Category: Macroinvertebrates

Return of the sponges

It was a bad afternoon for a river expedition. A light rain had started to fall, pushed by a gusty breeze. But this was about sponges, and it couldn’t wait. By tomorrow morning, the remnants of Hurricane Ida would sweep into Potomac country. Heavy rains would flood the river, producing swift currents that would scour away anything not firmly anchored to the bottom.This would be my last chance this year—perhaps forever—to see my delicate and enigmatic creatures.

The thrill of discovery. I call them “my sponges” because I had found them in 2011, the first sighting of sponges in the main stem of the upper Potomac River. I also call them “creatures,” because a lot of people don’t realize that they are animals, not plants, and are moreover the first animal to emerge on earth, possibly an astonishing 890 million years ago.

The most lively thing about D. radiospiculata are its jumping jack-like spicules. Otherwise it just sits there like a head of cauliflower.

I was thrilled by that discovery 10 years ago, and even more so when the sponge expert at the Smithsonian Institution identified the sponge as a species rarely found along the eastern seaboard. Its scientific name was Dosilia radiospiculata for the delicate radial structure of its skeletal parts. I proudly signed a release to donate my specimen to the Smithsonian collection.

I felt a kinship with the legendary naturalist explorers of former times, whose exploits I so admired. Now I had exploits of my own. But I quickly I learned that not all scientific discoveries are equal. A person can live their entire life without thinking even once about a freshwater sponge. When I told one person about my sponges, her response was, “Let’s go see what’s on the buffet table.”

Then the sponges disappeared, also after a flood, like the one that was threatening to hit now. In the following years I searched for them, all up and down the river, but found no trace.

But even though my discovery was perhaps not very interesting, it was significant. Look at Darwin and his earthworms! As it turned out, my discovery and the events that followed would transcend the subject of sponges, and help me to better understand the world and our place in it.

Here my guide would be Potowmack the Trickster, the spirit of the river, who excelled in exposing our foibles and puncturing our pride, particularly as regards our place in the natural world.

A nice afternoon for my son and his family of mucking around in the river turned into another chapter in the sponge saga.

Sponges redux. And now, after 10 years of absence, the sponges reappeared. I had taken my son Matt and his young family out for a Potomac afternoon of getting wet and muddy. As we were preparing to leave, Matt casually remarked, “Hey Dad, isn’t that one of those sponges you told me about”? I was astounded.

We were in a little cove, its water murky from the silt we kicked up boarding the canoe. But we could still make out the bottom and its tufts of algae, dead leaves, and smoothed river stones. And the sponges, a whole colony of them.

By now everyone was settled in the canoe and clearly anxious to be on their way. I snapped off a couple of more photos, and off we went. I would return as soon as I could to spend some quality time with these long lost creatures.

One last encounter. Now here I was by myself, paddling hard through the rain and the weeds and around the rock ledges. I heard some distant rumbling to the west, imagining that Hurricane Ida was already arriving. It turned out to be aircraft making their approach to Dulles Airport, some of them probably carrying Afgan refugees. Most of the world was focused on the Afgan evacuations, or the hurricane, or covid-19. My mind was on sponges.

I angled across the river heading for a gravel bar, a spot where you can still find remnants of the canal that George Washington built to skirt the rapids. I wondered if Washington also saw the sponges. Would he have cared?

The gravel bar was already occupied by an increasingly agitated flock of Canada geese. They bobbed their heads and honked and cackled and pooped and then took to the air, flying low over the water, still honking and pooping.

I pulled my kayak up on the shore, picking my way around the piles of goose droppings. The water was a little discolored, but clear enough to see.

Crushed and confused, I search the little cove for any sign of sponges.

The trickster returns. But there was nothing to see. The sponges had vanished, once again gone without a trace.

I was disappointed, but at the same time intrigued. No doubt about it. Here was Patowmack the Trickster at work once more.

