EVOLUTIONARY RESPONSE OF EARTHWORMS TO LONG-TERM METAL EXPOSURE

The occurrence of a number of soil invertebrate species in severely polluted soils raises the question whether they have evolved resistance, e.g. by adaptation or acclimatization. This was examined in experiments on the earthworm Dendrobaena octaedra (Sav.) using specimens from sites polluted by a brass industry. One site, exposed since the 17th century, was situated close to an old mill, another was near a modern mill that has been operating for the last two decades, and the third was a non-polluted reference site. Adults and juveniles of D. octaedra from each site were reared in each of the three different soils. Growth (increase of body weight), survival, and cocoon production of the populations were followed over 255 d. The presence of molecular size fractions of proteins binding Cu, Zn, and Cd (metallothioneins) and the relative composition of amino acids in worms from the polluted and non-polluted soils were analysed. Specimens obtained from the long-term exposure site and reared in soil from that site reached a larger average body size than specimens from the other populations and also lost weight more slowly. Regardless of origin, the earthworms survived equally well in the different soils. A 90% reduction in maximum reproductive output was observed in worms grown in the soil from the long-term exposure site, where individuals from the reference site laid most of their cocoons almost instantaneously. The population from the long-term exposure site exhibited a slightly extended reproductive period compared with other populations in the same soil type. The pattern of molecular size distribution of metal-binding proteins was similar for worms grown in the same soil irrespective of the population from which worms were sampled. Cu was bound to two molecular size fractions and Zn to a number of large and medium sized fractions, which partly overlapped the three fractions binding Cd. Two of the latter bound 73-89% of the Cd content. One of the Cd-binding fractions was more abundant in worms from the long-term exposure site. The relative concentration of cysteine and aspartic amino acids increased in D. octaedra reared in polluted soil. Though worms from the long-term exposure site differed in growth and reproductive pattern and in Cd-binding capability and amino acid composition, the experiments did not conclusively demonstrate that metals had exerted a significant selection pressure on D. octaedra. The two alternative explanations for the maintenance of a scattered and low-density D. octaedra population in the vicinity of the old mill are immigration of juveniles from less polluted areas, and spatial heterogeneity in metal distribution in the soil at the polluted site.