RESUMO
It has long been known that a high degree of "unexplained" residual variability can occur in the concentrations of some elements in some biological tissues. Until now, no reasons have been found for the presence of such high levels of variability. The present study describes a factor which can adequately explain this phenomenon. It was found that elements which are stored primarily in an insoluble form showed much higher degrees of variability than those stored in a soluble form. Elements found primarily in an insoluble form are often isolated from cellular metabolism including any regulatory processes and may build up to high levels in some individuals. The groups of elements showing the highest levels of residual variability were the heavy metals, lanthanides and actinides. These groups tended to be stored primarily in an insoluble form as determined by subcellular fractionation. The groups of elements showing the lowest levels of residual variability were the alkali metals and non-metals which were found to occur primarily in a soluble form in mussel tissue. The elements of the kidney generally had higher levels of residual variability than those of any other organ probably because of the kidney's ability to store high concentrations of elements in insoluble granules. A study of the behavior of aluminum in the digestive gland suggests that elements associated with insoluble granules of sediment passing through the gut could contribute to the residual variability of these elements. The highest levels of residual variability were observed for zinc, silver and lead in the kidney while the lowest for rubidium in the gills.(ABSTRACT TRUNCATED AT 250 WORDS)