What happened? I had read somewhere that ducks will eat sponges. I’d guess that geese eat a lot of the same things as ducks. I imagined a flock of geese coming upon a colony of sponges, and in no time devouring them all—but with an exception. For along with the sponges the geese also pick up the sponges’ gemmules, those tiny spherical reproductive bodies. The geese fly off and land somewhere else, always honking and pooping, and depositing the gemmules. If the current is swift, the gemmules just become part of the organic soup that washes downstream to who knows where. But if the geese land in some protected cove or back water, the gemmules could produce a new colony of sponges come next season.

I later asked my son Matt what he thought of my scenario. After all, he was the one who found the sponges. He also has a doctorate in ecology, so he should know about such things.

Matt listened politely to me, and then came up with a more prosaic explanation. The sponges looked pretty shopworn, he remembered, and they smelled bad. Later he had to wash his hands over and over to get out the stink. Probably the sponges were just dying, he said. Their season had come to an end, like the leaves starting to fall from the trees or mushrooms turning black and slimy. The bits and pieces of their bodies—including the gemmules—those tiny time capsules—probably lay  among the algae and dead leaves in that same little cove. With any luck, the gemmules could produce another colony when the conditions are right. I later learned that scientists describe this stage in the sponge’s life cycle as “gemmulation and degeneration.”

River keeps its secret. I like both explanations. I like mine because it illustrates the lengths the human imagination will go in finding a cause for anything and everything, even in the absence of any real evidence. The more complex the cause, even if it enters the realm of the conspiratorial, the greater its allure. Taken to the extreme, a search for causes could inspire elaborate myths and liturgies and the creation of priestly hierarchies. (If it seems like I’m straying a little far from a pooping goose, blame Potowmack the Trickster.)

In contrast, Matt’s explanation looked at the disappearance of the sponge from the standpoint of the sponge. The life cycles of natural creatures operate independently of our assumptions and expectations. It’s the creatures that are calling the shots, whether we like it or not.

Whatever actually happened in that little cove is the river’s secret, at least for now. I don’t have the persistence to figure it out, and Matt is mainly focused on getting tenure.

Let’s just call it a thought experiment. We can name it Schrödinger’s Sponges. It’s as if the sponges were locked away in a box, just like the famous physicist’s imaginary cat. Is the cat dead or alive? Unless we open the box to see, it occupies both states at the same time. The same goes for the fate of the sponges. Until we have data based on observation, all explanations are valid.

I think this would be good problem to leave to some graduate student with a lot of time for research and little interest in public recognition. 

Peaceful and easily overlooked, a little cove branching off from the river probably holds the answer to the mystery of the sponges.

The Potomac’s hard-working immigrant

Seeing the bottom of the Potomac River through a facemask is a little like looking through a microscope. It’s a new world down there, and you never know what you might find.

I was looking sponges. The previous year they were right here, at the head of Watkins Island, near Pennyfield Lock. It was apparently the first sighting of sponges in the river’s main stem. What happened to them? Would they come back?

I examined the bottom minutely, admiring the shimmering pebbles and the green patina on the mussel shells. I poked at anything that looked even the slightest bit spongy.

The clams suck water into one
opening and expel it out the other,
removing silt and other pollutants.

Then I saw something that made me come to a full stop. Wedged among the pebbles was a pair of tiny white tubes, then another, and another. Hundreds of them. I had found a colony of freshwater clams.

This was interesting, but not because clams are unusual in the Potomac. Far from it. The shells of this little bivalves literally pave parts of the river in Montgomery County. Along some stretches of shoreline, their shells crunch under every footstep.

But I had never seen these little creatures actually at work. And here they were, drawing water through one tentacle-lined tube, retaining nutrients other matter, and then expelling the same water through the other tube.

It struck me that they might be a little like the Higgs boson of the Potomac. Even though most people don’t know anything about them, their vast numbers could give them a decisive role in the river’s ecology. Are they another of Charles Darwin’s humble creatures that rule the earth, such as the  barnacles that were the subject of his first book and earthworms which he lovingly saved for his last?

