Yes, that’s a thing! When most people think of sponges, the living organisms, they think of coral reefs and oceans. But not many people (including us until recently) know that sponges also live in freshwater. They actually inhabit creeks, streams and the mighty Anacostia River. With the help of experts, we have identified, so far, 4 species of native freshwater sponges.
With funding from the Chesapeake Bay Trust we started a watershed-wide survey to inventory sponge species of the watershed. This is the most comprehensive assessment of freshwater sponges in this watershed to date. What started as an assessment of mussel populations in the non-tidal tributaries of the Anacostia River ended up becoming two-pronged mussels and sponges surveys. We actually have not found a significant number of mussels in the non-tidal streams, only in two tributaries out of 9+ tributaries. But we started seeing a curious presence, even abundant at some sites, of freshwater sponges. This sparked an interest to start a watershed wide sponge inventory. We have been lucky to partner with experts from the University of Olivet in Michigan. Their expertise not only with sponges but also with molecular biology has helped us identify 4 species of sponges. So far the following species have been identified (all are native and belong to the Spongillidae family): Trochospongilla leidyi, Trochospongilla horrida, Ephydatia fluviatilis, and Eunapius fragilis. They have been found throughout the watershed as well as the tidal Anacostia and Potomac Rivers. These findings expand our understanding of benthic biodiversity (the variety of life forms living on the riverbed) in the watershed. Not only that, freshwater sponges could be used as bioindicators for tracking water quality and ecological health in our waterways.
What is a Freshwater Sponge?
Sponges belong to the phylum Porifera — literally meaning “pore-bearers” — and represent one of the oldest and most primitive animal lineages on earth, with a fossil record stretching back more than 500 million years. They lack true tissues, organs, and a nervous system, yet they are wonderful filter feeders (just like the mussels): they pump water through an intricate network of canals lined with flagellated cells called choanocytes, filtering out bacteria, algae, and dissolved organic matter as they go. Because of their unique anatomy, they are able to filter very tiny particles.
Our freshwater sponges belong primarily to the family Spongillidae (order Spongillida). Unlike their marine relatives, spongillids have to deal with the highly variable chemistry, temperature swings, and sediment loads characteristic of rivers and streams. Add to that, all the pollution carried by stormwater and discharges coming from the lands.
Freshwater sponges are sessile animals, that is they anchor to hard substrates, such as rocks, submerged woody debris, trash, infrastructure, etc., and grow in forms ranging from thin patchy crusts to lobed masses and branching fingers (more reminiscent of their marine relatives), depending on water flow and light conditions.
One of the most distinctive features of freshwater sponges is their internal “skeleton”, made up of tiny silica rods with needle-like ends and stars called spicules. These microscopic structures, which give sponges its characteristic gritty texture, are durable enough to persist long after the living tissue has died. Spicule size and shape are the primary characteristics used to identify species, making a microscope with at least 40X magnification and a careful hand, essential tools for freshwater sponge identification.
Freshwater sponges also produce gemmules: resistant, seed-like overwintering bodies packed with special spicules often with spine-like structures on them making them look like medieval weapons. Gemmules are unique to freshwater sponges as they allow them to survive winter die-offs and drought, germinating when conditions improve.
What Can Freshwater Sponges Tell us About the Health of our Waterways?
Freshwater sponges are not commonly included in standard methods used to assess the health of a body of water. Benthic macroinvertebrate sampling, for example, focuses on indicators that consider insects, crustaceans, worms, and mollusks, but not sponges. This is a gap worth addressing. Sponges offer several qualities that make them valuable ecological indicators. Our goal is to encourage the inclusion of sponges in benthic macroinvertebrate sampling. However, we are not there yet. Freshwater sponges are much less common than mussels and there are still many unanswered questions regarding their ecology.
Freshwater sponges are filter feeders that target bacteria and plankton but they are also able to filter the smallest of critters and particles. In impaired urban streams where bacterial loads are elevated from stormwater and sewer overflow, sponges provide a potentially significant ecological service. For example, we have observed an important abundance of sponges in tributaries like the NW Branch and Sligo Creek. Measuring how sponge density and distribution changes over time, and correlating that with bacteria and other water quality parameters, is an interesting area of research. That is why we have been elevating the topic of sponges to folks in academia.
Freshwater sponges provide three-dimensional microhabitat structure on otherwise flat or featureless hard substrates. The internal canals and surface texture of living sponges are colonized by specialized invertebrates — including chironomid midges, ostracods, water mites, and the spongillafly larvae of the family Sisyridae, obligate predators that feed exclusively on freshwater sponges. So, just like mussels, freshwater sponges are like river reefs themselves!
Take look behind the scenes of one of our most recent freshwater sponge surveys, conducted in partnership with Dr. Mary Miller of the Louisiana Freshwater Sponge Project.
