Elusive Potomac River Sponge Reappears―But Just Barely

I’m floating through a weedy channel in the Potomac River, wearing a life vest, flippers and a facemask. The water is shallow, the current languid, yet I am concerned.

I examine the carpet of pebbles and shells. Sunfish and small bass eye me while keeping their distance.

I come upon a fat tire, festooned with algae. I knock on the tread to see if anyone is home. A small catfish pokes a whiskered snout over the rim, and then drifts back into its sanctuary.  

All the members of the Potomac family were there, but with one exception: sponges.

You may remember a column, written about a year ago, in which I reported the first recorded sighting of a fresh water sponges in the main stem of the Potomac River. I found them at the head of an island upstream from Pennyfield Lock.

They didn't look like much, really more plant than animal. Still, I felt a rush of excitement and a sense of kinship―admittedly very distant―with the great naturalist explorers. Like them, I was traveling the earth―in this case, Montgomery County―making discoveries that would change our ideas about the natural world.  

My first step was to learn this sponge’s name. I contacted the Smithsonian Institution’s sponge expert, Dr. Klaus Ruetzler, and he generously offered to help. The results came in: It was Dosilia radiospiculata, unusual along the eastern seaboard and notable for its tiny skeletal bodies called “spicules,” which look like crystal jumping jacks.

A series of floods and the onset of cold weather ended my explorations for that year. But I resolved that next year I would look for other colonies, perhaps enlist others to help, and begin mapping Dosilia’s distribution.

Something is different

In March, I headed for the gravel bar that had marked my sponges’ location. But it had vanished over the winter. I didn’t find a trace of sponges either, but concluded that it was still too early in the season.  

As spring turned into summer, weeds began to fill the river, practically shore to shore. The open shallows where I had previously found the sponges became clogged with plants, undulating in the gentle current.  I searched hard, but still no sponges.  

Other things had also changed. For example, I was finding large numbers of largemouth bass, which normally favor quieter, weedier places. Normally, this part of the river, with its moving water and rocky bottom, holds only smallmouth bass.

Still optimistic that something would turn up, I decided to unveil my sponge to the public. I chose an Earth Day exhibit at the Izaak Walton League chapter in Poolsville.

It didn’t go well. My exhibit neighbor was a personable young woman from the Maryland Department of sources. Her table’s centerpiece was a fox pelt, thick and luxurious. Mine was a dried out sponge that resembled a chunk of cinder.

What could I tell people about sponges that would make them feel my excitement? I had to think fast, since a mother with two children was approaching my table.

“It’s the first fresh water sponge ever recorded in the Potomac River,” I announced.  The little girl dropped her mother’s hand and headed off to stroke the fox pelt. Her brother followed.

One last chance:  “Sponges take in water through pores on their body,” I said. “They absorb the nutrients, and then expel the water.” The mother thanked me politely and joined her children.  

The river plays its tricks

Then in early August my luck changed. In a shallow channel, among the familiar clams and big, rough mussels, I spotted something different. It was no more than a clump of pebbles and shells, held together by what seemed to be something “spongy.” Then I found a second clump.

I sent a sample to Dr. Ruetzler, who later confirmed it was indeed a sponge, and the same species.

I have continued to scour that spot and elsewhere up and down the river. But those two tiny clumps were the river’s only assurance that the sponges were alive and presumably well.  

What happened? Probably nothing out of the ordinary. The river is always changing, from day to day, year to year. For those of us who were brought up believing in the balance of nature, the river challenges our assumptions. It tricks us and upsets our plans.

After a whole season of searching, the river teased me with those two tiny sponges. “Keep looking,” the river tells me. And I will.