of transgenic dwarf surfclams, Mulinia lateralis, with pantropic retroviral vectors. Proc. Natl. Acad. Sci. U.S.A. 93(8): 3482-3486. [https://www.vliz.be/en/imis?module=ref&refid=393327] 

[24]        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] 

[25]        Manchester, S.J.; Bullock, J.M. (2000). The impacts of non-native species on UK biodiversity and the effectiveness of control. J. Appl. Ecol. 37(5): 845-864. [https://www.vliz.be/en/imis?module=ref&refid=393328] 

[26]        Virnstein, R.W. (1977). The importance of predation by crabs and fishes on benthic infauna in Chesapeake Bay. Ecology 58(6): 1199-1217. [https://www.vliz.be/en/imis?module=ref&refid=393329]

[27]        Virnstein, R.W. (1979). Predation on estuarine infauna: Response patterns of component species. Estuaries 2(2): 69-86. [https://www.vliz.be/nl/imis?module=ref&refid=324693] 

[28]        McClintock, J.B.; Lawrence, J.M. (1985). Characteristics of foraging in the soft-bottom benthic starfish Luidia clathrata (Echinodermata: Asteroidea): prey selectivity, switching behavior, functional responses and movement patterns. Oecologia 66(2): 291-298. [https://www.vliz.be/en/imis?module=ref&refid=393330] 

[29]        Cronan, jr., J.M. (1957). Food and feeding habits of the scaups in Connecticut Waters. The Auk 74(4): 459-468. [https://www.vliz.be/en/imis?module=ref&refid=393331] 

[30]        Berlin, A. (2018). Foraging values of Mulinia lateralis and Ischadium recurvum: energetics effects of surf scoters wintering in the Chesapeake Bay. PhD Thesis. University of Maryland: Baltimore. [https://www.vliz.be/en/imis?module=ref&refid=394260] 

[31]        Britton, J.; Morton, B. (1989). Shore ecology of the Gulf of Mexico. University of Texas Press: Austin. 396 pp. [https://www.vliz.be/en/imis?module=ref&refid=393384] 

[32]        Smith, F.; Watts, B.; Lyons, J.; Keyes, T. (2016). Investigating population dynamics of red knot migration along the Georgia Coast through mark/recapture analysis of resights: 2015 Spring season. College of William and Mary/Virginia Commonwealth University: Williamsburg. 13 pp. [https://www.vliz.be/en/imis?module=ref&refid=393385] 

[33]        Abbott, R.T. (1974). American Seashells: the marine Mollusca of the Atlantic and Pacific coasts of North America. Second edition. Van Nostrand Reinhold: New York. ISBN 0-442-20228-8. 663 pp. [https://www.vliz.be/nl/imis?module=ref&refid=16563]

VLIZ Alien Species Consortium (2024). Mulinia lateralis – Dwarf surfclam. Introduced alien species of the Belgian part of the North Sea and adjacent estuaries anno 2024. Flanders Marine Institute (VLIZ). 6 pp.