Accumulation and excretion of manganese ion in the kidney of M ytilus galloprovincialis.

T 1-weighted magnetic resonance imaging (T 1w-MRI) was employed to detect the accumulation of manganese ion (Mn2+) in urine in the kidney of the mussel Mytilus galloprovincialis, and the longitudinal relaxation rates (1/T 1=R 1) were measured. When the mussel was exposed to seawater containing 10 µmol l-1 Mn2+, the T 1w-MRI intensity and R 1 of the kidney, stomach and digestive glands were increased. Mn2+ might be taken into the hemolymph via the gastrointestinal tract, and then filtrated into the pericardium via the auricles. Although the image intensity in the pericardium was not affected by manganese, an image intensity enhancement was observed in the distal part of the renopericardial communication canals between the pericardium and the kidneys, indicating Mn2+ was concentrated in the excretion pathway. As the seawater Mn2+ concentration ([Mn2+]SW) was increased from 3 to 50 µmol l-1, R 1 of the kidney (R 1K) was elevated. When the mussels were immersed in 3-10 µmol l-1 [Mn2+]SW for 24 h, the Mn2+ concentration in the kidney ([Mn2+]K) showed a 15-fold increase compared with the ambient [Mn2+]SW In the range of [Mn2+]SW from 10 to 50 µmol l-1, R 1K reached a plateau level that corresponded to 200 µmol l-1 [Mn2+]K As [Mn2+]K fell transiently, voluntary excretion of urine from the kidney was assumed. The decreases in intensity were not synchronized between the right and left kidneys, and the closure of the shells might not be essential for urinary excretion. The voluntary excretion suggested an additional explanation for the large range in metal concentratons in the kidneys of the mussel.