Rare earth elements and yttrium in suspension-feeding bivalves (dog cockle, Glycymeris glycymeris L.): accumulation, vital effects and pollution

Rare earth element (REE) and Y (REY) concentrations were determined in soft tissues and shells prepared from 9 to 93 years old dog cockles sampled alive from a marine site in NW Brittany, France. The concentrations shown by the different soft tissues and shells are extremely variable ranging from 9.10-5 to 10-1 x PAAS. The mantle is the organ with the highest concentrations, while the feet, the adductor muscles and the digestive glands display the lowest ones. Relative to the masses of the different organs, we show that the shell concentrates more than 85% of the REY contained in an animal, and the mantle, which represents less than 0.5% of the total animal mass (on a dry basis), contains 10% of the total REY. Organs accumulate REY throughout the life of the animal. REY concentrations in soft tissues increase by more than one order of magnitude in 90 years. The shells mirror the REY abundances of the mantle but are slightly less light-REE enriched than the latter. Furthermore, shells as well as feet and adductor muscles show a striking correlation between animal age and Ce/Ce* ratios. Ce/Ce* ratios evolve from 0.8 for animals aged ∼10 years to 1 for the oldest, with most of the variation during the first 25 years, indicating that Ce also accompanies biochemical reactions probably related to the growth or maturity of the animals. Soft tissues also show extremely variable positive Gd anomalies (Gd/Gd*= 1.04 - 1.94) which indicate environmental pollution by Gd based contrast agents (GBCA) used in medical imaging. Anomalies tend to be more important for organs poor in REY (like muscles and digestive glands). Low excesses of Gd, on the order of 1-3 ng/g (on a dry basis), account for these anomalies. There is no correlation between the age of the animals and the amount of anthropogenic Gd or Gd/Gd* ratios suggesting that GBCAs do not accumulate in the bivalve body on a long-term basis.