Mya arenaria - Steamer clam

The softshell clam, or sand gaper, is native to the Atlantic coast of America, from Labrador to the state of North Carolina. The current spreading in the Pacific Ocean along the coastline from California to Canada and in the south of Alaska is the result of intentional and unintentional introductions during the 20^th century, combined with natural spread ^{[2, 3]}.

Originally, the softshell clam also occurred in Europe. Here, the species was present during the Pliocene (5.3 to 1.8 million years ago) but went locally extinct during the ice ages of the Pleistocene (1.8 million years to 11,500 years ago) ^[4]. The species was reintroduced by humans to Europe during the Middle Ages. So, the species is regarded as a non-native species.

First observation in Belgium

For a long time, it was assumed that this species got introduced to Europe in the 16^th or 17^th century ^[5]. However, there are indications that the softshell clam was already present in Europe between 1245 and 1295 ^[6]. The species would have travelled from America – via Greenland ^[7] – with Viking ships ^[8]. This is tangible evidence that the Vikings discovered America before Columbus in 1492, or as the English scientists put it: ‘Clams before Columbus’ ^[9].

Old remains of marine shells, dating from 1721-1810, were found in Belgian polder areas (e.g. the Snaaskerkepolder). During this period, the polder was used as a flooding area ^[4], leaving the shells behind at the bottom. Research shows that the softshell clam was already a common species in Ostend in the 19^th century ^[10].

Spreading in Belgium

The softshell clam is still a common species in our ports and estuaries and is also regularly found along the coastline ^[8]. Because of its buried lifestyle, the species is often overlooked.

Spreading in neighbouring countries

The softshell clam has been observed in all European seas. It occurs from northern Norway to southern France (Arcachon, Bay of Biscay). The species has also reached the British and Irish isles, where it is widely distributed and common. The softshell clam also occurs in the Black Sea and the Mediterranean Sea ^{[2, 11]}.

According to scientists, the softshell clam got introduced to Europe via shipping ^[8]. The original introduction by the Vikings could have taken place via bilge water (the water that accumulates in the hold of the ship). Recent studies based on genetic analyses confirm that the species originated from North American populations ^[12]. Subsequent introductions could have occurred because adult individuals and/or larvae got transported in the ballast water ^[7]. The secondary spreading can take place by transporting larvae via the prevailing sea currents ^[7]. In America, there are cases where softshell clams got transported with American oysters Crassostrea virginica intended for aquaculture purposes ^[13].

The softshell clam thrives in both brackish and salt water, allowing it to survive in many places. In addition, these animals easily spread in soils of silt- and sand-rich waters, and they can survive in varying temperatures and salinities. Softshell clams can handle temperatures from -2 to 28°C and salinities as low as 4-5 PSU, allowing them to colonise brackish water ^{[2, 11, 14]}. By comparison, the North Sea has an average salinity of 35 PSU.

Its buried lifestyle (it can dig up to 40 cm deep) protects against predation and directly contributes to the species' success. In addition, the softshell clam feeds on both phytoplankton, zooplankton and dissolved organic material ^[4].

The softshell clam is a digging species and thrives in soft soils with silt, sand and gravel ^[11]. Because of this, the species does not occur in the rocky seabed of southern Europe. In coarser sediment, the softshell clam has a thicker shell and predation pressure is lower. Digging deeper in coarser sediment requires much more energy than in finer sediment ^[15].

The spreading of the softshell clam in an intertidal area is determined by the transport of sediment. After the larvae settle on soft substrate, they do not remain in the same location. Waves and tides stir up the sediment with the softshell clam and transport them to another site ^[16].

This species can adapt to different environmental conditions in terms of physiology, morphology and behaviour ^{[17, 18]}. Juvenile softshell clams are negatively affected by ocean acidification. Small softshell clams do not bury themselves as well in acidic sediment as in other sediments ^[19-21]. Additionally, increased turbidity leads to a decrease in the ventilation rate of M. arenaria. This reaction offers tolerance against intermittent turbidity but leads to mortality during prolonged exposure to turbid water ^[22].

Negative effects due to the presence of the softshell clam remain unknown. Because the species has most likely been native to Europe before the last ice age, it is often considered ‘autochthonous’ ^[23].

