SEM observation of structural (non-mineralogical) alteration inside the previously crystallized nacreous layer of Crenomytilus grayanus (Bivalvia: Mytilidae)

► Shells of mytilid bivalve Crenomytilus grayanus variously infested by boring organisms were studied. ► Structural (non-mineralogical) alteration inside the previously crystallized nacre was found. ► Alteration zones (nacre-prisms transition) are out of contact with the mantle or extrapallial fluid. ► Altered structures (aragonitic lenses of prisms) may be a part of anti-predator mechanism.