By Dr. David Wartinbee, for the Redoubt Reporter
Several weeks back I collected a bunch of critters and bottom sediments from a stream near the Swanson River. While poking around in this collection, I found a tiny hydroptilid caddisfly that I wrote about in a previous column. After I had briefly perused the collected mass, I dumped it all into an aquarium I had set up in the lab. This week I decided to dig through the aquarium contents and look at any other smaller creatures that might be buzzing around.
When I opened the top of the aquarium, there were a number of small midge adults flying around below the light. These adults had mistakenly emerged due to the room-temperature water and the continuous light. There were enough of them that some, if they found a member of their own species, might have mated and left some eggs in the water. I decided to see if I could find any chironomid eggs.
As I looked, I saw a tiny round object that looked like a miniature Frisbee, and it looked familiar. I hadn’t seen any of these for more than 35 years, so I discounted my identification. Then I looked at some bottom sediments under high magnification and I found long, pointed spines of silicon. Again, I hadn’t seen these since I was a grad student at the University of Pittsburgh, and it just couldn’t be what I was thinking.
I was really curious now and reached into the substrate to separate out a couple of the slender masses of algae I had collected. When I put them under the dissection scope, there they were. These were freshwater sponges!
I’ll bet that most people, including a great many biologists, don’t know there are a fair number of freshwater sponges in streams and lakes throughout the world. When we think of sponges, we think of what we wash our cars with, and they all come from the oceans. It turns out that many streams and rivers here in Alaska have sponges, too.
Sponges are members of the phylum Porifera, and they are the simplest of all multicellular animals. They only have a couple types of cells and we usually think of them as having an inner and outer layer with a little noncellular material in between.
The cells are arranged so as to create a series of passageways that water can flow through. Some inner cells have flagellae that wave around and move the water along these passageways.
Along the way, other sponge cells engulf organic particles that may be drifting in the water. That’s how sponges feed. They simply filter out fine organic particles that happen to pass by.
Some of the sponge cells are involved in creating the fine spicules of silicon dioxide called megascleres. There are also a number of smaller spines that are called gemmoscleres. Together, these two forms of spines are used to distinguish between the 300 species of freshwater sponges. These fine spines, combined with a collagen outer covering of the sponge, give it a fairly rigid structure that keeps the tubelike passageways open so water can move through.
Sponges can reproduce by releasing egg and sperm at various times during the year, but they also reproduce asexually by forming gemmules.
These were the brown-colored, almost microscopic disks I saw as soon as I started to look into the aquarium water. They are released at various times during the year and they germinate to start a new colony that constitutes a baby sponge.
The sponges I found looked like a tuft of miniature tree limbs that are no more than 3 inches long and probably no more than a couple millimeters in diameter. Besides being very small and easily overlooked, they were all green and look like stringy masses of algae.
Sponges are well-known to have a variety of algal symbionts growing inside and around the passageways. In some species these symbiotic associations are required for survival, while in others they are only a mutualistic relationship, and either party can live on its own.
While looking at a couple sections of sponge under the microscope, I noticed another sponge associate. Inside a couple of the passageways were super small midge larvae. These insects had taken up residence inside the narrow passageways.
While that may seem to be a major burden on water flow through the tubes, the midges undulate their bodies and actually help pump extra water through the sponge. The midge larvae feed on the symbiotic algae and then provide fine organic particles the sponge can absorb. This is another win-win relationship.
There have been relatively few attempts to quantify the role played by sponges in fresh waters. It is known that they are only found in unpolluted water and they pass a lot of that clean water through their bodies each day. There are estimates that the animal biomass in some small streams is actually dominated by sponge tissue. When sponges are a major biomass entity in a lake or small stream system, they are no doubt playing a major role in energy and nutrient cycling.
While freshwater sponges are used as a food source for a few specialized insects, most avoid them because of their sharp, protective spines. Those insects that target sponges are known as spongillid flies.
Freshwater sponges are an unheralded and usually unrecognized member of the aquatic community. While simple, diminutive and somewhat sessile, they can be a major player in the aquatic system.
David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Kenai River and Cook Inlet watershed.