They are mostly farmed these days owing to the demand for them, which started in the 1970s in North America and has increased since.
Small: 1.25″ (3.2 cm)
25-34 clams per pound or 57-66 per kg
Medium: 1.5″ to 1.75″ (3.8 to 4.5 cm)
18-24 clams per pound or 39-48 per kg
Large: 1.75″ to 2.25″ (4.5 to 5.7 cm)
10-14 clams per pound or 22-30 per kg
Native to South-East Asia. Brought to North America sometime around the late 1940s by mistake; was in with other clams being imported for seeding.