Ensis leei - Atlantic razor clam

The Atlantic jackknife clam is native to the Atlantic coast of the United States, from Labrador to North Carolina ^[2]. There, the species occurs on a sandy seabed, from the low-water mark to a depth of 20 to 30 meters.

First observation in Belgium

The first ‘Belgian’ individuals of E. leei were found on the beach of Oostduinkerke on the 2^nd of April 1987, between individuals of the local sword razor Ensis magnus and the minor jackknife clam Ensis minor. A few days later, individuals were found in Zeebrugge. Due to the large distance between the two observation sites, it was assumed that the Atlantic jackknife clam had colonised the entire Belgian coast ^[3].

Spreading in Belgium

In the months following the first observations (in 1987), a number of reports were received confirming the presence of the Atlantic jackknife clam along the entire Belgian coastline.

Even now, large numbers of living Atlantic jackknife clams still regularly wash ashore on our beaches. This is the result of the massive presence of the species in the subtidal sandbanks. In the autumn of 2006, up to 350 individuals per m² were found off the coast of Koksijde, at a depth of about 2 metres ^[4].

On the ‘Vlakte van de Raan’ – about 5 kilometres off the coast of Knokke-Heist – researchers found densities of 1,200 clams per m² in 1999. Such record numbers were not observed again in the following years ^[5]. However, even greater densities were found in Dutch waters: up to 2,000 individuals per m² ^[6].

Along the Belgian shoreline, three stable populations of the Atlantic jackknife clam have been observed: around the Nieuwpoort Bank, on the Ostend Bank and on the northern edge of the ‘Vlakte van de Raan’ ^[7]. Previously, the species was also observed on the northern edge of the Wenduine Bank. The status of this population is currently unclear ^[7]. The species often occurs in association with another bivalve species, namely the white furrow shell Abra alba ^[7].

The aforementioned densities are likely an underestimation, since the samples are typically taken with a Van Veen grab sampler. The Atlantic jackknife clam is located vertically in sandy and muddy soils, and when disturbed – e.g. falling in the bottom of a sampling instrument – these animals retreat instantly, up to 50 centimetres deep ^{[6, 8]}. A Van Veen grab sampler only takes samples from the top 10 to 15 centimetres of the seabed, causing an unknown amount of clams to escape the sampling ^[9].

Spreading in neighbouring countries

The Atlantic jackknife clam was observed for the first time in European waters in June 1979, in the German Bight, situated on the border between the German and Dutch Wadden Sea (figure 1). However, it is suspected that the introduction had already taken place in early 1978. In the subsequent years, the species expanded its range in northern and southern directions ^[10]. This gradual colonisation differs from the results of a genetic research suggesting that multiple introductions occurred in Europe ^[11].

Figure 1: Introduction pattern of the Atlantic jackknife clam in Europe. (Source: VLIZ adaptation of Severijns, 2002 ^[12]).

The first observation from the Netherlands dates from 1982. The observation took place on Schiermonnikoog, one of the Wadden Islands ^[13]. Since then, the population has grown dramatically and dense populations settled across the entire coastline.

In France, the first individuals were observed in January 1988 in Bray-Dunes, close to the Belgian border ^[14]. By now, the species has been observed up to Saint-Vaast-la-Hougue, in Normandy ^[15] and in the Bay of Biscay ^{[16, 17]}.

In the north, the species occurs along the Norwegian coast (to Oslo), in Danish and German coastal waters and along the western coast of Sweden to the southern Baltic Sea ^[12].

The crossing to Great Britain proved to be no problem for this clam. The Atlantic jackknife clam is now distributed from the Humber estuary in the north to Newhaven in the south ^{[12, 18]}. The species has been observed in the Firth of Forth estuary ^[19] near Edinburgh, in Angle Bay, the western part of the Milford harbour ^[20] and in Liverpool Bay ^{[16, 21]}.

Because E. leei got introduced to Europe in the colder part of its thermal niche, there is a good chance that the species will expand its habitat not only northwards but also southwards due to global warming ^[22].

It is assumed that the Atlantic jackknife clam came to our regions in the ballast water of ships. Scientists assume that the planktonic larvae were brought to Europe instead of the adults, due to the adult’s buried lifestyle ^{[12, 14]}. Further spreading along the European coasts occurred via passive transport of larvae with the ocean currents. Atlantic jackknife clams that were caught in the Netherlands but consumed and discarded in southern Europe, may promote the introduction of this species in these latter countries ^[16].

The Atlantic jackknife clam is a strong opportunist. This mollusc is found in sandy and muddy soils, and thrives also on tidal sandbanks ^[2]. This explains its abundance in Belgian waters ^{[14, 23]}. Because this clam digs quickly, the species can settle on dynamic spots with strong tidal currents and waves ^{[24, 25]}. It is also a fast growing species – up to 14 centimetres in two years ^[24] – and can reproduce throughout the entire year ^{[6, 26]}. The absence of its natural predators (e.g. the common northern moon snail Polinices heros and the milky nemertean Cerebratulus lacteus) in Europe further contributes to its successful spread ^{[16, 27]}. Nonetheless, the Atlantic jackknife clam has become part of the diet of some bird species in our region ^{[6, 28, 29]}.

