| Summary: | peer reviewed New results confirm earlier findings on the importance of tiemannite acumulation on speciation and inter-tissue relationships. It is hardly surprising that, as opposed to what was concluded earlier, major inter-regional differences in Hg accumulation can be demonstrated when comparing several populations of harbour porpoise on an age corrected basis. Important inter-species differences, probably depending on prey choice, are found, even after correction for "relative age". Regional differences are more important than the inter-species variability, at least within both classically described sub-orders of cetaceans. Based on actual estimates of MeHg exposure and tissue concentrations of Hg and MeHg in harbour porpoise from the Southern North Sea, Hg detoxification through precipitation of tiemannite is evaluated to neutralise on average 12% of the overall MeHg intake at age 7. Based on the idea of a 3 fold excess in toxicity of mercury over selenium and a 5 to 15 molar concentration excess of selenium over Hg in fish, Hg seems to play the major part in the mutual detoxification. The formation of tiemannite was previously described as resulting from a two-step accumulation mechanism appearing at a threshold level of Hg 100 g/g fw in liver. The reaching of an equimolar Hg to Se ratio can, however, be fully explained by the gradual increase of tiemannite levels in liver only. The new view is that molar Hg to Se hepatic ratios go towards equimolarity along with the slow nature of the tiemannite detoxification process, with tiemannite gradually taking the upper hand over MeHg, IHg2+ and a surplus of "free" selenium with the increase of the total Hg load.
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