A beautiful symbiosis: Eels and mussels

Allow me to elaborate on something I report in my Sun column today: The significance of a project underway in Maryland to move hundreds of thousands of eels upstream of the Conowingo hyrdoelectric dam on the Susquehanna River.

If Democrats and Republicans could work as well together as Anguilla rostrata and Elliptio complanata do, we’d probably have a better country. They don’t have to like each other much — Anguilla and Elliptio aren’t especially fond of each other, either — but if the leaders of the two major parties could at least strive toward the kind of symbiosis found in nature, we might see some progress on all sorts of issues confronting the nation.

Consider what goes on between the American eel (Anguilla rostrata) and a certain Eastern fresh-water mussel, Elliptio complanata.

When it’s feeling particularly reproductive, Elliptio spreads its larvae, called glochidia, into the water. Fish swim through the glochidia, and they end up carrying it with them as they migrate.

Of all the fish in the rivers, the American eel seems to be the one that gets the most glochidia. In fact, it’s believed that the American eel is the primary host of the hitchhiking E. complanata.

The eels swim here and there and they spread the glochidia, and pretty soon more mussels are growing up and thriving in various river beds, where they can live for decades.

My column in The Baltimore Sun today explains how Maryland agencies are trapping and stocking eels upstream of Conowingo Dam on the Susquehanna River.

“As eels mature in the locations where they are stocked,” the Department of Natural Resources explains, “then they have a better chance of glochidia attaching to their gills.  Data has shown that eels transported and stocked upriver have traveled significant distances both up and down the river and even into other tributaries, significantly increasing not only the opportunity for glochidia to attach to their gills but to spread those mussels throughout the watershed. Before the trap and transport program, the mussel populations were old (and large), and without eels were not producing new juveniles. Now they are showing signs of reproduction. 

“A federal study in 2018 showed how the size structure of Eastern elliptio mussel populations was positively affected after stocking American eels in several streams. Additional, unpublished data has shown a continued trend of increasing numbers of small mussels where populations were dominated by very large individuals prior to eel stocking.”

And here’s why this is important: Various scientists have looked at the filtering power of Elliptio and found that they’re really good at removing suspended solids and nutrients, and controlling algal growth. They play a vital part in making rivers healthy. An adult mussel can filter several gallons of water per day.

Several years ago William Lellis of the U.S. Geological Survey estimated that the Delaware River had about 2 million mussels per mile, and that translated to the natural filtering of between 2 billion and 4 billion gallons of water per day.

So the Elliptio clean up rivers.

Therefore, the further Rostrata can travel while hosting the glochidia, the better. That would mean more miles of river being filtered and cleaned, and if you clean the rivers that feed the Chesapeake — the Susquehanna or the Patapsco, to name just two — you’ll have a cleaner bay.