Endangered SpeciesA branch of The Humanity Project

Freshwater Species

Winged Mapleleaf

Quadrula fragosa

A freshwater mussel once found across a dozen states, now surviving in a handful of river reaches, principally the St. Croix River on the Minnesota-Wisconsin border.

Illustration of a winged mapleleaf mussel ambassador — thick squarish dark brown shell with rows of raised bumps — half-buried in clean gravel on the bed of a clear flowing northern river.
Illustrated Species Ambassador — original artwork, not a photograph

Illustrated Species Ambassador

Croix — Winged Mapleleaf

I'm Croix, an illustrated ambassador for the winged mapleleaf. I was once found across a dozen states; now I hold on in a handful of river reaches.

One fact worth knowing
The winged mapleleaf survives principally in the St. Croix River on the Minnesota–Wisconsin border, and depends on catfish to carry its larvae.
What I depend on
I depend on clean, well-oxygenated flowing water, stable silt-free gravel and sand, and catfish host species living in the same reach.
My greatest threat
My greatest threat is river degradation and sedimentation, along with invasive zebra mussels.
One way you can help
Clean, drain and dry every boat, trailer and piece of gear between water bodies — zebra mussel spread is the single most preventable threat — and never collect mussels or shells from rivers.
Freshwater species focus area →

This ambassador is an original illustration created for this project. It is an educational character, not a documentary image, and its statements follow the sourced record on this page. Artwork and rights record

Why this species matters

The St. Croix is one of the last large rivers in the upper Midwest with a nearly complete native mussel community. The winged mapleleaf's survival there is a measure of how much a protected river can still hold.

If it disappears: The upper Mississippi drainage would lose a species already reduced to a fraction of its range, and one of the strongest indicators of remaining high-quality river habitat.

Conservation status

U.S. Endangered Species Act
EndangeredListed 1991.Last verified 2026-09-08
IUCN Red List
Critically EndangeredLast verified 2026-09-08
State
Endangered in Minnesota and WisconsinLast verified 2026-09-08

Population

The St. Croix River holds the largest known population, with additional populations documented in a small number of Arkansas, Oklahoma, and Kansas river reaches. Populations are assessed by survey density rather than total count.

Trend: Unknown

As reported: USFWS recovery reporting and state mussel surveys

Source: U.S. Fish and Wildlife Service; Minnesota Department of Natural Resources

Uncertainty: Discoveries of additional southern populations changed the understanding of this species' status after listing; treat the range as subject to revision.

Historical: Historically recorded from at least twelve states across the Mississippi River drainage.

Fragmentation: Surviving populations are widely separated with no possibility of natural exchange.

Range and distribution

Historic range: Rivers throughout the Mississippi River drainage from Minnesota to Alabama.

Current range: The St. Croix River in Minnesota and Wisconsin, plus reaches of the Ouachita, Little, and Saline rivers in Arkansas and Oklahoma and the Verdigris system.

Range contraction: Lost from the great majority of its historic drainage.

Regions occupied

  • St. Croix River, Minnesota and Wisconsin
  • Ouachita River basin, Arkansas
  • Little River, Oklahoma
  • Kansas river reaches

Habitat

Primary habitat: Clean, flowing rivers with stable gravel and sand substrate and good water quality.

Breeding habitat: Larvae attach to the gills of channel catfish and blue catfish, the known host species.

Feeding habitat: Filter feeds on suspended algae and organic particles.

Critical habitat: No critical habitat designated.

Habitat dependencies

  • Stable, silt-free substrate
  • Catfish host species present in the same reach
  • Clean, well-oxygenated flowing water

Biology and ecology

Filters water, stabilizes substrate, and provides habitat structure for aquatic insects.

Key relationships: Reproduction depends entirely on catfish hosts; the timing of glochidia release is synchronized to host availability.

Important adaptations: Autumn release of larvae distinguishes it from most co-occurring mussels and reduces competition for host fish.

Size
Shell up to about 4 inches.
Lifespan
Potentially several decades.
Diet
Suspended algae, bacteria, and detritus.
Reproduction
Releases glochidia in autumn, unusually late in the season, which must attach to a catfish host.
Offspring
Large numbers of glochidia with extremely low survival.
Breeding frequency
Annually.
Age at maturity
Several years.

Primary threats

  • Invasive species

    Zebra mussels attach to native mussel shells in dense clusters, smothering them; a St. Croix infestation prompted major response efforts.

  • Sedimentation

    Agricultural and construction runoff buries substrate and clogs filtering structures.

  • Water quality

    Nutrient loading and chemical contaminants affect a filter feeder directly.

  • Dams and barriers

    Impoundments block host fish movement and convert flowing reaches to still water.

  • Channel modification

    Dredging and bank armoring remove the stable substrate the species requires.

  • Small isolated populations

    Widely separated populations cannot recolonize one another after a loss.

  • Climate change

    Increased extreme rainfall drives sediment pulses and scouring flows.

Conservation and recovery

Programs and efforts

  • USFWS winged mapleleaf recovery program
  • Captive propagation and juvenile release at the Genoa National Fish Hatchery
  • St. Croix River zebra mussel monitoring and control efforts
  • Reintroduction of propagated juveniles to historic reaches
  • Watershed sediment reduction programs

Protected areas

  • St. Croix National Scenic Riverway
  • Ouachita National Forest river reaches

Legal protections

  • Endangered Species Act
  • State endangered listings in Minnesota and Wisconsin
  • National Scenic Riverway protections

Measurable progress

  • Captive propagation has succeeded in producing juveniles for release
  • Discovery of additional southern populations improved the outlook relative to what was known at listing

Continuing challenges

  • Zebra mussels cannot be eradicated once established
  • Propagation is technically demanding
  • Populations remain few and isolated

How people can help

What individuals can do

  • Clean, drain, and dry all boats, trailers, and gear between water bodies — zebra mussel spread is the single most preventable threat
  • Never collect mussels or shells from rivers

What communities can do

  • Support watershed sediment and nutrient reduction
  • Fund and staff boat inspection stations at river access points

Educational actions

  • Use the catfish host relationship to teach that protecting a mussel means protecting a fish

Habitat support

  • Support riparian buffer establishment in agricultural watersheds

Organizations and programs to learn about

  • USFWS Midwest Region
  • Genoa National Fish Hatchery
  • Minnesota and Wisconsin departments of natural resources

Educational notes

  • Releases its larvae in autumn, unlike most co-occurring mussels.
  • The St. Croix River is one of the most intact large-river mussel systems remaining in the United States.

Sources and references