Creature Feature: Winged mapleleaf mussels cling to existence in a short stretch of the St. Croix

Endangered mollusks only live in a fraction of their historic range, while scientist seek to better understand and protect their delicate population.




7 minute read

Via the U.S. Fish & Wildlife Service:

USFWS biologist Chris Davidson holds three winged mapleleaf mussels, which are tagged with identification numbers for monitoring purposes. Photo credit: Sarah Sorenson – USFWS

The winged mapleleaf is an endangered species. Endangered species are animals and plants that are in danger of becoming extinct. Threatened species are animals and plants that are likely to become endangered in the foreseeable future. Identifying, protecting and restoring endangered and threatened species is the primary objective of the U.S. Fish and Wildlife Service’s endangered species program.

What is the winged mapleleaf?

There are an estimated 11,000 winged mapleleaf mussels growing in a six-mile stretch of the St. Croix River near Taylors Falls.

Scientific Name: Quadrula fragosa

Appearance: Winged mapleleaf grow up to four inches long. They have thick shells that are greenish brown, chestnut, or dark brown in color. Their shell, like that of a few other native freshwater mussel species, has several rows of bumps running from the hinge (umbo) to the edge of the shell. The patterns of these rows of bumps, or tubercles, help biologists differentiate this from other, similar mussel species. Faint rays are visible in small shells.

Geographic distribution of winged mapleleaf mussel (Quadrula fragosa (Conrad, 1835) in the St. Croix River, Minnesota and Wisconsin. Wisconsin Department Natural Resources. 2002.

Range: When the recovery plan was approved in 1997 only one population of winged mapleleaf was known to be extant – in the St. Croix River on the border between Minnesota and Wisconsin. Since then four additional populations have been found – in the Saline River and Ouachita River in Arkansas, in the Little River in Oklahoma, and in the Bourbeuse River in Missouri. Additional survey effort is needed to assess and monitor the status and distribution of each population.

Habitat: Winged mapleleaf are found in riffles with clean gravel, sand, or rubble bottoms and in clear, high quality water. In the past, it may also have been found in large rivers and streams on mud, mud-covered gravel, and gravel bottoms.

Feeding Habits: To feed, the winged mapleleaf siphons in water and filters out food particles. It is thought that most of the particles that are actually used as food are phyto- and zooplankton – tiny organisms that drift with river currents.

Reproduction: Winged mapleleaf reproduction is similar to many other freshwater mussels. The males shed sperm into the water. Eggs on the gills of females are fertilized when sperm is collected as the female siphons in water. After fertilization, the females store the developing larvae (glochidia) in their gills until they’re expelled into the river current. These glochidia must attach to the gills or fins of a fish to complete development. Glochidia can only develop on certain species of fish which are called host fish. Known host fish for the winged mapleleaf are channel and blue catfish. Glochidia continue growing on the fish and transform into juveniles, then they drop off and land on the river bottom where they mature into adults. The life span of the winged mapleleaf is not known, but the oldest known individual in the St. Croix is 22 years old.

Why is the winged mapleleaf endangered?

Locations of extant populations of winged mapleleaf (Quadrula fragosa). (USFWS)

Habitat Fragmentation and Small Population Size: The five remaining populations are largely or entirely isolated from one another and appear to be highly differentiated genetically. Thus, each may possess important local adaptations. Propagation of winged mapleleaf and its reintroduction into historically occupied river reaches could reduce the impact to the species’ genetic diversity that might result from the loss of any of the remaining populations. In addition, it may be an important tool to increase the viability of one or more populations. The Service and its partners have not yet demonstrated the ability to produce winged mapleleaf in the numbers that would be necessary to effectively reintroduce the species. Significant progress toward this end has been made recently, however, and several agencies and scientists are now cooperating to establish successful protocols.

Zebra Mussels: Zebra mussels are an exotic species of mussel that threaten native freshwater mussels in the Mississippi River watershed. They were inadvertently introduced into North America during the late 1980’s and became established in the lower St. Croix River in 2000. Zebra mussels attach to any hard surface and breed so prolifically that they smother or otherwise harm native mussels. It is essential to the conservation of winged mapleleaf that zebra mussels are not allowed to invade any of the remaining winged mapleleaf habitat.

Land Use Changes: The population of winged mapleleaf in the St. Croix River is near the major metropolitan area of Minneapolis and St. Paul, Minnesota. As sprawl from this urban area continues, more and more of the St. Croix River watershed will be developed, which could result in increasing levels of contaminants and sediments in runoff that drains into the river. Researchers have recently documented an increase in fine sediments in the winged mapleleaf habitat in the river (Hornbach & Hove 2008). In Arkansas, Missouri, and Oklahoma agriculture and industry are abundant in the watersheds where winged mapleleaf are present. These activities can destabilize river corridors and increase runoff of harmful pesticides, chemicals, and sediment.

What is being done to prevent the extinction of the winged mapleleaf?

Listing: The winged mapleleaf was added to the U.S. List of Endangered and Threatened Wildlife and Plants on July 22, 1991. It is illegal to harm, harass, collect, or kill the mussel. Scientific studies with winged mapleleaf or take that is incidental to an otherwise legal activity may be allowed by obtaining a permit from the U.S. Fish and Wildlife Service.

