Metal levels in tissue granules of the freshwater bivalve Hyridella depressa (Unionida) for biomonitoring: the importance of cryopreparation

The freshwater bivalve Hyridella depressa accumulates metals in extracellular granules distributed through its tissues and the elemental content of these granules was examined by X-ray microanalysis. The influence of two specimen-preparation techniques, chemical fixation and freeze-substitution, on the elemental data generated by granule microanalysis was compared to assess their application for environmental monitoring. Chemical fixation and wet sectioning resulted in lower proportions of Ca and Mn, and higher proportions of Fe, P, Cu, Zn and Pb compared with freeze-substitution and dry sectioning. Chemical fixation and wet sectioning also resulted in an increase in among-mussel variability. Wet sectioning of freeze-substituted tissue yielded similar results to chemical fixation and wet sectioning, suggesting that much of the dissolution and redistribution of elements associated with the chemical fixation procedure was due to aqueous processing. Granule microanalysis is a new approach to monitoring metal levels in the environment. Comparison of the data obtained by the two preparatory methods by analysis of variance and multi-dimensional scaling revealed the necessity of cryopreparation for maintaining elemental distribution and concentration. Use of the granules as biomonitors may be particularly useful when used with the mantle biopsy method for non-destructive sampling of mussel populations.