Oysters of the Upper Potomac. It seems so, at least according to biologist Harriette Phelps, a professor emeritus at the University of the District of Colombia. Phelps is widely known for her work with this clam species in the tidal Potomac.

Biologist Phelps sorts through
a haul of Asian clams destined
for research on pollution.

She told me the clam’s scientific name is Cobicula fluminea. Its common name will have to wait, so as not to give away the second part of the story.

“Hard working Corbicula,” she calls them. Put a layer of them on the bottom of a pail of murky water, and an hour and a half later the water will be clear. Multiply this effect by the countless millions of clams in the river, and you have what Phelps calls a “key species.”

Forget the feisty bass, the graceful heron, the majestic eagle. One of the real drivers of life in the river is this humble mollusk, most of them no bigger than a quarter. Darwin would have understood.

Corbicula almost seems designed with the express purpose of purifying water. The particles it extracts from the river are not simply cast back again. Instead, they are combined with mucus to build up the sediment in which they live.

Does this sound familiar? Elsewhere on this site I wrote how oysters were the cheapest and most efficient water purifier system in the Chesapeake Bay, until they were nearly wiped out.

Stands of stargrass grow so thickly
that canoes barely penetrate.

Clearing up the water kicks off a chain of happy events. More sunlight penetrating the water fuels the growth of aquatic plants. With oxygen-producing plants come fish and birds, and pretty soon, the river environment becomes whole again.

Phelps saw this happen in the tidal Potomac.  There’s “absolutely every reason” to assume that Corbicula is providing the same service in our upstream portion as well, she says.

Certainly something good is going on out there. In some places in the river, plants are so thick that you can barely plow your kayak through them. In the channels through the vegetation you see darting shoals of minnows and bass slipping back into the cover. Big catfish chug along the bottom like monster tadpoles.

A dark secret. But these sunlit waters contain a dark secret.  Corbicula is not a native of the Potomac region or anywhere else in North America. It originally came from eastern Asia, and is know by the popular name Asian clam. It arrived first on the West Coast around 1930, then showed up in the Potomac in 1977.

Once in the Potomac the clams rapidly increased their numbers, carpeting many parts of the river bottom. This would seem to qualify Corbicula for membership in that increasingly familiar “genus” of the world of advocacy ecology called “invasive.” Here we have another foreign creature that has invaded our own natural communities, like a shiploads of Vikings burning monasteries, slaughtering townspeople, and carrying off treasure.

Clam gathering isn’t much
sport, but it’s a sure thing.

But not according to Phelps. “Corbicula were not invaders,” she said emphatically. “They occupied areas that were previously unoccupied.” And like many immigrants, they got their start by doing jobs that natives wouldn’t or couldn’t do themselves.

In the tidal Potomac, Corbicula inhabits the upper layer of the river bottom, she explained. The estuary’s native clam lives deeper down. “Even with Corbicula, the native clams are all over,” said Phelps.

Paradoxically, Phelps worries that this “invasive” species might disappear. It already has in her former study areas in the tidal Potomac. The same could happen in the upper river as well.

If Corbicula disappears, the river’s ecological clock could start to go in reverse. With no more clams filtering out so much stuff that gets washed into the river, the water could again lose its clarity, and with it, many of its plants, fish, birds, and other life forms.

Tiny, but considered a
perfect addition to soup.

The clams’ disappearance would also be felt by one group of local people whose ties to Corbicula extend to long ago and far away. You can sometimes see them, the men bent over the water, raking out clams with fan covers, the women sitting in the river, plastic baskets resting on their outstretched legs, picking the clams out from the stones and empty shells.

These people are also immigrants, harvesting the same clams in the same way they they and their families have done for many generations in their southeast Asian homelands. Following age-old traditions they will cook the clams in a wok with ginger, chile peppers, basil, and soy sauce.

Evidently it’s an acquired taste. “They’re good,” says Phelps, “but not really clam-like to me.  I like clams that are saltier.”