STRUCTURAL-CHANGES IN THE DIGESTIVE LYSOSOMAL SYSTEM OF SENTINEL MUSSELS AS BIOMARKERS OF ENVIRONMENTAL-STRESS IN MUSSEL-WATCH PROGRAMS

A field study has been carried out to validate the measure of structur al changes in the digestive lysosomal system of sentinel mussels as bi omarkers of environmental stress. Previous laboratory studies demonstr ated that the digestive lysosomal system of molluscs reponds to a vari ety of pollutants and to different stress situations by exhibiting sig nificant changes in its structure. Mussels were collected monthly over 1 year at the Abra estuary (Bizkaia, Biscay Bay) from six sites with different degrees of pollution. The changes in the structure of the di gestive lysosomes were quantified on cryostat sections of the digestiv e gland by means of automated image analysis. Four stereological param eters were recorded: lysosomal volume density, surface density, surfac e-to volume ratio and numerical density. A seasonal pattern in the str ucture of the digestive lysosomes was evidenced with reduced volume, s urface, size and numbers of lysosomes in winter-spring; increased volu me, surface, size and numbers in summer and an intermediate situation in autumn. The structure of digestive lysosomes was also dissimilar am ong sites, the most significant differences being found between Plentz ia (nonpolluted site) and Galea (polluted site). The digestive lysosom es of mussels collected from Galea were smaller and more abundant than in Plentzia's mussels in most sampling times. The basis of these diff erences are discussed to conclude that organic chemical pollution migh t be the cause for these specific changes which are different from the enlargement of digestive lysosomes described as a result of various s ources of environmental stress. It is concluded that structural change s in the digestive lysosomes of sentinel mussels are sensitive to poll ution-induced environmental stress even in the complex situation of th e field where many factors may interact to affect the structure of the digestive lysosomal system.