Clamming Up: the Wide World of Mollusks
Thursday, December 28th, 2006by Andy McAllister, Watershed Coordinator
Limpets, pond snails, sheepnose, heelsplitters, pocketbooks, pistolgrips: the names are as numerous and diverse as the mollusks themselves. The scientific divisions within the Phylum Mollusca are numerous as well.
In Pennsylvania, the most common mollusks are Gastropods (snails) and Bivalves (clams and mussels). The mussel fauna of the family Unionidae in particular, can be found in portions of the Ohio and Susquehanna River drainage basins in Western PA. Freshwater gastropods (snails and limpets), on the other hand, are found practically everywhere.
The feeding habits of the two types of mollusk vary as well. Clams and mussels burrow in the stream bottom and position themselves to pull in water through tubes called siphons and filter out food particles, whereas most snails scrape algae off of rocks and logs with a sort of rasping tongue.
As sedentary filter feeders, clams and mussels are particularly susceptible to being smothered by sedimentation. Mussel populations in Virginia, Tennessee, Kentucky, and other areas of the country with a historically rich mollusk fauna, have declined significantly due to sedimentation from mining sites, such as coal fines and AMD precipitate.
Acidity from mine drainage is another factor that can affect mollusks in a stream. Low pH from acid mine drainage has been shown to reduce the viability of glochidia, the tiny larval forms of mollusks, which in turn, affects the re-population rate (also known as recruitment).
Not only do our native mollusks have to contend with various types of pollution in their habitat, but recently they’ve also had to compete with invaders from other waters. Zebra mussels and Asiatic clams are two mollusks that have invaded the freshwater of the United States. Asiatic clams are small, round clams that compete with native fingernail clams for food and available habitat. The rapidly reproducing Zebra mussel has become the bane of water intake structures throughout the Ohio River and Mississippi River basins, clogging these structures as they attach by the thousands in search of a good spot to filter out their food. Zebra mussels are so abundant and so efficient at filtering food particles from water, that in some areas, they leave water crystal clear with almost no food particles left for the native mussels.
