[myriam (OP)]

what makes some kind of bivalves relatively resistant  to some forms of  marine pollutants

[Don_1]

In the bad old days when raw sewage was discharged into the sea, Mussels were to be found in great numbers around the pipes. Since the Mussel is a filter feeder, this would not just be case of capable of surviving the polluted water, but a distinct advantage. Bacterioplankton would have been in their ideal position for feeding on the organic waste and they, in turn, would be fed on by the Mussels.

[Oceans Helen]

yuck! That's why I steer clear of eating mussels that have grown near sewage outlets!

But other than feasting on nutrient goodies in sewage effluent, mussels can resist other marine pollutants, essentially by spitting them out using something called a Multi-Drug Resistance pump, or MDR pump. These are proteins imbedded in some cells walls that act as an inbuilt self-defense mechanism.

These were first discovered in cancer cells, but have been found in mussels, which explains why they can tolerate some pollutants. And it also means we may have been underestimating how polluted the seas are: mussels have been used as bio-indicators for gauging levels of pollution. Since they filter sea water it was assumed you could measure the levels of toxins in the water by measuring the amount absorbed into mussel tissues. But if they are selectively getting rid of some nasties using their MDR pumps, then the picture isn't nearly as straight forward.