Species description: A solid, broadly oval shell that can reach almost 13 cm, although is more commonly smaller. Colour is usually quite dull, ranging from a dirty white through to grey brown or grey. Growth stages are conspicuous. There are concentric lines and some ridges on the outside of the shell, as well as fine radiating lines. As the animal ages these are often worn away and in older specimens the sculpturing is only still visible near the margins of the shell. 3 cardinal teeth in each valve, no lateral teeth. There is a noticeable rough area behind the beaks which has the appearance of a broken tooth. The inside of the shell is white but may sometimes have some purple marking at some of the muscle scars. Pallial sinus shallow and triangular in shape.

Habitat: Mud, amongst stones and shells, from the lower intertidal to around 8 m depth.

Distribution: A few reported from 1860-1890s. Introduced deliberately to a few experimental sites in 1960s and 1970s. Seems to now be established in southern and south western parts of Britain, with a few scattered records further north. So far only 1 record from island of Ireland on west coast. Any new records should be reported.

Similar Species: Large robust clam that is unlikely to be confused with any other species.

Key Identification Features:

• Large, solid shell reaching almost 13 cm.
• Dull colouration of dirty white to dull grey-brown.
• Shell sculpture often eroded and only visible at margins.
• 3 cardinal teeth in each valve, no laterals.
• Shallow, triangular pallial sinus.

Distribution Map from NBN: Mercenaria mercenaria at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Mercenaria mercenaria at iNaturalist World Species Observations database.

WoRMS: Mercenaria mercenaria at World Register of Marine Species. Accepted name: Mercenaria mercenaria (Linnaeus, 1758). AphiaID: 141919.

Classification: Biota; Animalia; Mollusca; Bivalvia; Autobranchia; Heteroconchia; Euheterodonta; Imparidentia; Venerida; Veneroidea; Veneridae; Venerinae; Mercenaria