Petricolaria pholadiformis - American piddock

The American piddock originally occurred exclusively along the east coast of America, from the Gulf of Saint Lawrence in Canada to the Gulf of Mexico ^[3].

This bivalve drills a tunnel in peat layers, wood, hard clay and limestone, where it remains for the rest of its life. Young individuals are also sometimes observed between mussel beds. Their habitat extends from the extreme low-water mark to the middle of the intertidal zone ^{[4, 5]}.

First observation in Belgium

The American piddock was first observed near Nieuwpoort in 1899 ^[6]. The species was reported again in 1900 and 1901, in Koksijde and Wenduine. One year later, this alien species was found between Klemskerke and Blankenberge ^{[7, 8]}.

Spreading in Belgium

Since 1901, the American piddock is considered a common species along the Belgian coast ^[8]. A study of the clay and peat banks at Raversijde (Ostend) showed that the American piddock was very common during the first half of the 20^th century ^[9] and more recent studies also confirm the success of this species in the Belgian coastal area ^[10].

Data from the Flanders Research Institute for Agriculture, Fisheries and Food (ILVO-Fisheries) shows that the American piddock is mainly found in the coastal area of the Belgian part of the North Sea, more specifically in the eastern part – from Zeebrugge to the Scheldt estuary – in the proximity of old or new dumping sites for dredged materials. Researchers from the ILVO-Fisheries have found that the occurrence of the species shifted eastwards since 1990s, from Zeebrugge towards the Scheldt estuary, where the highest numbers are observed today. At the end of the 1980s, the species was sporadically observed along the western coastal area, but no observations were made over the following years ^[11]. In December 2012, another 100 individuals of the American piddock were counted at ‘Ster der Zee’ in Koksijde (West coast), of which 14 were alive ^[12]. This alien species can reach high densities locally, ranging from hundreds ^[13] (south-east of Thornton Bank) to one thousand individuals per m² (period 1994-2001) ^[10].  

Spreading in neighbouring countries

The American piddock was first observed in Europe (around 1890) in Essex (South East England). From there, the species spread naturally in a northern direction to the Humber estuary. The larvae of the American piddock live freely in the water column and are passively transported by sea currents. In this way, the species reached the European continent for the first time in 1899 at the Belgian coast. From here, the American piddock spread mainly northward. In 1906, the species was already spotted in the north of the Netherlands (Vlieland) and then it spread into the German Wadden Sea region. In 1907, this mollusc reached the Danish North Sea coast, and the Skagerrak between Denmark and Sweden in 1910. Twenty years later, this species was found in the western part of the Baltic Sea ^{[4, 5, 8]}.

In a southern direction, the spreading of the American piddock was much more difficult. In 1903, the first individuals were found in Dunkirk. In 1910, this species was also observed in Calais, and in Boulogne-Sur-Mer in 1914, its southernmost point so far. Scientists attribute the difficult southern distribution to the prevailing currents along our North Sea coasts, which move from the southwest to the northeast, making it difficult to transport free-living larvae to the south ^{[4, 5, 8]}.

Since the rapid expansion of its range at the beginning of the last century, little has changed in the distribution of the species ^[5]. However, in some places the species has declined in number or even disappeared, such as in the Danish Wadden Sea. This was presumably due to a lack of adaptability, local parasites or competition ^{[14, 15]}.

The introduction in South East England happened simultaneously with the import of American oysters. The American piddock settled and expanded its range by transport of its larvae with sea currents. Because of the southward currents along the east coast of Great Britain, this species quickly reached the Belgian coast where it settled ^{[5, 17]}.

The female American piddock produces about 3 to 3.5 million eggs annually. The larvae can spread over large distances by floating passively in sea currents ^[5]. Adult individuals of this species can also spread through the currents by drilling themselves into floating wood.

The American piddock depends on the presence of peat or clay banks to settle. The species is only found in fine sediments with a median particle size of less than 250 µm and does not occur in the absence of silt ^[10]. The species drills a small burrow in the sediment and stays there its entire life. The species is very successful at places where sand accumulation on top of the peat and clay banks is limited.

The buried bivalve has two tubes or siphons that rise above the ground surface: one for the inflow of water and food; the other for draining water and food waste. In places where there is a lot of sand accumulation, the siphons can be buried under the sand. As a result, the drilling mussel loses contact with the seawater (its oxygen and food source) and dies ^{[4, 5, 9]}. The American piddock is occasionally also found in sandy soils where many tubes of the sand mason worm Lanice conchilega are present which retain the sand ^[18].

