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December 07, 2016
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Hidden Treasures of the Delaware

This beautiful green-rayed specimen of an eastern elliptio mussel exhibits a color variation found in some juveniles from the Delaware and Neversink Rivers. Eastern elliptios make up the greatest animal biomass in the Delaware, and are the only mussel species here with consensus on their conservation status being “Secure.”
Photo by Erik Silldorff, DRBC

Mussels filter water. They do this by ingesting suspended particulate matter through their incurrent siphon, and running it through internal gills that strain out food particles. They then expel the newly-cleansed, filtered water back to the river through their excurrent siphon. Mussels subsist on a mixed diet of algae, detritus (decaying plant and animal matter) and microbes that are digested and assimilated into their body tissue. Undigested materials are excreted as bio-deposits—organic-rich materials full of nutrients—that enter the river’s food chain to become incorporated into the bodies of aquatic insects, or swim back out into the ocean in the biomass of out-migrating fish. (These include immense numbers of young-of-year American shad, or the still-significant numbers of mature American eels that manage to avoid the hazards of predators and eel weirs on their end-of-life journey back to the Sargasso Sea in the North Atlantic, where they will breed and begin a new generation.) Biogeochemical cycles that have occurred for thousands of years are still relatively intact in the free-flowing Delaware, maintaining an age-old balance through biomass interchange with the ocean that benefits both systems. Freshwater mussels are central to sustaining this balance.

Freshwater mussels, like many bivalves, are regarded as “ecosystem engineers” for their ability to modify habitat complexity and improve water quality. Their bio-deposits enrich sediments with organic material and biochemical compounds, enabling enhanced river-bottom algal growth and greater food resources for aquatic insects, fish, and other fauna.

Mussels also help stabilize substrates and stream channels and reduce streambed transport of sediments during high-flow events. They are important links in aquatic food webs as well, feeding on microscopic matter at the base of the food chain, then themselves being eaten by secondary consumers.

The sheer numbers of freshwater mussels in the Delaware River magnify their positive influence here. Though little noticed, freshwater mussels make up the greatest animal biomass in the river. Quantitative sampling done here by scientists from the U.S. Geological Survey’s Northern Appalachian Research Lab in 2002, and based on nearly 16,000 random sample plots, documented an average of 76 mussels per square meter of riverbed. Some sections of the river had over 634 mussels per square meter of substrate.