Fig. 2
Acetate peel replicas and surface growth rings in the shell of Dosinia exoleta: a Acetate peel of sectioned valve illustrating the sequential deposition of growth marks during ontogeny (from right to left). Annual growth rings are associated with one (single arrow) or two (double arrows) clefts on the shell surface. b Annual growth mark with the formation of one cleft (arrow), being possible to observe a slow-growth band where microgrowth increments become narrow. c False growth ring (arrow) without narrowing of microgrowth increments. d Comparison between growth rings observed on the shell surface and growth marks revealed by the acetate peel replica of the same shell (a). PL outer prismatic layer, MN middle nacreous layer, IN inner nacreous layer, P periostracum, U umbo