The shell is roundly triangular in shape strong and heavy, with radiating ridges and varies in colour from greyish white through yellow to buff brown. The shell also has distinctive black and white markings. The interior surface of the shell is smooth with a deep purple band. It can grow up to 6 cm in length. The tip of the siphon is split. The body tissue of live specimens, especially the foot, are orange.Manila clams are indigenous to Japan and are widely used in commercial fisheries. It can tolerate water temperatures ranging from 0 to 23 °C. The Manila clam is superficially similar in shape and size to the paloured clam Tapes decussatus, however the flesh of live Manila clams, especially the foot is orange whilst in the paloured clam, the foot is white. The shell of the Manila clam also has distinctive black and white markings. When observed feeding underwater, the siphon of the Manila clam is joined but separate in the paloured clam.
Biology/Natural History: The siphon of this clam is not long (even though it has a well-developed pallial notch) and the clam buries only to 10 cm depth. This clam is introduced from Asia. It was first accidentally introduced into British Columbia in 1936 along with Pacific Oyster spat. It has now become well established and is one of the main species of clam harvested along the coast of Washington and British Columbia. Moon snails Polinices lewisii don't often attack this species because the species lives so high in the intertidal. Breeds in the summer. May contain pea crab symbionts Pinnixia faba or P. littoralis. Tolarates salinity as low as 1/3 seawater. May live 14 years. May contain red tide toxins.
As with all members of Family Veneridae, this species has no winglike extensions at the umbo, the two shells are of similar shape and the anterior and posterior ends are not highly differentiated from one another. Both valves have three well-developed cardinal teeth (photo). They have two adductor muscle scars of similar size. The umbones are near the middle of the shell. In this species the shells are oval (longer than high) and it has concentric ridges and strong radial ribs which are strongest at the posterior end. The inner ventral margin of the shell is not serrated. Color variable gray or brown, often with streaks (photo) or angular patterns. Frequently there is a purple (photo) or yellow color (photo) inside the shell. Shell has a pallial sinus (photo). The siphons are short, the black tips are separated at the end (photo), and they can be withdrawn entirely within the shell. Length up to 7.5 cm.
The overall shape is ovate. The valves are the same size and shape (equivalve), and there is no gape when they are closed. Ornamentation consists of growth lines, comarginal and radial ribs. The radial ribs are more prominent than the comarginal ribs. All ribs become stronger anteriorly and posteriorly. The central area of the shell is smoother, but the comarginal ribs become slightly stronger as the animal grows. The lunule is elongate, varies in width and can extend up to 2/3 of the anterior dorsal margin. The umbo points slightly anteriorly. The escutcheon is not well defined, but can be elongate and smoother than the overall shell; it also may be more prominent on the left valve.
The exterior coloration is highly variable and not indicative of the species (Shaw 1956). Generally, the exterior color is a light tan or brown with a dark brown lunule. There may be radial or comarginal brown stripes, or brown mottling. The interior color is white and there may be purple coloration around the margin. The purple is stronger on the posterior margin, under the ligament and in the pallial sinus.
The ligament is exterior and supported by nymphal ridges. There are three cardinal teeth on each valve. On the left valve, the central tooth is bifid, and the anterior and posterior teeth are not bifid. On the right valve, the posterior and central teeth are bifid, and the anterior tooth is not bifid. There are no lateral teeth. The anterior adductor muscle scar is slightly smaller than the posterior adductor muscle scar. The pallial sinus is rounded and extends to approximately the end of the ligament in length.
How to Distinguish from Similar Species: Butter clams Saxidomus gigantea have only concentric sculpture. Native littleneck clams Protothaca staminea are as high as they are long, their ridges are less prominent, their siphons are fused all the way to the tips, and the inner ventral margin of the shell is finely serrated. These other species do not have purple markings inside the shell. Another introduced species, Nuttallia obscurata, has purple markings inside the shell but the light brown periostracum is smooth and shiny.
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