Dann et al. (2020) conclusively distinguished, using mitochondrial data, the closely related Mya japonica from M. arenaria. Furthermore, they showed that the former is established in south-eastern Tasmania where it was believed to have been introduced at least a decade prior to that study (Grove et al., 2018).

While Dann et al. (2020) recently confirmed the presence of the closely related Mya japonica in the southern hemisphere, including Australia (southeast Tasmania), Mya arenaria is not known to occur in Australia.

Summary Description

Mya arenaria is a bivalve, more commonly known as soft-shell clam. The thin fragile valves of M. arenaria are roughened, grey or chalky white and covered with concentric rings and a brown periostracum along the hinge ligament. The brittle shell can grow up to 75-150 mm in size (Abraham and Dillon, 1986; Council PWSRCA, 2004; Database GIS, 2005).

The interior left valve has a large, spoon-shaped internal tooth (the chondrophore) which distinguishes this species from many Pacific bivalves. The extremely long siphons that filter water are fused and too large to be completely withdrawn within the shell, and as a result the clam can be submerged 20-35 cm below the surface and live anaerobically for several days (Cowles, 2007; Abraham and Dillon, 1986).

Its native range encompasses eastern North America, from New Brunswick to South Carolina and eastern Canada. The soft-shell clam has been introduced to many countries, including; England, Spain, Denmark, Estonia, Finland, Germany, Iceland, Latvia, Lithuania, Poland, Sweden and Norway (including Svalbard and Jan Mayen), as well as the Mediterranean (Olenin and Leppakoski, 1999; Abele et al., 2002; Database GIS, 2005; Repetto et al., 2005; Crocetta and Turolla, 2011). In addition, this species has also been reported in western North America from Alaska down to California (Database GIS 2005).

Mya japonica has been reported from Tasmania (see Grove et al., 2018; Dann et al., 2020). It is almost identical to Mya arenaria in appearance, and is best distinguished using molecular techniques. However, Zhang et al. (2018) distinguishes M. arenaria from M. japonica based on the following subtle morphological differences: M. arenaria possesses a generally more elongated shell, with a longer and thinner posterior end and regular thin commarginal growth lines compared to M. japonica. Conversely, they noted that M. japonica is ‘higher, with rough commarginal wrinkles and a relatively shorter and more rounded posterior end. In addition, the pallial line of M. arenaria was less impressed than in M. japonica’. Nevertheless, they noted considerable variation in the shape, relative size and orientation of the left valve chondrophore based on their observations. Consequently, we here advise for conclusive identification to be based on molecular barcoding, and for these morphological characters to serve as supplementary identification tools.