Optimum conditions using manganese as a shell marker for abalone age validation studies

Abstract

Blacklip abalone, Haliotis rubra (L.), were immersed for 48 to 144 hours in buckets containing sea water to which MnCl₂.4H₂O was added at concentrations ranging from 9.5 to 1890 mg L^-1, in order to determine the optimum concentrations for marking. Manganese concentrations above 945 mg L^-1 over the immersion period were lethal to abalone. Cathodoluminescence microscopy was used to detect manganese mineralized within shell layers in situ. Although the average length and width of manganese carbonate marks at the outer growing margin of the abalone shell increased with higher concentrations, marks in the shell area under the spire appeared to be maximal at intermediate concentrations. Better marks were found at the growing edge than under the spire, although this appeared to depend on the seasonal timing of marking. Manganese concentrations of 378 mg L^-1 or less in sea water for 48 h were not lethal to abalone after 10 days, but the scores of marks indicated a possible sub-lethal toxic effect of 144 h immersion at 272 mg L^-1. In a field trial we obtained 93% survival after 14 days of abalone exposed to 200 mg L^-1 of manganese for 48 h and of these 60% showed marks in the spire region, where age is estimated, so this technique can be used in age validation studies. However, the effectiveness of manganese as a time-marker for increment analysis appears to depend on the growth of abalone when exposed to the marker and the ambient levels of manganese in the natural habitat.

Keywords:

• manganese
• time-marker
• abalone
• shell
• cathodoluminescence
• age validation