In America, this species is eaten a lot. There, the softshell clam has a great economic value. The molluscs are often cooked by steaming, also referred to as clam bakes. This method is similar to our preparation of mussels ^[24].

The softshell clam has a firm oval shell, of which the outside shows irregular concentric growth lines. When closed, the two shells do not fit perfectly, hence the name ‘gaper’. The shell can grow up to 15 cm long and usually has a white to beige colour, depending on the surface in which it is dug ^{[11, 14]}.

The softshell clam has a buried lifestyle, in which its depth in the sediment is proportional to its age: one-year-old clams are usually not found deeper than 5-10 cm, while clams of 10 years old can be found up to 40 cm deep.

The species collects its food by suspension or filter-feeding: they absorb seawater, filter it over their gills and take up the necessary food. They mainly absorb organic matter, algae and microscopic plants and animals. Since softshell clams dig themselves in, they are not directly in contact with the seawater. Thanks to two ‘funnels’ or siphons, they can interact with the seawater: one funnel serves as an inlet, while the other pumps the water out of the animal. An adult softshell clam can filter up to 50 litres of seawater per day ^{[25, 26]}.

[1]          World Register of Marine Species (WoRMS) (2020). Mya arenaria Linnaeus, 1758. [http://www.marinespecies.org/aphia.php?p=taxdetails&id=140430] (2020-11-17).

[2]          Strauch, F. (1972). Phylogenese, Adaptation und Migration einiger nordischer mariner Molluskengenera (Neptunea, Panomya, Cyrtodaria und Mya). Abhandlungen der Senckenbergischen Naturforschenden Gesellschaft, 531. Verlag Waldemar Kramer Frankfurt. ISBN 3-7829-2531-9. 211, ill. tab., pl. pp. [http://www.vliz.be/en/imis?module=ref&refid=70559]

[3]          Powers, S.P.; Bishop, M.A.; Grabowski, J.H.; Peterson, C.H. (2006). Distribution of the invasive bivalve Mya arenaria L. on intertidal flats of southcentral Alaska. J. Sea Res. 55(3): 207-216. [http://www.vliz.be/en/imis?module=ref&refid=98222]

[4]          Strasser, M. (1999). Mya arenaria: an ancient invader of the North Sea coast. Helgol. Meeresunters. 52(3-4): 309-324. [http://www.vliz.be/en/imis?module=ref&refid=119128]

[5]          Hessland, I. (1946). On the Quaternary Mya period in Europe. Ark. Zool. 37A(8): 1-52. [http://www.vliz.be/nl/catalogus?module=ref&refid=115904]

[6]          Essink, K.; Oost, A.P. (2016). De Strandgaper Mya arenaria al in Nederland ‘vóór Columbus’. Spirula Newsletter = Spirula Mededelingenblad 408: 1-13. [http://www.vliz.be/nl/catalogus?module=ref&refid=300292]

[7]          Wolff, W.J. (2005). Non-indigenous marine and estuarine species in the Netherlands. Zool. Meded. 79(1): 3-116. [http://www.vliz.be/en/imis?module=ref&refid=101200]

[8]          Kerckhof, F.; Haelters, J.; Gollasch, S. (2007). Alien species in the marine and brackish ecosystem: the situation in Belgian waters. Aquat. Invasions 2(3): 243-257. [http://www.vliz.be/en/imis?module=ref&refid=114365]

[9]          Petersen, K.S.; Rasmussen, K.L.; Heinemeier, J.; Rud, N. (1992). Clams before Columbus? Nature (Lond.) 359: 679. [http://www.vliz.be/en/imis?module=ref&refid=113658]

[10]        Forbes, E.; Hanley, S. (1853). A history of British Mollusca, and their shells: IV. Pulmonifera and Cephalopoda. , 4. John Van Voorst: London. 301, plates I-CXXXIII pp. [http://www.vliz.be/en/imis?module=ref&refid=115993]

[11]        Hayward, P.J.; Nelson-Smith, A.; Shields, C. (1999). Gids van kust en strand: flora en fauna. Tirion: Baarn. ISBN 90-5210-327-5. 352, ill. pp. [http://www.vliz.be/en/imis?module=ref&refid=72948]