Petricolaria pholadiformis spread rapidly along our coast in the early 20^th century ^[8] and may have contributed to the decline of the native white piddock Barnea candida during this period, although there is no conclusive evidence for this ^{[4, 14]}. The white piddock is on the rise again since 1950 ^{[5, 9, 17]}. In 1993, the white piddock was observed at various locations along the Belgian coast, and both our indigenous species and the alien species were found occurring side by side. The American piddock prefers peat and clay banks, while the white piddock endures more sand accumulation ^{[5, 9]}. The population of American piddocks is declining near Raversijde due to sand accumulation on the peat and clay banks ^[9].

Measures against the American piddock have not yet been taken, as the species does not appear to be a demonstrable problem anywhere. In some locations, the population is declining naturally ^[19].

The shell of the American piddock is oblong cylindrical and has a white colour on the outside. The outside of the shells is rather off-white to flesh-coloured, the inside is glossy white. The American piddock reaches a size of 5 to 6.5 centimetres ^[4].

Ribs leave from the top of the shell, which are thick in the front part of the shell and become thinner towards the rear end. These ribs are crossed by growth rings, with a thickened knot at the location of the crossing. It is with these knots that the mollusc digs a burrow.

The American piddock looks very similar to our native white piddock Barnea candida, and can easily be confused with it. However, the native species has a clearly folded front edge or lip and has an additional shell piece on the top of the shell. The thickened knots are much sharper on the white piddock than on the American species. Another difference between both species is the presence of ‘lock teeth’: the white piddock has none, while the American piddock has two on the right valve and three on the left valve ^{[4, 20, 21]}. A clear third difference is visible on the inside of the shell. The white piddock has a long protrusion, called the ‘apophysis’, below the top. The American piddock has no ‘apophysis’ ^[19].

Both species are often found in extended or compacted shapes, depending on the type of soil in which they live ^[5]. They often live in moderately hard to hard substrate in the seabed, such as peat, wood and limestone. The harder the substrate, the more compact their shape.

The American piddock occurs from the tidal zone to a depth of approximately 15 m. It can also be buried in firm clay in mudflat areas and live in mussel beds. It makes mechanical movements with the shells during drilling.

The species has separate sexes and are, at the earliest, sexually mature at the end of their third year. Eggs and sperm are released into the water between July and September. This species has a lifespan of up to 10 years ^[2].

During a beach walk you can often find holes in the sand. This is often the inflow opening of a buried worm or mollusc. If you find an 8-shaped hole, it could be the burrow of an American piddock ^[17]. Further, during a beach walk, only left or right valves will be found. This is the result of shell-current interactions, where the currents send the two different shell valves in a different direction due to their unequal shape ^[5].

[1]          World Register of Marine Species (WoRMS) (2020). Petricolaria pholadiformis (Lamarck, 1818). [http://www.marinespecies.org/aphia.php?p=taxdetails&id=156961] (2020-11-17).

[2]          Stichting ANEMOON (2018). Amerikaanse boormossel Petricolaria pholadiformis (Lamarck, 1818). [http://www.anemoon.org/flora-en-fauna/soorteninformatie/soorten/articletype/articleview/articleid/32] (2018-08-16).

[3]          Cohen, A.N.; Carlton, J.T. (1995). Non indigenous 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. [http://www.vliz.be/en/imis?module=ref&refid=117462]

[4]          Van Benthem Jutting, T. (1943). Mollusca(I) C. Lamellibranchia. Fauna van Nederland, 12. A.W. Sijthoff: Leiden, The Netherlands. 477 pp. [http://www.vliz.be/imis/imis.php?module=ref&refid=56751]

[5]          Wouters, D. (1993). 100 jaar na de invasie van de Amerikaanse boormossel: de relatie Petricola pholadiformis Lamarck, 1818 / Barnea candida Linnaeus, 1758. De Strandvlo 13(1): 3-39. [http://www.vliz.be/imis/imis.php?module=ref&refid=18790]

[6]          Loppens, K. (1902). Petricola pholadiformis L. Bull. Séances Soc. (R.) Malacol. Belg. 37: 41-42. [http://www.vliz.be/imis/imis.php?module=ref&refid=70030]

