The Mediterranean Mussel is widespread on the coasts of the Mediterranean and Black seas and it popular in many countries. There is an opinion that the colouring of the mussel shell can change dependent upon the conditions of the marine environment in which it resides.
This question generated the interest of biologists from the Lomonosov MSU. They endeavoured to clarify the nature of the shell colouring and to this end they gathered mussels from the Black Sea near Anapa and the Bolshoi Utrish Cape, from various depths. In all the scientists studied 115 molluscs and they discovered three variations of colouring. In one sort of mussel the colour of the chalky layer of the shell is almost white and the radial stripes are not visible. In others the chalky layer is deep purple in colour. There are also shells with alternating purple and white stripes. The scientists believe that the colour of the deep mineral layers of the shell is determined genetically. There could be a different intensity of synthesis of the organic pigment that has impregnated the chalky layer and which determines the colour of the entire shell. The “stripy” shells arise because the density of the chalky layer varies; and it varies in all molluscs, only these stripes are not evident to the eye in the white and purple varieties. And if the organic pigment is present in an average quantity, the shell gains a contrast and becomes “stripy”.