If you live on a river delta or coastline, you may not be able to eat the local shellfish because of high levels of toxins present in the water. why? the toxins contained in the shellfish can't be broken down by your stomach. shellfish regularly consume poisonous red tide organisms. clams seasonally produce toxic substances. most harmless native clams have been replaced by bad species. clams filter-feed and concentrate the dilute pollutants from the water, thus acting as early warning indicators of severe pollution.