Marine bivalve mollusc. Shell up to 260 mm long, 110 mm wide and 90 mm deep. More elongated and curved than Mytilus edulis. Shell colour of juveniles bright green, wild adults often dark purple to black; farmed adults green to yellow-brown, often with brown stripes radiating from the umbo or brown spots. Inside of shell milky white and slightly iridescent. All specimens possess distinctive green lip along inside margin of shell.