For many years Mya japonica was considered to be a synonym of M. arenaria Linnaeus, 1758. While there are only minor differences in shell morphology, Zhang et al. (2018) combined the shell differences with extensive DNA analysis and spermatozoan structure to demonstrate they are in fact distinct species. As M. arenaria is a widespread, commercially important, species there is a substantial scientific literature available on the species. In contrast, the distribution of M. japonica is relatively small and as a recently recognised species, there are few data available. Because of this, the sections below rely heavily on information obtained on M. arenaria. The assumption is that because the two species are closely related their biology will be similar, but this is not always the case and needs to be tested with studies on M. japonica. There are seven extant species in the Mya genus (WoRMS 2023) but only M. arenaria and M. japonica have been reported to become invasive.

Mya arenaria has become widespread through deliberate and inadvertent human introductions, and dense populations have become established in many areas. The ecological effects of dense populations of M. arenaria have been extensively studied and are summarised by NEMESIS (2023). In contrast, M. japonica is known primarily from its native range. Two isolated individuals have been identified from British Columbia, Canada (Zhang et al. 2018) but it is not known whether populations have been established. The population of M. japonica recorded in the Prosser River in southeastern Tasmania in 2018 (Grove et al. 2018; Grove 2020; Dann et al. 2020; NRE 2023) is the first known outside the native range of the species. Dead shells of M. japonica were also found in November 2019 at One Tree Point, Louisville, a few kilometres north of the Prosser River and closer to Triabunna.  This suggests the species might first have colonised the Triabunna area and then later colonised the mouth of the Prosser River a few kilometres to the south (Dr. S. Grove, pers. comm.).  

Mya japonica has a thin, brittle, chalky-white or grey shell. The shell is oval, rounded and slightly elongate in outline and gapes at both ends. Adult length is 75-100 mm but may reach 150 mm. It has a distinguishing leathery tube that encases well developed and retractable siphons.