Mapping Mussels Leads to Sponge Discovery

During the month of May we launched Mussel May Mapping, a collection project on the free iNaturalist application, the goal of the project was to document freshwater mussels and other organisms living in the riverine ecosystems of the Anacostia River during the month of May (2023). We selected 18 taxa to be collected during that timeframe, the taxa ranged from fish, crustaceans, mussels and other mollusks; to birds, mammals and plants. The project area was an existing map of the main stem of the Anacostia River and surrounding areas in Maryland and DC.

Map of the iNaturalist collection project named “Mussel May Mapping”. All observations within the polygon along the main stem of the river, were collected during the month of May.

A total of 1,173 observations were made by 124 observers and identified by 299 identifiers, the latter are people that use the application and have experience to identify species observed by other app users. This includes both amateur experienced naturalists but also subject experts. A total of 476 species were identified during the month giving us a snapshot of the biological diversity during the month. And it clearly gave us an interesting snapshot, the most observed species was the Lilypad forktail (Ischnura kellicotti), a small species of damselfly listed as  a District of Columbia Species of Greatest Conservation Need in the District’s 2015 Wildlife Action Plan. It is also listed as such in several Mid-Atlantic and New England states. The other most observed species, making the top five were the Eastern mosquitofish (Gambusia holbrooki), Osprey (Pandion haliaetus), Eastern Kingbird (Tyrannus tyrannus) and the Spring peeper (Pseudacris crucifer), all native species! This is a fun snapshot of the biological activity during the season in a different way from our overarching collection project called Biodiversity of the Anacostia River which shows cumulative since 2017.

Another thing to note is that 59% of the observations are of “Research Grade” which in iNaturalist means that at least two other people have agreed on the identification of the species. This is like the social network part of the app where other people can help you identify the observations you make using the app. 

Unsurprisingly, Painted turtle (Chrysemys picta) and the Common watersnake (Nerodia sipedon) were the most observed reptile species. The latter is a common non-venomous snake often confused with the water moccasin, a species we DO NOT have in our region.  

As for the actual mussel species, it looks like all our 8 native mussel species were observed mostly during our community science mussel surveys during the month. 

Orchard Orioles (Icterius spurious) was one of the most observed bird species. This close relative of our Baltimore Oriole, both migratory species, loves river edges as a nesting and foraging habitat.

Perhaps one of our most exciting observations happened during the last day of the month at Bladensburg Waterfront Park. There we observed a freshwater sponge (yes, that’s a thing!) growing on one of our mussel culturing baskets. We had previously observed one in the upper part of the watershed in the NW Branch. That’s how we learned how to identify it, of course with the help of the great iNaturalist! It turns out they have been growing on our mussels baskets unbeknown to us!

Picture of a mature sponge growing on the mesh lid of one of our mussel culturing baskets. According to experts it is without a doubt a freshwater sponge in the Spongillidae family. However identifying it by species it’s a difficult task since these creatures are poorly known so far. Note the holes used for filtering water. 

Freshwater sponges live attached to surfaces in rivers, streams and lakes, and like our mussels, they are also filter feeders and bioindicators. They are eaten by fish, birds, crayfish and other invertebrates. Only a few species live in freshwater, and most sponges live in marine ecosystems. Freshwater sponges filter organic particles, plankton and other organisms from the water column but also ingest some products produced by their symbiotic algae. The algae is what give some of these sponges the green color that makes them look like “pond scum”. The one observed in the picture above did not seem to have algae in its systems just by judging from the dull coloration (?). They reproduce sexually, or asexually when pieces are broken off and grow into new sponges or when the sponge forms gemmules, which are tiny reproductive buds that can overwinter and eventually form new sponges. Overall they feel fibrous or “spongy” to the touch and have pores and cavities to let the water sift through their bodies for filtration. 

Right now we are just wrapping up the Anacostia River Bioblitz, so look for more updates about the Anacostia River's wildlife soon!