[12]        Cross, M.E.; Bradley, C.R.; Cross, T.F.; Culloty, S.; Lynch, S.; McGinnity, P.; O’Riordan, R.M.; Vartia, S.; Prodöhl, P.A. (2016). Genetic evidence supports recolonisation by Mya arenaria of western Europe from North America. Mar.  Ecol. Prog. Ser. 549: 99-112. [http://www.vliz.be/nl/catalogus?module=ref&refid=300295]

[13]        Stearns, R.E.C. (1881). Mya arenaria in San Francisco Bay. American Naturalist 15(5): 362-366. [http://www.vliz.be/en/imis?module=ref&refid=115919]

[14]        Hayward, P.J.; Ryland, J.S. (1995). Handbook of the marine fauna of North-West Europe. Oxford University Press: Oxford, UK. ISBN 0-19-854054-X. XI. 800 pp. [http://www.vliz.be/en/imis?module=ref&refid=10501]

[15]        Thomson, E.; Gannou, D.P. (2013). Influence of sediment type on antipredator response of the softshell clam, Mya arenaria. Northeastern naturalist 20(3): 498-510. [http://www.vliz.be/nl/catalogus?module=ref&refid=297537]

[16]        Morse, B.L.; Hunt, H.L. (2013). Impact of settlement and early post-settlement events on the spatial distribution of juvenile Mya arenaria on an intertidal shore. J. Exp. Mar. Biol. Ecol. 448: 57-65. [http://www.vliz.be/nl/catalogus?module=ref&refid=297535]

[17]        Lasota, R.; Pierścieniak, K.; Miąc, J.; Wołowicz, M. (2014). Comparative study of ecophysiological and biochemical variation between the Baltic and North Sea populations of the invasive soft shell clam Mya arenaria (L. 1758). Oceanological and Hydrobiological Studies 43(3): 303-311. [http://www.vliz.be/nl/catalogus?module=ref&refid=297532]

[18]        Bayne, B.L. (2014). Comparisons of measurements of clearance rates in bivalve molluscs. Mar. Ecol. Prog. Ser. 276: 305-306. [http://www.vliz.be/nl/catalogus?module=ref&refid=300299]

[19]        Clements, J.C.; Hunt, H.L. (2014). Influence of sediment acidification and water flow on sediment acceptance and dispersal of juvenile soft-shell clams (Mya arenaria L.). J. Exp. Mar. Biol. Ecol. 453: 62-69. [http://www.vliz.be/nl/catalogus?module=ref&refid=297526]

[20]        Green, M.A.; Waldbusser, G.G.; Reilly, S.L.; Emerson, K.; O'Donnell, S. (2009). Death by dissolution: sediment saturation state as a mortality factor for juvenile bivalves. Limnol. Oceanogr. 54(4): 1037-1047. [http://www.vliz.be/nl/catalogus?module=ref&refid=300297]

[21]        Green, M.A.; Waldbusser, G.G.; Hubazc, L.; Cathcart, E.; Hall, J. (2013). Carbonate mineral saturation state as the recruitment cue for settling bivalves in marine muds. Est. Coast. 36(1): 18-27. [http://www.vliz.be/nl/catalogus?module=ref&refid=300298]

[22]        Grant, J.; Thorpe, B. (1991). Effects of suspended sediment on growth, respiration, and excretion of the soft-shell clam (Mya arenaria). Can. J. Fish. Aquat. Sci./J. Can. Sci. Halieut. Aquat. 48(7): 1285-1292. [http://www.vliz.be/en/imis?module=ref&refid=119130]

[23]        Backeljau, T. (2011). Persoonlijke mededeling

[24]        Global Invasive Species Database (2007). Mya arenaria. [http://www.issg.org/database/species/ecology.asp?si=1159&fr=1&sts=sss] (3/1).

[25]        The Marine Life Information Network (MarLIN) (2018). Sand gaper (Mya arenaria). [http://www.marlin.ac.uk/species/detail/1404] (2018-08-16).

[26]        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. [www.exoticsguide.org] (2018-08-08).

VLIZ Alien Species Consortium (2020). Mya arenaria – Softshell clam. Non-indigenous species in the Belgian part of the North Sea and adjacent estuaries anno 2020. Flanders Marine Institute (VLIZ). 6 pp.