When last week’s rains whipped the Potomac into a mud-colored torrent, my thoughts went to a certain spot on the river that’s home to some very special creatures.
The spot is a little channel between two islands, halfway between Riley’s and Pennyfield locks. The creatures are sponges.
A week ago, when the river was low, I could see them in the shallow water, safely fastened to the bottom. How were they managing now? I imagined the powerful current scouring the bottom with blasts of sand and pebbles.
These sponges are special to me because I discovered them. Not in the sense that they’re new to science, but rather new to the Potomac River. At least it appears so.
They’re also new to most people, it seems. Hardly anyone even thinks that there might be such a thing as freshwater sponges. It was the same with me, even after decades of poking around rivers and streams.
In fact, I became aware of freshwater sponges for the first time this past August, not on the river, but in an exhibit hall where I was attending a symposium on stream conservation.
As I paused before a poster, a young woman named Tonya Watts began telling me about her sponge research project in Virginia. She was from the U.S. Park Service’s Center for Urban Ecology, based on MacArthur Boulevard. I listened politely.
Then she said something that caught my attention. The last reports of sponges in the Potomac River area were from the early 1970s, she said. They came from several tributaries to the Potomac, but not actually in the river itself.
“Tell us if you find any,” she said.
A couple of days later, I was paddling at the head of Watkins Island. Through the clear, shallow water, I saw a wonderful still life of golden pebbles and opalescent shells shimmering in the light. Small bass and sunfish poked out from behind flowing locks of stargrass as they undulated in the gentle current.
Then I saw it. For lack of a more uplifting image, it looked like a small, greenish cow pie. I spotted more and more.
I picked one up, along with the pebbles and shells on which it was fastened. It felt firm, yet crumbly; a little like rusty #4 steel wool. It smelled fishy, like the river. I got it to pose for photos and then put it back.
Once off the river I emailed Tonya with some photos. “This is really exciting,” she shot back. We set a date to visit the site, and then another, each time foiled by a hurricane or tropical storm.
But she did tell me some things about these creatures, to which I added a little research of my own.
No matter how much they look like plants, sponges are animals. They feed on microscopic organisms, which they filter out in cavities in their bodies. In turn, sponges are fed upon by fish, crayfish, waterfowl, and particularly by certain kinds of aquatic insects.
Their “skeletons” are composed of tiny structures made from silica. The size and shape of these “spicules” (see photo) is the surest way to determine a sponge’s species.
My sponges were greenish. This color comes from algae that lives in their bodies, where they produce byproducts that sponges use as nutrients.
When I looked closely at a specimen I could see masses of tiny spheres, smaller than poppy seeds. These are gemmules that overwinter and produce new sponges. They are tough. Like seeds, they can be stored for years and still remain viable.
We still don’t know what kind of sponge this is. After the river levels drop, Tonya and I will get to the site. Hopefully the sponges will have held their ground against the torrent. Then we will take a sample to an expert to determine its species.
Finding its name will answer the easy question. But how about the harder ones?
For one thing, sponges are sensitive to pollution. Is it just coincidental that the last sightings in the Potomac Basin were in the 1970s, when the quality of the river was reaching its lowest point? Are the seemingly healthy populations I saw an indication of how much the river has improved?
Another question: Just how widespread are these sponge populations? I found a lot, but only in a few spots in one stretch of the river. When I checked out other places of similar depth, bottom composition and current speed, I came up spongeless.
Getting answers will take some real work and maybe even a graduate student or two. It will also take readers like you.
What interesting discoveries have you made in the Potomac River? Any sponges?