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December 09, 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

To the casual observer, these numbers may seem inconceivable, as most people only notice the shiny inner shells of dead mussels sparsely strewn across the bottom of the Delaware. But having worked alongside the dedicated USGS crew here during much of the sultry summer of 2002, I can attest to the astounding numbers of mussels found in the riverbed. The live mussels are much less visible, mostly buried in the substrate, oriented vertically, often with only the posterior end of their shells and their siphons exposed, and require an attuned eye to detect.

There are literally hundreds of millions of mussels in the Delaware River, each with the ability to filter multiple gallons of water a day. Collectively, they filter billions of gallons of water on a daily basis, greatly influencing and contributing to the superb water quality found here and on down through the entire non-tidal Delaware. The flow volume of the Delaware River is filtered many times over through the bodies of freshwater mussels on its way to the ocean.

Connectivity with the ocean is key in maintaining the Delaware’s robust mussel fauna, as well as the rest of its biodiversity and resilience. During most of the period of escalating dam-building in the last century, we weren’t aware of important tradeoffs being made. Freshwater mussels, in order to reproduce, require a fish host for their larvae to live on briefly during the early stages of their development, without harm to the fish. This fish host is often specific for each species of mussel (the Delaware has 12 of the 14 species found on the Atlantic Slope), and is usually a fish species with which they’ve co-evolved over thousands of years. When they’re de-coupled from their fish hosts, mussels can’t reproduce.

The most important fish host for eastern elliptio mussels (Elliptio complanata), which make up 98.7% of the Upper Delaware River’s mussels, is the American eel, a sea-run migratory fish that is still able to access its important historic habitat here (all the more unique to this system, when 84% of historic stream habitat on the Atlantic Coast is blocked by dams). Consequently, we have hundreds of millions of eastern elliptio mussels each filtering multiple gallons of water per day and providing other important and economically valuable ecosystem services for free, just because we’ve kept the system healthy enough to sustain the native biota of the Delaware River.