Raising Mussels at the Urban Stream Research Center

Since 2012 Forest Preserve District ecologists have been raising freshwater mussels at its Urban Stream Research Center at Blackwell Forest Preserve. The process allows them to increase populations in a relatively short period of time.

The main goal is to raise juveniles to a size that will increase their chances of survival in the wild. Juvenile mussels can be as small as 0.15 millimeter (far too small to see with the naked eye), placing them at risk from aquatic organisms such as hungry flatworms or creatures called “rotifers,” which at only 0.01 millimeter themselves can colonize en masse around juvenile mussels, cutting off their food supply.

To start, ecologists first need to don waders and slowly move through rivers and streams to find pregnant, or “gravid,” female mussels. By gently prying open the shells they can look for inflated “marsupial gills,” which indicate a mussel is full of larvae called "glochidia."

Of the nine types of mussels that live in DuPage, though, seven are only gravid in winter. This means someone needs to get cold as well as wet.  These mussels often burrow in the streambed in winter to avoid near-freezing temperatures, so they can be incredibly hard to find, so ecologists go out in summer to record where they find them, occasionally moving individuals to a certain stretch of a river to ensure they can find them later during the small window when females are gravid, which can be as short as four weeks.

The gravid mussels go to the Urban Stream Research Center, where ecologists then remove and count the glochidia. The most common extraction technique uses water to flush out the marsupial gills, easily removing large amounts of glochidia without causing significant damage to the adult. Ecologists then examine the glochidia under a microscope to make sure they’re fully developed and healthy. A little salt in the observation dish will cause healthy glochidia to quickly snap shut, showing they should be able to successfully latch onto the gills of their host fish when the time comes.

Some glochidia will only attach to the gills of certain species of fish; others are less picky. For common fish such as largemouth bass, smallmouth bass, and yellow perch, the Forest Preserve District can purchase the stock it needs from a hatchery, but for smaller nongame species such as creek chubs, darters, and minnows, it may have to collect the fish from local rivers and streams.

To get glochidia to pair with a host fish, both go in a small water-filled bucket, and the contents is lightly to ensure the microscopic mussels remain suspended. (If they settle on the bottom they’re less likely to come in contact with a fish’s gills.) During this process, the fish are monitored to ensure an ample amount of glochidia have latched on, at which point they go into a larger tank. There, the mussels harmlessly feed off nutrients from their unsuspecting host.

After one to three weeks the mussels’ vital organs fully develop, and they detach. They’re now considered day-old juveniles. They move into specialized pans, each set up with a sandy bottom, water temperature, flow rate, and algae diet that’s appropriate for the particular species. They’re measured weekly and placed in new containers as they grow. Once they reach 2 to 5 millimeters (no bigger than a pencil eraser), though, they’re ready for a more natural environment.

For this second-to-last step, ecologists number the mussels and move them into floating containers lined with sand, which go in a spring-fed pond that connects to a river. Staff can still monitor the mussels, but the animals now have a more natural wild-water diet, one they can’t receive in an artificial setting.

After four to six months, the mussels, now “subadults,” measure anywhere from 30 to 60 millimeters (1 to 2.25 inches) and are ready to move into their forever homes along the streambed, where staff will continue to check in on them over the years.

How to Identify Freshwater Mussels

 To make sure they collect the right species, ecologists need to be able to reliably identify mussels in the field without guides or microscopes — a skill that also helps them get out of winter water as quickly as possible! The thing about mussels, though, is that some have distinguishing features while others are more difficult to label.

Of the 20 or so species ecologists work with on a regular basis (the District works with partner agencies to raise more than the nine that currently live in DuPage), the mapleleaf is one of the easier to ID. District ecologists first recorded the mapleleaf mussel in DuPage in 2023 as they were working in an unsurveyed stretch of the East Branch DuPage River. They quickly identified it by its distinct appearance, which includes a double row of raised knobs or bumps, called “tubercles,” separated by a shallow, elongated depression in the shell, known as a “sulcus.”

The mapleleaf was an exciting find, but more common species have equally striking appearances. One, the white heelsplitter, gets its common name from a part of its shell called “the posterior wing,” a flattened, often pointy extension at the back of the mussel’s body that can jut out of the riverbed as the mussel burrows. According to legend, the mussel earned its name when the scientist who first described it was walking barefoot through a stream and stepped on one, splitting his heel open. Since then, any mussel with a prominent posterior wing has been called a “heelsplitter.”

In wider northern Illinois, mussels have even more distinct features. One exemplary species (and one of my personal favorites) is the pistolgrip. This large mussel has a thick, elongated, laterally compressed shell with irregularly shaped raised bumps called “pustules” that cover about two-thirds of both halves of its shell. It’s one of the few freshwater mussels that’s sexually dimorphic, which means you can identify males and females just by looking at the shell. Males have shortened, squared-off shells. Female shells are longer and rounder so their gills have the space they need when they’re gravid. (Females carry developing larvae called “glochidia” in their gills before releasing them into the water. Ecologists can tell if a mussel is gravid by looking at its gills.)

Although some mussels are easy to ID, others have just one or two minute discerning features. The giant floater and the creeper are prime examples. Even to the trained observer the two are nearly identical. Only tiny ridges on the “umbo” or “beak” of the shell, the hinge where the two parts of the shell meet, offer a clue. On a giant floater, these markings are called a “double-loop sculpture.” Picture a curvy M or a child’s drawing of a seagull. In comparison, the markings on the beak of a creeper are thickened, raised U-shaped lines.