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Round Hickorynut (Obovaria Subrotunda)

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Executive Summary

Round Hickorynut
Obovaria subrotunda

Species Information

The round hickorynut, Obovaria subrotunda (Rafinesque, 1820), is a small freshwater mussel (maximum length in Canada ~ 60 mm) that is easily distinguished from other Canadian species by its almost perfectly round shape. The shell is thick, solid, and dark brown in colour, with a band of lighter colouration along the posterior dorsal surface.

Distribution

The round hickorynut was found throughout the Tennessee, Cumberland, and Ohio river systems in the United States, and also occurred in Lake Erie, Lake St. Clair, and some of their tributaries. In Canada, it was known from the western basin of Lake Erie, Lake St. Clair, and the Welland, Grand, Detroit, Thames and Sydenham rivers. The species is declining throughout its North American range, and has been extirpated from New York. In Canada, it is still extant in the Sydenham River and Lake St. Clair.

Habitat

The round hickorynut is typically found in medium-sized to large rivers, but also occurs in Lake Erie and Lake St. Clair. Its preferred habitat is generally described as rivers with steady, moderate flows, and sand and gravel substrates at depths of up to 2 m. In southeastern Michigan and southwestern Ontario, however, it has mainly been found in murky, low-gradient rivers with clay/sand or clay/gravel substrates. In Lake St. Clair, it currently occupies shallow (<1 m) nearshore areas with firm, sandy substrates.

Biology

The round hickorynut has separate sexes, with females smaller than males. The lifespan is probably at least 10 years. Like other freshwater mussels, O. subrotunda is parasitic on fish during its larval stage. Spawning occurs in the summer, and the female mussel holds the embryos over winter until they reach the larval stage the following June. The larvae are then released into the water where they attach to the gills of an appropriate fish host and form a cyst. After a period of time, the larvae transform into juveniles that drop off the fish and fall to the substrate to begin life as free-living mussels. The host fish for the round hickorynut is unknown, but may be the eastern sand darter (Ammocrypta pellucida). Round Hickorynuts, like all freshwater mussels, feed on bacteria and algae that they filter from the water with their gills.

Population Sizes and Trends

Obovaria subrotunda is a very uncommon species that is clearly declining throughout most of its North American range. In Canada, It has been lost from Lake Erie, the Detroit River, and the offshore waters of Lake St. Clair. It has also been lost from the Grand and Thames rivers, and has significantly declined in the Sydenham River. The only significant population of O. subrotunda left in Canada occurs in the shallow waters of the St. Clair delta, but it is not known if the population will continue to survive. The round hickorynut has been lost from ~ 90% of its former range in Canada.

Limiting Factors and Threats

The round hickorynut has been lost from most of its former range in the Great Lakes due to impacts of the zebra mussel, and the remaining population in the St. Clair delta may be at risk. Populations in the Grand and Thames rivers were likely extirpated due to the combined effects of sewage pollution and agricultural impacts in these heavily populated watersheds. The decline of the Sydenham River population is likely due to agricultural impacts. Predation by muskrats or raccoons may also play a role. If the eastern sand darter is the host of the round hickorynut, then the decline of this threatened fish would affect the mussel’s survival.

Special Significance of the Species

There are six species in the genus Obovaria, but only the hickorynut (O. olivaria) and round hickorynut (O. subrotunda) have ranges extending into Canada. The ring pink (O. retusa) is listed as federally endangered in the United States. The American Fisheries Society also considers the round ebonyshell (O. rotulata) to be endangered, and three other species, including the round hickorynut, to be of special concern. Thus, most members of this genus appear to be sensitive to environmental degradation.

Existing Protection or Other Status Designations

The round hickorynut is listed as endangered in Illinois, Michigan and Alabama, threatened in Tennessee, and special concern in Indiana, and is therefore afforded some protection in these states. There is currently no protection for the round hickorynut in Canada.

Summary of Status Report

Obovaria subrotunda historically occurred in 12 states and the province of Ontario.  Most populations in the United States are in decline. The round hickorynut has been lost from approximately 90% of its historical range in Canada due to impacts of the zebra mussel and poor land use practices. The Sydenham River population is probably close to extirpation, with only scattered individuals still found alive in the system. The only significant population left in Canada is located in a shallow area of Lake St. Clair near Walpole Island. It is not clear at present if this population is successfully reproducing.