Mussels and Oysters don't actually feed on sh1t, they eat plankton. Plankton feed on sh1t, or the nutrients therein. However, shellfish certainly SWALLOW a lot of sh1t, as they filter gallons of water every day. Some of the bacteria and viruses and other contaminants lodge in the shellfish. I believe an oyster virus did for Michael Winner, so it's not all bad. Red tides are unusually high concentrations of particular phyto-plankton. Read this, scroll down to the Discovery
section - horrible!
Commercial shellfish farms are tested for red tide contaminants and other pollutants. The fish are also all quite young and this must help. When the fish are harvested, they undergo purification to reduce the levels of bacteria etc in them.
There is NO WAY I'd touch a mussel or oyster from an industrialised or over populated area (i.e. England)