[7]          Dupuis, P.; Putzeys, S. (1902). Note concernant la découverte du Petricola pholadiformis en Belgique. Ann. Soc. Roy. Zool. Bel. 37: 4. [http://www.vliz.be/imis/imis.php?module=ref&refid=70189]

[8]          Schouteden, H. (1907). Distribution géographique actuelle de Petricola pholadiformis en Europe. Ann. Soc. R. Zool. Malacol. Bel. 42: 64-66. [http://www.vliz.be/imis/imis.php?module=ref&refid=11788]

[9]          Jocqué, R.; Van Damme, D. (1971). Inleidende oecologische studie van klei- en turfbanken in de getijdenzone te Raversijde (België). Biol. Jb. Dodonaea 39: 157-190. [http://www.vliz.be/en/imis?module=ref&refid=3427]

[10]        Degraer, S.; Wittoeck, J.; Appeltans, W.; Cooreman, K.; Deprez, T.; Hillewaert, H.; Hostens, K.; Mees, J.; Vanden Berghe, E.; Vincx, M. (2006). De macrobenthosatlas van het Belgisch deel van de Noordzee. Federaal Wetenschapsbeleid: Brussels, Belgium. ISBN 90-810081-5-3. 164, photographs, 1 cd-rom pp. [http://www.vliz.be/imis/imis.php?module=ref&refid=100230]

[11]        Groep Biologische Milieumonitoring van het Instituut voor Landbouw- en Visserijonderzoek. Onderzoeksdomein Visserij (ILVO - Visserij) (2012). Ongepubliceerde data, geanalyseerd in functie van monitoringsopdrachten voor de Federale Overheid (FOD Economie, in kader van zandwinning) en Ministerie van de Vlaamse Gemeenschap - Administratie Waterwegen en Zeewezen - Afdeling Maritieme Toegang (in kader van baggerwerken en loswallen).

[12]        Vanhaelen, M.-T. (2012). Strandwaarnemingen tijdens de winter 2011-2012 aan de Westkust. De Strandvlo 32(3): 111-113. [http://www.vliz.be/en/imis?module=ref&refid=219863]

[13]        Rumes, B.; Di Marcantonio, M.; Brabant, R.; Dulière, V.; Degraer, S.; Haelters, J.; Kerckhof, F.; Legrand, S.; Norro, A.; Van den Eynde, D.; Virgin, L.; Lauwaert, B. (2011). Milieueffectenbeoordeling van het NORTHER offshore windmolenpark ten zuidoosten van de Thorntonbank. Beheerseenheid van het mathematisch model van de Noordzee (BMM): Brussel. VIII, 190 pp. [http://www.vliz.be/en/imis?module=ref&refid=245187]

[14]        Jensen, K.T. (1992). Macrozoobenthos on an intertidal mudflat in the Danish Wadden Sea: comparisons of surveys made in the 1930s, 1940s and 1980s. Helgol. Meeresunters. 46(4): 363-376. [http://www.vliz.be/imis/imis.php?module=ref&refid=125923]

[15]        Wouters, D. (1993). Petricola pholadiformis Lamarck, 1818 en Scrobicularia plana (Da Costa, 1778) verdwenen uit de Deense Waddenzee. De Strandvlo 13(2-3): 84-85. [http://www.vliz.be/imis/imis.php?module=ref&refid=18803]

[16]        Zenetos, A.; Ovalis, P.; Vardala-Theodorou, E. (2009). The American piddock Petricola pholadiformis Lamarck, 1818 spreading in the Mediterranean Sea. Aquat. Invasions 4(2): 385-387. [http://www.vliz.be/nl/catalogus?module=ref&refid=254687]

[17]        Van Campenhout, B. (1963). Présence, apparition et disparition de Barnea (Barnea) candida (Linné) et de Petricola (Petricolaria) pholadiformis (Lamarck). Les Naturalistes Belges 44(7): 350-353. [http://www.vliz.be/imis/imis.php?module=ref&refid=71463]

[18]        Swennen, C. (1959). Iets over Petricola pholadiformis en andere boorders. Het Zeepaard 19(5): 71-72. [http://www.vliz.be/imis/imis.php?module=ref&refid=114592]

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

[20]        Kaas, P.; Ten Broek, A.N.C. (1942). Nederlandse zeemollusken. De Wereldbibliotheek: Amsterdam, The Netherlands. 232 pp. [http://www.vliz.be/imis/imis.php?module=ref&refid=25183]

[21]        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]