Planogyra asteriscus (E.S. Morse, 1857)
Basis for Listing
The Eastern Flat-Whorl snail (Planogyra asteriscus) extends from the maritime provinces of Canada and the New England states, west through the Great Lakes region to the north shore of Lake Superior (Hubricht 1985; Nekola 2009). While once considered globally threatened, it has recently been downgraded to being "apparently secure" (NatureServe 2013) due to its documented abundance in Maine, northern Michigan, and eastern Canada (Nekola 2008; 2009). Not known from Minnesota until 1998, it is currently known from only four sites in Cook County (Nekola et al. 1999). These represent the western known limit for this species’ global range. The Eastern Flat-whorl was listed as a species of special concern in 2013.
The Eastern Flat-whorl has a shell that is 1.8 mm (0.07 in.) wide by 1 mm (0.04 in.) tall. The shell has a lustrous brown color with a very low spire, making it almost flat on top. It has a simple aperture, lacking any thickening or lamellae. It is most easily recognized by the numerous elevated thin external ribs that run across the whorls. Also, the initial whorls have a granulose appearance at high magnification.
In Minnesota, the Eastern Flat-whorl occurs on mossy rock outcrops in white cedar and yellow birch in northern mesic hardwood forests and leaf litter accumulations in northern poor conifer swamps in the North Shore Highlands Subsection of the Northern Superior Uplands Section of the Laurentian Mixed Forest Province (Nekola et al. 1999). In these sites, the Eastern Flat-whorl lives in damp accumulations of northern white cedar (Thuja occidentalis) litter.
Biology / Life History
Little is known, specifically, about the life-history behavior of the Eastern Flat-whorl. Field observations suggest that this species is a generalist consumer of fungus, and perhaps algae, that cover decaying pieces of damp plant leaf litter, in particular needles and branches of western white cedar. Because individuals at all stages of development can be found at any given time within a colony, it is likely that they reproduce throughout the growing season, with individuals taking no more than eight weeks to reach adult maturity from hatching. It is also likely that individuals are able to live longer than one growing season.
Throughout its range, the Eastern Flat-whorl is limited to sites that not only support western white cedar but also possess a maritime climate due to their proximity to large water bodies: the Atlantic Ocean in the northeast and the Great Lakes in the continental interior. It thus seems likely that this species is unable to withstand extreme temperatures or long periods of drought.
A closely related species, the Western Flat-whorl (Planogyra clappi), is found in the temperate rain forests of the Pacific Northwest, where it too is limited to sites supporting cedar, in this case western red cedar (Thuja plicata) (Nekola et al. 2012). This suggests that the close relationship between these two genera extends far back in evolutionary time.
Conservation / Management
Three of the four locations for the Eastern Flat-whorl in Minnesota exist within the Cascade River and Temperance River state parks, with the remaining being found on a private inholding along the Superior Hiking Trail in Grand Portage State Forest.
Maintenance of Eastern Flat-whorl populations requires the persistence of western white cedar trees in conjunction with humid ground conditions. Activities that would open the ground layer to increased sun and wind and would thus decrease humidity at the ground level would be detrimental to populations. Additionally, activities which either increase or decrease water levels of the lowland forest / fen site should also be avoided.
Conservation Efforts in Minnesota
No conservation efforts are currently being undertaken on behalf of the Easter Flat-whorl in Minnesota; however, some of the known sites for this species exist on publically-owned lands, which should offer the species some protection.