The cytology of heavy metal accumulations in the digestive glands of three marine gastropods

Abstract

In the digestive glands of gastropod molluscs, metals are metabolized in the sense that they are subject to inorganic biochemical processes within the epithelial cells and lumen of the digestive tubules and the pore cells in the intervening connective tissue. These systems have been examined in the tower shell Cerithium vulgatum Bruguieres, a sediment feeder, the top shell Monodonta articulata Lam., a grazing herbivore, and in the whelk Murex trunculus L., a carnivore whose prey includes Cerithium . These animals were taken from a Mediterranean environment polluted by heavy metals. In all three species the metals are compartmentalized within mineralized granules as phosphates and within lysosomal residual bodies in association with sulphur. However, the extent to which a particular metal is accumulated and the relative proportions that are bound within the different compartments are factors that are primarily determined by the species and not the concentration available in the environment. Thus Cerithium accumulates high concentrations of a wide range of metals from the ingested sediment and these are rendered insoluble and non-toxic in the digestive gland. This unavailability is transferred to Murex when it eats Cerithium because Murex does not accumulate the full range of metals from its prey. Indeed, for some metals there appears to be bioreduction rather than bioamplification. The grazing herbivore Monodonta articulata accumulates a third distinct spectrum of metals. The accumulations in each species do not reflect the levels of all the metals in the environment. The presence of metals in the digestive glands is associated with the removal of magnesium from the phosphate granules but also with an increase in the concentration of magnesium in the tissue. It is proposed that metals induce the formation of magnesium phosphate as a source of metal-binding phosphate ions.