Recovery Plan: A recovery plan has been prepared for the winged mapleleaf. It identifies and prioritizes actions that are necessary to recover this species.

St. Croix Falls Hydro Generating Station, Wisconsin (Tony Webster/Flickr)

Habitat Protection: Xcel Energy, which operates a dam just upstream of the winged mapleleaf habitat in the St. Croix River, recently agreed with Wisconsin Department of Natural Resources to manage flows from its St. Croix Falls dam in a run-of-river mode. This effectively ended peaking operations at this facility that adversely affected the immensely important mussel beds just downstream. In addition, the National Park Service, the states of Minnesota and Wisconsin, the U.S. Army Corps of Engineers, and others are cooperating with the U.S. Fish and Wildlife Service to keep zebra mussels out of habitat occupied by winged mapleleaf in the St. Croix River.

What can I do to prevent the extinction of species?

Learn: Learn more about the winged mapleleaf and other endangered and threatened species. Understand how the destruction of habitat leads to loss of endangered and threatened species and our nation’s plant and animal diversity. Tell others about what you have learned.

Join: Join a conservation group or volunteer at a local Refuge, nature center, or zoo.

Protect: Protect water quality by minimizing use of lawn chemicals (i.e., fertilizers, herbicides, and insecticides), recycling used car oil, and properly disposing of paint and other toxic household products. If you boat, do whatever you can to prevent spreading zebra mussels within or between water bodies.

Highlights from a five-year review of winged mapleleaf restoration issued in 2015:

A brooding female winged mapleleaf mussel is displaying her lure for catfish, the host fish for her larvae. Catfish like to eat dead things and the mussel mimics a dead mussel to attract the host fish. The lure resembles rotting flesh and the female is gaping similar to a recently deceased mussel. Credit: Chris Barnhart/Missouri State University

From the Winged Mapleleaf (Quadrula fragosa) 5-Year Review: Summary and Evaluation, U.S. Fish and Wildlife Service, Twin Cities Field Office, Bloomington, Minnesota, May 2015 (PDF)

Our understanding of the breeding behavior of Q. fragosa has improved substantially since the species was listed in 1991and even since the recovery plan was approved in 1997.

Q. fragosa was assumed to behave in a manner typical of members of the subfamily, Ambleminae – i.e., to brood its young and to infest its host in the summer… They found no evidence of brooding until 31 August and none after 6 October, confirming Q. fragosa as a fall short-term (tachytictic) brooder.

For a few days during its approximately six-week brooding period the posterior mantle around the excurrent aperture of brooding Q. fragosa becomes “greatly expanded” with swelling and development of “black-ridged crenulations overlaying the gray mantle”

Q. fragosa emerges from the substrate when brooding glochidia; during the brooding period a greater proportion of Q. fragosa are exposed at the surface than outside the brooding period

The earliest reproduction may occur before age 8 – as early as age 4 to 6

“Increasing levels of fine sediments and increasing flow velocities may be developing threats to Q. fragosa in the St. Croix River.”

Channel catfish (Ictalurus punctatus) and blue catfish (I. furcatus) are the only known suitable reproductive hosts for Q. fragosa.

The development of Q. fragosa that are attached to their host fish may be lengthy and is temperature dependent… glochidia that are attached to host fish cease development at water temperatures of about 9.24°C (48° F)… total encapsulation periods during the two tests were 262 and 260 days, respectively.

Juveniles in the St. Croix River are likely to remain on host fish throughout winter and detach in the spring of the following year.

Q. fragosa encapsulated on their fish hosts grow considerably before metamorphosis is complete – increasing about 2 to 4 times in length…

Temperatures in the St. Croix River are typically below the minimum for glochidia development for five to six consecutive months. Nevertheless, encapsulated glochidia may continue to develop for about five weeks in the spring.

Water temperatures should be carefully controlled to ensure normal development and survival of glochidia… Shells of juvenile Q. fragosa that transformed from fish held at water temperatures higher than in the St. Croix River typically appeared “lopsided” and were smooth, lacking the surface features that typify Q. fragosa.

Nearly all of the Q. fragosa population [in the St. Croix River] occurs in a 9-km (6-mile) reach that comprises only about 9% of the species’ original distribution in the river

A recent study conducted in the St. Croix River suggests that Q. fragosa there may be threatened by reduced genetic diversity that may have resulted from inbreeding.

Only found Q. fragosa in areas with high mussel density in the St. Croix River where hydraulic measurements were indicative of low sediment deposition rates… Increasing levels of fine sediments and increasing flow velocities may be developing threats to Q. fragosa in the St. Croix River.

Decrease in the density of juvenile mussels, increases in proportions of fine sediments, and signs of increasing flow velocities in the reach of the St. Croix River inhabited by Q. fragosa.

Threats to the St. Croix River population include human-mediated introduction of zebra mussels (Dreissena polymorpha) and increasing levels of fine sediments


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Creature Feature: Winged mapleleaf mussels cling to existence in a short stretch of the St. Croix