The Potomac River’s First Sponge Acquires Name, Smithsonian Placement

In science, every answer leads to more questions. The same goes for , that domain of sharp-eyed people who explore the natural world with cameras and notebooks at the ready.

It happened to me last summer when I was fishing upstream from Pennyfield Lock. Rounding a gravel bar in my kayak, I spied some strange, bulbous forms on the river’s rocky bottom. What were they?

Up to then, . What a surprise!

An even bigger surprise was learning that these were likely the first freshwater sponges ever seen in the main stem of the Potomac River. The last sightings in a few of the river’s tributaries date back to the early 1970s.

I duly informed the scientific world of my discovery in a non-peer reviewed article in Potomac Patch.  

But what were they? If I had spotted some exotic bird, I would have been practically mobbed by experts volunteering answers. But no such luck with creatures that at best resemble cow pies. In fact, some local experts I contacted didn’t even know that there are such things as freshwater sponges.

One thing led to another, and then to a mountainside gathering at the home of the National Geographic’s geotourism expert and originator. One of his guests was a specialist in insects at the Smithsonian. He put me in touch with Klaus Reutzler.

A research zoologist, Reutzler is also the Smithsonian’s curator of Porifera (sponges). He invited me to stop by with a sample.

Tell-tale clues

Reutzler led me to his office, through a crowd of visitors listening to their docent explain about human origins, then down passageways that the public never sees, filled with mysterious cabinets. Gaining the inner sanctum, Reutzler’ office was just as I had imagined: shelves of specimen jars, yards of books, pieces of lab equipment. We sipped coffee and talked about sponges.

He said that identifying my specimen would involve taking a bit of tissue and treating it with bleach. This would free up its spicules, which are tiny silicon structures that help to add rigidity to the animal’s body. Individuals of a sponge species may vary considerably in external characteristics, but the distinctive shapes and sizes of its spicules are diagnostic.

The results came a few days later. Reutzler identified the sponge as Dosilia radiospiculata. He pronounced this species as “quite interesting” for several reasons.

First, it is one of the few in the world whose tiny spicules radiate (as in “radiospiculata”) from a central point, like the arms of jacks in the old-fashioned playground game. It also has two sizes of another kind of spicule called birotulates, which are shaped like apple cores. 

Ruetzler’s assistant, Sarah Klontz, determined that the Smithsonian collection has very few examples of this species from the eastern seaboard. Now it has another.

But now come the real questions. Is D. radiospiculata truly rare in the Potomac, or merely overlooked? What is its population size and preferred habitat? What role does it play in the ecosystem? And intriguingly, Does its presence say something about the river’s water quality?

Some of these questions are made to order for local citizen scientists and river geeks. Others will require hard-won scientific expertise. 

In the end, I know we will get some answers―along with more questions.