Round Hickorynut

Obovaria subrotunda

SARA Status
No Status
NS
Special Concern
SC
Threatened
TH
Endangered
EN
Extirpated
EX

SARA Status

  • No Status NS
  • Special Concern SC
  • Threatened TH
  • Endangered EN
  • Extirpated EX
COSEWIC Status
Not at Risk
NR
Special Concern
SC
Threatened
TH
Endangered
EN
Extirpated
EX

COSEWIC Status

  • Not at Risk NR
  • Special Concern SC
  • Threatened TH
  • Endangered EN
  • Extirpated EX

Description

The Round Hickorynut (Obovaria subrotunda) is one of Canada’s 54 freshwater mussel species, and is one of only two species of the genus Obovaria found in the country (both are listed as endangered). It is a small mussel, easily recognized by its almost perfectly round shape, with the following features:

  • thick, solid shell is olive-brown to dark brown in colour, with a light-coloured band along the top front surface;
  • shell is mostly smooth;
  • maximum length about 6 cm (in Canada);
  • entrally located beak (raised part at the top of the shell) curves inward and is raised well above the hinge line;
  • heavy, strong hinge teeth; and
  • the inside of the shell (nacre) is silvery-white, sometimes with a tinge of blue or pink.

Habitat

In Canada, the Round Hickorynut was historically found in western Lake Erie, Lake St. Clair and the Welland, Grand, Sydenham, Thames and Detroit rivers of Ontario. It has been lost from Lake Erie, the Detroit River and the offshore waters of Lake St. Clair due to the impact of the invasive Zebra Mussel (Dreissena polymorpha). It is no longer found in the Grand and Thames rivers and has significantly declined in the Sydenham River. Overall, the Round Hickorynut has been lost from about 90% of its historical range in Canada and is considered critically imperiled. This species is now limited to a 12 km² region of the Canadian waters of the St. Clair River delta and a 60 km reach of the East Sydenham River. In the U.S., the Round Hickorynut was historically found in 13 states, but most populations are now in decline and it has been lost from Georgia, Illinois and New York.

The Round Hickorynut lives mainly in medium- to large-sized rivers with steady moderate flows, and sand and gravel bottoms (substrates) at depths up to 2 m. However, in Ontario it has mainly been found in murky, slow-moving rivers with clay/sand or clay/gravel substrates. The Round Hickorynut is a moderately long-lived species with a lifespan of at least 10 years. Spawning likely occurs in late summer and the glochidia (the mussel’s larvae) are released the following June. Like most other freshwater mussels, the glochidia are parasitic on fish - the glochidia attach to the gills of their host fish until they reach their juvenile, free-living stage and drop off onto the substrate below. Adult mussels are sedentary and spend their lives partially or completed buried in the substrate.

The host fishes for the Round Hickorynut in Canada are the Blackside Darter (Percina maculata), Fantail Darter (Etheostoma flabellare) and Iowa Darter (Etheostoma exile).

Like all species of freshwater mussels, the Round Hickorynut filters its food from the water. Bacteria and algae are its primary food source.

Threats

Invasive Zebra Mussels (Dreissena polymorpha) compete with the Round Hickorynut for food and also attach to its shells by the hundreds-to-thousands, killing the Round Hickorynut by interfering with its ability to feed, breathe and move. Intensive agriculture and increasing urbanization also threaten remaining populations, particularly heavy loadings of silt, sediments, nutrients and pollution that destroy mussel habitat, clog their gills and reduce water quality.

Further Information

Under SARA, a recovery strategy and an action plan have been developed to prevent the loss of the Round Hickorynut in Canada. Research, land and water stewardship, monitoring and awareness activities are underway. Provincial legislation regulating agricultural nutrients and protecting clean water in Ontario have been enacted, while habitat needed to recover the Round Hickorynut is now legally protected under SARA.

For more information, visit the Species at Risk (SARA) Public Registry Profile.

Round Hickorynut

Hickory Nut (Obovaria subrotunda)
Obovaria subrotunda
Photo credit: Environment Canada

Scientific Name: Obovaria subrotunda
SARA Status: Endangered
COSEWIC Status
: Endangered
Region: Ontario

Round Hickorynut distribution as described in the following paragraphs

Map showing the Round Hickorynut distribution in Canada. The map indicates areas of current and historic sites in southern Ontario.

Did You Know?

Freshwater mussels are molluscs, soft-bodied animals without a skeleton (invertebrates), that live on the bottom of streams, rivers, lakes and ponds. They use a muscular foot to burrow and crawl and have a pair of hinged shells. Mussels are filter feeders — nature’s water purifiers — and are food for other wildlife like fishes, otters, mink, muskrats and some birds. They are also among the most endangered creatures in the world.

Round hickorynuts

Round hickorynuts

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