The “mussel shed,” as it’s affectionately called, at the Minnesota Zoo may not portray the most typical laboratory scene, but it’s a bustling hub of scientific research, nonetheless. Rather than lab benches full of beakers and microscopes, the shed houses rows of simply designed black trays and large orange buckets, which at first glance appear to contain only water flowing over sandy and rocky sediment. But on closer inspection, and if you move with the right amount of stealth and patience, you are likely to see these containers come alive.
Conservation scientists at the Minnesota Zoo are partnering with the Minnesota Department of Natural Resources (DNR) to conduct research on a variety of topics concerning freshwater mussels. The mussel shed at the Minnesota Zoo, a space that is barely the size of a single car garage, is now home to an astounding 20,000+ individual mussels and an impressive amount of data is collected here on a daily basis.
As you enter the mussel rearing facility at the Zoo, you are likely to first notice the rows of black circular trays to your left. Each tray contains some sandy sediment and has a constant flow of water cascading into and out of it. This is the basic setup for one of the scientific studies conducted in the shed. Here, among the dozens of nearly identical trays, hundreds of freshwater mussels are being reared. Both juvenile and adult mussels of three different species are hard at work, growing rapidly by filtering food from the freshwater that is pumped in from the nearby pond. Zoo researchers are studying the federally endangered Higgins’ eye pearly mussel, the Minnesota threatened mucket mussel, and the plain pocketbook mussel, in an effort to better understand what possible impacts adult and juvenile mussels have on each other when sharing habitat. Adult mussels burrow in the sandy bottoms of rivers and lakes and this activity changes the immediate environment around them. By raising individual mussels together of varying size, age and species, researchers can document how behaviors of adult mussels may influence the survivorship and growth of juveniles in rearing facilities to suggest how this relationship may help mussels thrive in nature.
“The results of this study could improve rearing methods and release success,” explains Ben Minerich, mussel conservation biologist with the Minnesota Zoo. “In order to effectively rear and release an endangered species such as the Higgins’ eye, it is imperative to understand under what conditions they thrive. We want to do our best to ensure that when the DNR releases them into a river, these mussels will survive and reproduce. And that may mean rearing and releasing them along with another species, if that’s what the science demonstrates they need.”
This study, which was set up in November 2020 with help from the DNR, is scheduled to run for 2 years, until the mussels have grown adequately in size, at which time all mussels will then be returned to the DNR to be released into Minnesota rivers, with the hope of establishing new and healthy populations.
In addition, Zoo researchers are also working hard to develop new protocols on rearing young juvenile mussels that are so tiny they are nearly microscopic in size. Working with mussels this young presents many challenges and rearing methods are constantly being improved upon to increase survivorship. This is due in part to the fact that these mussels are small enough that they may become easy food for a variety of predators. Freshwater mussels also require a fish host to complete their development. As young microscopic mussels, they latch on to the gills of an unsuspecting fish where they can hitch a ride for a few weeks and safely eat and grow. This means that rearing mussels from their earliest life stages requires that you also raise their fish host.
The spectacle case mussel is a federally endangered species and there has been little luck in rearing them in controlled settings, in part because their fish host species was unknown. One breakthrough came in 2018 when the DNR confirmed that these mussels require one of two fish species, the mooneye or goldeye, for their development. With much effort and innovation, the DNR has now seen success in maintaining these fish species in the lab, a critical step to raising spectacle case mussels for reintroduction efforts. Once the mussels have entered into their next life stage and left their fish host, they can be moved to a variety of rearing conditions, including the Zoo’s mussel shed. Here, Minerich has set up rearing buckets to filter and add lake water to provide the tiny mussels with ample food while also keeping them safely inside their rearing buckets.
“By trying to replicate the suspected habitats of these young mussels, we hope to develop effective rearing methods so that these mussels can eventually be released into Minnesota rivers and lakes,” Minerich explains. “These reintroduction efforts are critical to the survival of these species in the wild and to the health of our waterways.”
Freshwater mussels are crucial components of healthy, stable waterways. They filter and clean water, help support healthy populations of fish, turtles, and aquatic vegetation, and are a food source for species such as otters, muskrats, ducks and gulls. They also stabilize riverbeds and lake bottoms which helps control erosion. Unfortunately, freshwater mussels are among the most threatened animals in North America. Historic harvesting, pollution, and invasive species have driven many to the brink of extinction. It is the hope of scientists like Minerich and his collaborators that by improving rearing methods they can release even greater numbers of these important and fascinating animals into our waterways to reverse this troubling trend and help ensure healthy populations of freshwater mussels for generations to come.
Funding and support for this project was provided by: the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR); and the Clean Water, Land & Legacy Amendment.