The Science of the Total Environment, Supplement 1993 Elsevier Science Publishers B.V., Amsterdam 1283 Mytilus galloprovincialis as a bioindicator of lead pollution: biological variables and cellular responses Francesco Regoli and Enzo Orlando Dipartimento di Biornedicina Sperimentale Infettiva e Pubblica, Unioersith di Pisa, Via A. Volta 4, 1-56100 Pisa, Italy ABSTRACT The mussel Mytilus galloprovincialis was used as a bioindicator species in a lead-polluted area of the North Tyrrhenian Sea. A pronounced seasonality was observed in Pb concentration within the digestive gland, whereas the total Pb content of the organ appeared less variable. This difference is mainly due to the influence of gonadal development on digestive gland weight. Control mussels transplanted for 6 months into the polluted area showed a strong increase of Pb, which reached a steady state after 4 weeks. Subcellular responses were investigated in mussels sampled from the polluted environment. A severe disturbance of the lysosomal system was demonstrated, indicating the utility of stress indices based on the responses of these organelles to pollutants. Our results are discussed in terms of their implications for biomonitoring programs. INTRODUCTION Mussels have received considerable attention in pollution studies because they are universally considered to be suitable as bioindicators of heavy metal pollution. In fact they present many features which are prerequisites for a biological indicator, including world-wide distribution, sedentary filter-feeding habit, easy sampling, reasonable size and heavy metal levels which represent time-integrated measurements of environmental pollutants (Goldberg, 1980; Phelps and Galloway, 1980; Ritz et al., 1982; Farrington et al., 1983; Roesijadi et al., 1984; Goldberg, 1986). However, there are many factors which can influence heavy metal concentrations in mussel tissues and they must be considered when comparing samples from different areas or samples collected during different periods of the year (Phillips, 1976). Some understanding of natural fluctuations in body concentrations of metals is therefore important for the assessment of environmental pollution. 1993 Elsevier Science Publishers B.V.