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Round Hickorynut (Obovaria Subrotunda)

Habitat

Habitat Requirements

The round hickorynut is typically found in medium-sized to large rivers (van der Schalie 1938; Strayer 1983; Parmalee and Bogan 1998), but also occurs in Lake Erie and Lake St. Clair (Clarke 1981; Strayer and Jirka 1997). During his study of the mussel fauna of the Huron River in southeastern Michigan, van der Schalie (1938) found O. subrotunda only near the mouth of the river and described it, as well as Quadrula pustulosa and Ligumia nasuta, as “invaders from Lake Erie.” Ortmann (1919) reported collecting this species in “smaller branches” of the Ohio River. The preferred habitat of the round hickorynut is generally described as sand and gravel substrates with steady, moderate flows at depths of up to 2 m (Ortmann 1919; Gordon and Layzer 1989; Parmalee and Bogan 1998). In southeastern Michigan, however, it has mainly been found in turbid, low-gradient, hydrologically unstable rivers with clay/sand or clay/gravel substrates (van der Schalie 1938; Strayer 1983). In Lake St. Clair, O. subrotunda currently occupies shallow (<1 m) nearshore areas with firm, sandy substrates (Zanatta et al. 2002).


Figure 3: Historical (1890-1990) and Current (1991-2001) Distributions of Obovaria subrotunda in Ontario (based on records from the Lower Great Lakes Unionid Database)

Figure 3: Historical (1890-1990) and current (1991-2001) distributions of Obovaria subrotunda in Ontario(based on records from the Lower Great Lakes Unionid Database).

The habitat preferences of juvenile mussels are believed to be different from those of adults, but there have been few studies on this topic (Gordon and Layzer 1989). The juvenile life stage is certainly more vulnerable than the adult stage, because juveniles have no control over the habitat into which they are released by their host and may die quickly in unsuitable habitats. The glochidial (larval) stage is the most vulnerable and specialized life stage, because the glochidia must successfully attach to an appropriate host in order to complete their metamorphosis to the juvenile stage. The extent and quality of habitat in the Sydenham River is probably insufficient to maintain viable populations of O. subrotunda, and it is not known at present if the St. Clair “refuge” will persist (see section on Population Sizes and Trends).


Trends

Habitats for O. subrotunda and other unionids in Lake Erie and Lake St. Clair have been largely destroyed by the zebra mussel. Native mussel communities were virtually extirpated from the offshore waters of western Lake Erie by 1990 (Schloesser and Nalepa 1994) and the offshore waters of Lake St. Clair by 1994 (Nalepa et al. 1996). The mussel communities of Lake Erie were already in decline, probably due to a general decline in water quality over the past 40 years (Nalepa et al. 1991), but Lake St. Clair still supported an abundant and diverse mussel assemblage as recently as 1986 (Nalepa and Gauvin 1988). Unionids continue to survive in some nearshore areas with very shallow water, a high degree of connectivity to the lake (which ensures access to host fishes), and harsh conditions for zebra mussels (high water temperatures and considerable wave action in summer; ice scour in winter). However, such “refugia” are rare, and most of the unionid habitat in the Great Lakes has been permanently lost.

The round hickorynut has apparently been lost from the Thames and Grand rivers, and has declined significantly in the Sydenham River. It has also declined throughout most of its range in the United States, particularly in the Tennessee River system (Parmalee and Bogan 1998). Agriculture is believed to be the main cause of the destruction of mussel habitat across North America (Strayer and Fetterman 1999). Since agriculture accounts for 75-85% of land use in the Grand, Thames and Sydenham River basins, it is likely that agricultural impacts (e.g., runoff of sediment, nutrients and pesticides, increased water temperatures due to loss of riparian vegetation, destruction of habitat by tractor crossings and cattle) are primarily responsible for the loss of mussel habitat in these rivers.


Protection/Ownership

Most land along the reach of the East Sydenham River where a few live specimens of O. subrotunda have been found in recent years is privately owned and in agricultural use. Only two small properties, the 7 ha Shetland Conservation Area and the 20 ha Mosa Township forest, are publicly owned and thus somewhat protected. It should be noted, however, that a recovery strategy has been developed for the aquatic ecosystem of the Sydenham River, and a number of landowners are participating in riparian rehabilitation projects and improved land use practices that will benefit O. subrotunda and other aquatic species at risk in the Sydenham River (Staton et al. 2002).

The most significant population of O. subrotunda left in Canada is located in the Canadian waters of the St. Clair delta, within the territory of the Walpole Island First Nation. The area is largely undisturbed and is likely to remain so in the future. The Walpole Island Heritage Centre is aware of the presence of O. subrotunda within their territory, and of the national significance of the population.