A comparison of fluorochromes for marking abalone shells

Abstract

This study assessed the potential of five fluorochromes in marking shells of the abalone Haliotis rubra, using an immersion technique. Such marks are required to 'time stamp' the shells and thus determine whether shell layers are deposited regularly enough to be used to age abalone. The stains used were: oxytetracycline and tetracycline at 300-1000 mg L^-1; calcein at 10-120 mg L^-1; alizarin red S at 10-60 mg L^-1; and xylenol orange at 20-100 mg L^-1. Immersion times were 12, 24 and 48 h. Mortality rates were low in all treatments, and clearly discernible marks were produced when abalone were immersed for 24 and 48 h at high concentrations in all the stains. Three problems were encountered when tetracyclines were employed: (i) the solutions were acidic, so the pH had to be adjusted with NaOH to prevent mortality; (ii) there was excessive foaming of the solutions; and (iii) a natural fluorescence in the shells closely resembled that of the tetracyclines. Problems also arose in assessing alizarin red and xylenol orange because they have long emission wavelengths, so that simultaneously viewing natural layers on the sections is difficult. Calcein, although expensive, was the most effective, as at high concentrations it consistently produced bright, extensive marks. The success of marking appeared to depend on the growth rate of the abalone, as feeding before staining increased the intensity of marks, and marking varied between batches of abalone collected at different times.