Cherrystone drop  (Hendersonia occulta)

The cherrystone drop has yet to be classified by the IUCN.

The only species in its genus, the cherrystone drop (Hendersonia occulta) is a tiny terrestrial snail which is named for its resemblance to the stone or pit of a cherry (3). Its minute, domed shell varies in colour from cinnamon-red or reddish-brown to pink, tan, orange or pale yellow (2) (3) (4) (5), and is marked with fine, spiral lines across its surface (2) (3) (4) (6).

The shell of the cherrystone drop is wider than it is high, and consists of 4.5 to 6.5 whorls, which coil to the right (2) (3) (4) (5). It is rather thick and has a crescent-shaped opening, surrounded by a greatly thickened lip that forms a projecting ridge (2) (3) (4). The cherrystone drop is able to seal itself inside its shell using a tiny plate known as an ‘operculum’ (2) (3) (5).

Unlike many other snails, the cherrystone drop does not have a hole, known as an umbilicus, at the base of its shell (3) (4).

The cherrystone drop has a widespread but patchy distribution in the north-eastern United States, from southwest Pennsylvania, western Virginia and North Carolina, west to Oklahoma, and north to Minnesota, Wisconsin and Michigan (2) (3).

The cherrystone drop is unusual among the members of the Helicinidae family in occurring in temperate rather than tropical areas (3). It typically occurs in cool, moist, leafy and shady habitats near streams, rivers or lakes (3) (4) (5) (6). This small snail is often associated with calcium-rich soils and is commonly found on steep slopes, such as cool, limestone talus (loose rock) slopes, as well as in ravines, floodplains and forest (3) (4).

During dry weather, the cherrystone drop can usually be found sheltering under twigs, logs, leaf litter or stones (3) (6). However, it is often seen crawling on leaves, rocks, logs or tree trunks after rain (3).

Very little is currently known about the biology and life history of the cherrystone drop. It is usually seen between April and September, and is likely to become inactive in autumn and winter, when it probably hides in small crevices (3).

The cherrystone drop feeds on vegetable matter, which it scrapes using a rasping ‘tongue’, or radula, that is covered in rows of hard teeth (3) (5).

Although many snails are hermaphroditic and are capable of self-fertilisation, in the cherrystone drop individuals are either male or female. This species lays its eggs in moist locations, such as beneath leaf litter or logs (3) (5).

This tiny, little-known snail is listed as ‘Threatened’ in the states of Michigan and Wisconsin (3) (4) (5). Its populations are small and isolated, and appear to have quite specific habitat requirements, making the cherrystone drop particularly vulnerable to any changes in its local environment (2) (5) (7). Its small size and low mobility also mean that it would not easily re-colonise areas from which it has been lost (3) (7).

The main threats to the cherrystone drop are the loss and disturbance of its habitat, for example through timber harvesting, changes to the water cycle, or the dumping of rubbish (3) (5) (7). The cherrystone drop is also vulnerable to trampling by visitors at some sites (3) (4) (7), although in others it occurs on private land (3).

In Pennsylvania, an important cherrystone drop population may be threatened by a proposed highway bridge (7) (8). Although the proposed bridge was moved a short distance away to avoid disturbing a rare plant species, road salt may still wash from the highway in winter, and it may also provide a corridor for invasive species (8).

The most important conservation measure for the cherrystone drop will be the protection of its habitat (3) (4) (7). In particular, it may need the presence of an intact tree canopy, while measures to restrict visitor numbers may be necessary at some sites (3) (7). Where the cherrystone drop occurs on private land, its conservation will rely on informing and working with the landowners (3).

Further research is needed into the biology, distribution and habitat requirements of the cherrystone drop, in order to better inform conservation measures for this rare snail. Studies are also needed into the effects of various management practices, such as timber harvesting, on its populations (3).

In Pennsylvania, a project studying land snails in limestone areas has been producing updated distribution maps which may help in assessing conservation priorities for the state’s snail species, including the cherrystone drop (9).

Find out more about the cherrystone drop and its conservation:

• Lee, Y. (2001) Special animal abstract for Hendersonia occulta (cherrystone drop). In: Michigan Natural Features Inventory. Michigan Natural Features Inventory, Lansing, Michigan. Available at:
  http://web4.msue.msu.edu/mnfi/abstracts/zoology/Hendersonia_occulta.pdf
• Wisconsin Department of Natural Resources - Cherrystone drop (Hendersonia occulta):
  http://dnr.wi.gov/ORG/LAND/ER/biodiversity/index.asp?mode=info&Grp=24&SpecCode=IMGAS03010