Metal bioaccumulation and storage forms in the shrimp, Rimicaris exoculata, from the Rainbow hydrothermal field (Mid-Atlantic Ridge); preliminary approach to the fluid-organism relationship

At the Rainbow hydrothermal site the concentrations of metals in the fluid are the highest observed in the Mid-Atlantic Ridge hydrothermal areas, and the shrimps Rimicaris exoculata were observed forming dense swarms in small depressions on the chimney walls. In order to understand the fluid-organism relationship, we investigated the relative abundance of metals in the water surrounding R. exoculata and their bioaccumulation by the shrimps. This first approach stresses a net metal enrichment around R. exoculata. Precipitation, scavenging and oxidative redissolution processes in the mixing zone are controlling the concentrations of metals. Their bioaccumulation varies according to their bioavailability in the surrounding water. Metals are abundant as insoluble forms in the tissues but the hypothesis of external deposits on the gills and the cuticle of the abdomen cannot be rejected. The regular lysosomal breakdown of metallothioneins may participate to the relative abundance of these insoluble forms. Improvements concerning the preparation of the biological material and the use of a complementary histological technique are needed to answer the questions emerging from our work. The study of the distribution of metals between dissolved and particulate fractions in the water surrounding the shrimps has to be performed, in order to provide data on the bioavailability of metals.