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COSEWIC assessment and update status on the round hickorynut Obovaria subrotunda in Canada - 2003



The basic life history of the freshwater mussel is applicable to the round hickorynut, and is described briefly as follows (adapted from Kat 1984, Watters 1999, and Nedeau et al. 2000): during spawning, males release sperm into the water and females living downstream filter the sperm out of the water with their gills. Ova are fertilized in a specialized region of the female gills, called marsupia, where they are held until they reach an intermediate larval stage termed the glochidium. The female mussel then releases the glochidia, which must attach to an appropriate host and become encapsulated. The glochidia remain attached and are nourished by the host’s body fluids until they metamorphose into juveniles. The juveniles then break free of the capsule and fall to the substrate to begin life as free-living mussels. The proportion of glochidia surviving to the juvenile stage is estimated to be as low as 0.000001%. Mussels overcome the extremely high mortality associated with this life cycle by producing large numbers of glochidia.


The round hickorynut, like most freshwater mussels, is considered to be dioecious. Hermaphroditism has not been observed in this species. The lifespan of O. subrotunda is not known, but members of the subfamily Lampsilinae generally grow more rapidly and have shorter life spans than members of the Ambleminae, which can live for over 40 years (Stansbery 1967). For comparison, life spans of three other COSEWIC-listed lampsilines are: 10-20 years for L. fasciola (Metcalfe-Smith et al. 2000c), more than 15 years for Epioblasma torulosa rangiana (Staton et al. 2000), and up to 11 years for V. fabalis (Woolnough and Mackie 2002).

Obovaria subrotunda is a long-term brooder (bradytictic). Gravid females have been observed in every month except July in the Huron River, Michigan (van der Schalie 1938), and in all months of the year in the Cumberland River system of Tennessee and Kentucky (Gordon and Layzer 1989). According to Clarke (1981), the gravid period extends from about September to June in Canada. Clarke (1981) describes the glochidia as ovate in shape with a nearly straight hinge line, without hooks, and measuring 180 mm long and 200 mm high (200 mm long and 230 mm high, according to Hoggarth 1993). The lack of hooks suggests that they are gill parasites. The host fish is unknown; however, Clark (1977) noticed an association between the eastern sand darter (Ammocrypta pellucida) and the round hickorynut in the St. Joseph River system, a tributary to the Maumee River in the Lake Erie drainage. The eastern sand darter was designated as threatened in Canada in 1994, and many Canadian populations have declined or been extirpated. However, it still occurs in the East Sydenham River and Canadian waters of the St. Clair delta (Holm and Mandrak 1996), providing additional support for a possible host/parasite relationship with O. subrotunda. Furthermore, two other darter species, the naked sand darter (Ammocrypta beani) and southern sand darter (Ammocrypta meridiana), are known hosts of the Alabama hickorynut, Obovaria unicolor (Haag and Warren 2001).


In the adult form, freshwater mussels are basically sessile; movement is limited to a few metres of the lake or river bottom. The only time that significant dispersal can take place is during the parasitic phase. Infected host fishes can transport the larval unionids into new habitats, and can replenish depleted populations with new individuals. Dispersal is particularly important for genetic exchange between populations (Nedeau et al. 2000). The Sydenham River population of the round hickorynut is isolated and may be close to extirpation. The remaining population in Lake St. Clair is located entirely within Canadian waters. Thus, there is no natural means by which individuals from American populations could bolster the Canadian populations or repopulate the Canadian range if the Canadian populations should disappear.

Nutrition and Interspecific Interactions

Round hickorynuts, like all species of freshwater mussels, are filter feeders as adults. Their primary food sources are bacteria, algae, particles of organic detritus, and some protozoans (Nedeau et al. 2000). Food availability may be a limiting factor for the Lake St. Clair population due to the presence of high densities of zebra mussels, which are extremely efficient filter feeders. During the parasitic larval stage, glochidia feed on the body fluids of the host.