A − Exemplary overview of a cross-section under fluorescent (top) and reflective (down) light (Treatment 200/12). In situ fluorescence markings are visible and congruent with the growth line. Identification of growth lines without in situ fluorescence marking is challenging due to disturbance lines (Treatment 200/12). B − Cross-section along the line of strongest growth viewed under fluorescent lights (B1) and reflected light (B2). In situ fluorescent marking is visible as a green line under fluorescent lights. C − Mean maximum brightness of in situ fluorescent markings in the cross-sections of O. edulis shells. Staining was performed for nine treatments using three different calcein concentrations (100 mg l⁻¹, 150 mg l⁻¹, 200 mg l⁻¹) and three immersion times (6 h, 12 h, 24 h). The higher the brightness the higher the readability and the applicability for measurements. Markings were hard to read at a brightness ≤ 45 (lower dotted line), good to read at a brightness of 45–75, and very good to read at ≥75 (upper dotted line). The mean maximum brightness showed no significant difference between the treatments. Abbreviation: DOG, Direction of growth.