The Atlantic jackknife clam digs vertically into sandy and muddy soils. This means that the species is not found on rocky shores. However, rocky shores do not form a barrier for its dispersal because of the larval stages. These stages are planktonic, which means that the larvae can float freely in the water and get carried away by the prevailing sea currents. The planktonic stage lasts about two to four weeks, and during this time the larvae may have moved tens of kilometres away from their original area, beyond the rocky barriers ^[12].

According to habitat models, the ideal depth for E. leei is between 0-67 m and the species thrives at a minimum water surface temperature of 3-18°C and a maximum of 20-26°C ^[30].

Almost anywhere where the Atlantic jackknife clam settles, the population grows so fast that in a few years their numbers are ten to a hundred times higher than the native species. Although no direct negative effect of E. leei has been demonstrated on the indigenous species (the local sword razor E. magnus, the sword razor E. ensis and the minor jackknife clam E. minor), the three indigenous species have declined in many places in Western Europe since the arrival of this American species ^[12]. Officially, no extinctions have yet been recorded, but it seems that E. minor and E. magnus are completely outcompeted along the Belgian coast ^[16]. In addition, some other species of bivalves, e.g. Spisula subtruncata, Mactra stultorum and Cerastoderma edule, have decreased in numbers ^[7]. High densities of the Atlantic jackknife clam can change the original habitat ^[31]. Furthermore, the increased accumulation of sediment can affect the polychaetes. For example, a shift from Lanice conchilega to Owenia fusiformis was observed in our waters ^{[7, 32]}.

The huge numbers of Atlantic jackknife clams found in recent years are a potential source of food for birds and fish. At sea, birds like the common eider and the common scoter can hunt for the clams. But because it is not an easy task to loosen the clams – which anchor themselves in the seabed with their ‘foot’ – it is mainly the shells that came loose from the bottom that are eaten. Eurasian oystercatchers and seagulls are often seen eating the washed-up clams after a storm ^[6]. Plaice, dab, sole ^{[7, 8]} and Atlantic cod ^[16] also feed on this bivalve.

During fishing, large clusters of empty clamshells may end up in the fishing nets. The sharp edges of the shells can severely damage the nets ^[33]. However, in some regions they are actively fished as they are highly appreciated for their culinary qualities ^{[12, 34]}. In Europe, this is the case with the sword razor E. ensis, which in many places led to deterioration of this species. Especially in southern Europe, many clams are eaten. In the Netherlands, the Atlantic jackknife clam is fished but the shells are mainly exported to southern Europe. Since clams are found deep in the soil and are difficult to catch with traditional fishing gear, special gear has been developed that can penetrate up to 30 centimetres into the seabed, so the animals can be caught undamaged ^[6]. The clams are also offered in Belgian (specialized) fish shops, but these are mainly imported from the Netherlands ^[35]. Fishing for these clams is currently prohibited in Belgium. It is expected that the impact of such fisheries on the ecosystem would be minimal if small-scale fishing would happen in Belgium, such as in the Netherlands ^[7].

In Europe, the Atlantic jackknife clam can grow 12-17.5 centimetres long ^[12], while the clam in its original distribution range (North America) grows up to 25 centimetres. There is usually a strong to moderate curvature in both shelf halves and the shell has openings at both ends. The average length-to-width ratio of the shells is 6.2.

The local sword razor E. magnus and the sword razor E. ensis – the two native clams – have a greater average length-to-width ratio of 7.4 and 8.0, respectively, and are generally slimmer than their American cousin. A distinctive feature that is only observed in the Atlantic jackknife clam is the clear curvature in the pallial sinus (see figure 2). Individuals older than 5 years are rarely seen in the North Sea ^[7].

Figure 2: Inside of the left valve of the Atlantic jackknife clam Ensis leei with the muscular indentations of the front (red) and back (green) muscle, the mantle cavity (light blue) and pallial sinus (dark blue) ^[12].

The Atlantic jackknife clam usually lives in sandy and muddy soils. The species is vertically dug into the bottom, with only the posterior end and the two siphon openings visible. When in danger, they quickly retract into the sediment by means of their well-developed foot ^[12]. During storms and strong onshore winds, the shell banks may let loose and huge numbers of dead clams are scattered in large packages all across the beach.

Like many other bivalves, this species mainly feeds on plankton ^[12], which is filtered from the water with its gills. This way of feeding is also called filter-feeding.

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VLIZ Alien Species Consortium (2020). Ensis leei – Atlantic jackknife clam. Non-indigenous species in the Belgian part of the North Sea and adjacent estuaries anno 2020. Flanders Marine Institute (VLIZ). 8 pp.