The section of the St. Croix River below the dam at St. Croix Falls to the mouth at the Mississippi is proposed to be designated by the federal government as “critical habitat” for two kinds of mussels, or clams, that call the river home. The U.S. Fish and Wildlife Service is now accepting feedback on the proposed designation as part of a plan to protect and restore populations of the endangered species.

The spectaclecase (Cumberlandia monodonta) and snuffbox (Epioblasma triquetra) mussels are two of the 40-some species of mussels that have been found in the St. Croix, which is considered one of the richest habitats for freshwater clams in the world. Two other species not found in the St. Croix are also part of the critical habitat proposal.

While many of the St. Croix’s mussel species have abundant, widespread populations, there are several that are small and imperiled. The spectaclecase and snuffbox are two of the five species found in the St. Croix that have Endangered Species Act protection, having both been listed in 2012. The three other species are winged mapleleaf (Quadrula fragosa), sheepnose (Plethobasus cyphyus), and Higgins eye (Lampsilis higginsii). Another mussel species found in the St. Croix, the salamander mussel (Simpsonaias ambigua), is currently being considered for Endangered Species Act protections, as St. Croix 360 reported in 2023.

Critical habitat designation would affect potential projects on public lands and waters undertaken by the federal government or with federal funding, and not any activities on private lands unless it’s funded or authorized by the federal government. The designation recognizes areas that contain “habitat features for the survival and recovery of species listed under the Endangered Species Act.”

Mussels have many specific habitat requirements, including clean water, substrate, flow levels, and more. They also have another unique need: the presence of certain fish species essential to the mussel’s reproductive cycle. Mussels depend on fish to carry mussel larvae on their gills until the creatures are able to survive on their own. Different mussels depend on different fish, and those fish have to be around during the time of year when the mussels are reproducing.

Part of the work to protect the spectaclecase and snuffbox mussels has been to identify the fish that host these mussel species’ larvae. The Minnesota Department of Natural Resources mussel laboratory in Lake City succeeded in doing that in 2017, determining that the fish species mooneye and goldeye were the host for spectaclecase larvae. While classified as “rough fish” by many anglers and fishery managers, these species are important parts of the river ecosystem.

A survey of the St. Croix River for spectaclecase mussels by the Wisconsin Department of Natural Resources in 2019, which was the subject of a St. Croix 360 story, was intended to inform the Endangered Species Act protection effort. Biologists using SCUBA gear conducted systematic searches for the creatures in areas of likely habitat, briefly bringing them to the surface for measuring and documenting.

Spectaclecase are notable for preferring rocky areas to call home, where they can hide in crevices. While other mussels are seen on shallow, sandy bottoms, the spectaclecase is rarely observed because of its reclusive ways. DNR biologist Lisie Kitchel told St. Croix 360 in 2019 that it’s because the mussels have thin, delicate shells compared to other species.

Also in 2019, scientists re-located a population of spectaclecase mussels in the St. Croix River at Wild River State Park. Because the section of river has been cut off from migrating mooneye and goldeye since completion of the St. Croix Falls dam in 1907, the specimens are believed to be more than 100 years old.

That’s right in line with what are considered the biggest threats to spectaclecase mussels: dams.

“Major threats to the spectaclecase mussel include dams, small population size and fragmentation, sedimentation and pollution,” the USFWS says. “Population losses due to dams have contributed more to the decline and potential extinction of the spectaclecase than any other factor.”

Spectaclecase can grow up to nine inches in length and have oblong shapes that seem like they would hold a pair of eyeglasses pretty well. They were once found in at least 61 rivers and streams, but still live in only 40 waterways, with the overall population estimated to have declined by 60 percent. The government has proposed designating about 1,143 miles of river in 12 different units as critical habitat for the species.

Snuffbox mussels are much smaller than spectacleclase, reaching three inches in size at most. The species once lived in 210 rivers and streams or more, but are now limited to just 85 waters. The government proposes 2,427 miles of river in 38 different units for critical habitat. The St. Croix would comprise 53 miles of critical habitat for both species.

Comments on the critical habitat designation are being accepted until Feb. 11. Click here for more information and to submit comments.