Since many chemical contaminants often are present in sediments, benthic fauna often are exposed to and can be harmed by these pollutants. Some benthic organisms are widely distributed. Others are limited more by salinity. For example, hard clams and oysters require higher saline waters. Brackish water clams also are found in lower salinities, along with freshwater mussels.
Salinity also determines the distribution of certain benthic predators, parasites and diseases.
MSX, a lethal parasite, and Dermo, a disease caused by another parasite, have decimated oyster populations of the mid and lower Bay, respectively. Oyster drills and starfish, which feed on oysters, are less of a problem in upper Bay waters because of their intolerance to low salinities.