Recovery Strategy for the Hickorynut and the Kidneyshell [Final Version]
1. Species information - Round Hickorynut
Figure 1 : Two Round Hickorynut specimens from the Lake St. Clair delta. Note the characteristic lightening of the posterior slope. Photo credit : D. McGoldrick, Environment Canada.
COSEWIC Assessment Summary - May 2003
Common Name: Round Hickorynut
Scientific Name: Obovaria subrotunda
COSEWIC Status: Endangered
COSEWIC Reason for designation: This species has been lost from 90% of its former range in Canada. Populations in the Grand and Thames rivers are extirpated and populations in the Sydenham River are declining, all due to the combined effects of pollution and agricultural impacts. Most of the Great Lakes populations have been lost due to impacts of the zebra mussel, and the remaining population in the St. Clair delta near Walpole Island may be at risk. If the Eastern Sand Darter were the host of this species, then the decline of this threatened fish would affect the mussel's survival.
Occurrence: Ontario COSEWIC
Status history: Designated Endangered in 2003.
The Round Hickorynut is one of only 6 species in the genus Obovaria. Only two of these species, O. subrotunda, and O. olivaria, have distributions which extend into Canada where both species are restricted to the Lower Great Lakes/St. Lawrence River drainage. The Round Hickorynut is considered globally secure (G4) and nationally secure (N4) within the United States although the American Fisheries Society has listed it as a species of special concern. The species is beginning to show declines across its entire American distribution. It is considered endangered within Michigan and is believed to be extirpated from Illinois (G. Kruse, Illinois Department of Natural Resources, pers. comm. February, 2004)) and New York (D. Strayer, Institute of Ecosystem Studies, pers. comm., February 2004). In Canada, the Round Hickorynut is considered critically imperiled (N1) and was designated as endangered by the Committee on the Status of Endangered Wildlife in Canada in 2003. The Canadian distribution of this species has always been restricted to southwestern Ontario where it was once found in the Welland, Grand, Sydenham, Thames, St. Clair and Detroit Rivers as well as the waters of Lake St. Clair and western Lake Erie. Declining water quality and the introduction of dreissenid mussels have resulted in a sharp decline in the Canadian distribution of the Round Hickorynut and it is now only found in the waters of the Lake St. Clair delta and a small portion of the East Sydenham River.
The Round Hickorynut is a small mussel reaching a maximum size of 60 - 65 mm in Canada. The mussel is readily recognized by its round shape and prominent centrally located, inward curving beaks that are elevated well above the hinge line. Beak sculpture is slight, consisting of 4 to 5 weak double bars which are sinuous centrally and angled posteriorly (Parmalee and Bogan 1998). The shell is generally dark in colour ranging from olive-brown to dark brown and is relatively smooth except for prominent growth rests. The posterior slope is often distinctly lighter than the rest of the shell (COSEWIC 2003a) (Figure 1). The hinge teeth of this species are heavy and strong. The left valve has two thick, roughened, triangular pseudocardinal teeth and two slightly curved, short, strong lateral teeth. The right valve has one large, massive serrated triangular pseudocardinal tooth, usually with a small, low compressed tubercular tooth on either side. There is one short, curved, thick, roughened lateral tooth and often an incomplete secondary lateral tooth in the right valve (Parmalee and Bogan 1988).
GlobalRange: The global distribution of the Round Hickorynut is restricted to eastern North America (Figure 2). In the United States the Round Hickorynut is considered nationally secure but is showing declines across its range. This species is historically known from the Ohio, Tennessee, Cumberland and Mississippi River systems as well as the St. Lawrence, Lake Erie and Lake St. Clair drainages. It is currently found in Alabama, Indiana, Kentucky, Michigan, Mississippi, Ohio, Pennsylvania, Tenessee and West Virginia and is believed to have been extirpated from New York and Illinois. In Canada the Round Hickorynut is considered critically imperiled, classified as endangered by COSEWIC and found only in southwestern Ontario.
CanadianRange: In Canada, the Round Hickorynut is historically known from the waters of western Lake Erie, Lake St. Clair and the Welland, Grand, Thames, Sydenham and Detroit Rivers (COSEWIC 2003a). Since 1996, live specimens have only been reported from the Sydenham River and Lake St. Clair (Figure 3).
Percent of GlobalRange in Canada: Approximately 1% of the global range of this species occurs in Canada.
Figure 2: Global distribution of the Round Hickorynut (modified from Parmalee and Bogan 1998)
Distribution Trend: Since the invasion of the Great Lakes by dreissenid mussels the Canadian geographical distribution for this species has been reduced by 90%.
GlobalRange: In the United States the Round Hickorynut is seldom a significant component of the mussel community, typically representing between 0.1 and 1.4% of the species present (COSEWIC 2003a).
CanadianRange: The largest Canadian population of the Round Hickorynut occurs in the delta region of Lake St. Clair where it comprises 0.011% of the overall mussel community and occurs at a density of 0.0006/m2. In the Sydenham River the Round Hickorynut represents approximately 0.0024% of the mussel community.
Percent of Global Abundance in Canada: Less than 1% of the global abundance of this species occurs in Canada.
Population Trend: It is estimated that the population of Round Hickorynut in Canada has declined by 90% since the invasion of the Great Lakes by dreissenid mussels. This estimate is based on the number of historical records that occur in waters that now contain dreissenid mussels.
Biological Limiting Factors
Reproductive Attributes: The reproductive biology of the Round Hickorynut follows the general reproductive biology of most unionid mussels. During spawning, male mussels release sperm into the water column and females filter it out of the water with their gills. Fertilization is then able to occur in specialized regions of the gills known as marsupia. Immature juveniles, known as glochidia, develop in the gill marsupia and are released by the female into the water column to undergo a period of parasitism on a suitable host fish species. Further development to the juvenile stage can not continue without a period of encystment on the host. The hookless glochidia become encysted on the gills of the host and are encapsulated in a fluid filled sac where they are nourished by the host until they metamorphose and break free, settling to the substrate to begin life as free-living juveniles. The host fish species for the Round Hickorynut has not been confirmed for Canadian populations although 5 host species have been identified in the United States. These 5 species include: varigate darter (Etheostoma variatum); frecklebelly darter (Percina stictogaster); speckled darter (E. stigmaeum); greenside darter (E. blennioides); and emerald darter (E. baileyi) (M. McGregor, Kentucky Department of Fish and Wildlife Resources, pers. comm., January 2004). Only the greenside darter is found in Canada where its range appears to be expanding. Interestingly, the known current and historic range of the greenside darter does not completely overlap with the historic range of the Round Hickorynut (i.e., greenside darters are not known from the Grand River prior to 1990 and no records exist for this species from the Welland River or Lake Erie (A. Dextrase, Ontario Ministry of Natural Resources, Peterborough, pers. comm.)) suggesting the existence of an additional host. Round Hickorynuts are known to be gravid between September and June and may be using host fish during this time.
Dispersal: Like most native freshwater mussels, Round Hickorynut adults are essentially sessile with movement limited to only a few meters on the river/lake bottom. Although adult movement can be directed upstream or downstream, studies have found a net downstream movement through time (Balfour and Smock 1995; Villella et al. 2004). The primary means for large scale dispersal, upstream movement, and the invasion of new habitat or evasion of deteriorating habitat, is limited to the encysted glochidial stage on the host fish.
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