Stock Island Tree Snail
Stock Island Tree Snail
Orthalicus reses reses
|Listed||July 3, 1978|
|Family||Bulimulidae (Tree snails)|
|Description||Large conical snail with a long, white or buff shell and narrow, flamelike purple stripes.|
|Habitat||Tropical hardwood forests.|
|Food||Minute fungi and algae.|
|Reproduction||Mating occurs in September, producing 8-21 eggs the following June.|
|Threats||Limited distribution, residential and recreational development.|
The Stock Island tree snail is a large, conical tree snail, which attains a mature shell length of 2.2 in (5.5 cm). The translucent shell is thin and lightweight compared to most other snails of this genus. The color is white to buff with three poorly developed bands, and narrow flamelike purple stripes. The seven whorls, slightly rounded, increase regularly in size with a modestly impressed suture. The last whorl or two sometimes has two to three faint, narrow, brown spiral bands. There are also one to four dark brown bands on the shell that occur at irregular intervals. The interior of the shell is white, but distinctly shows the exterior markings through the translucent shell.
The Stock Island tree snail lives exclusively in trees, hiding in holes, bark crevices, and leaf clusters. It feeds on lichen, fungi, and algae, and is most active between June and December after a rainfall.Nests are constructed in soft dirt that is rich in leaf-mold directly at the base of the host tree. The nests are excavated initially with the posterior part of the snail foot to about 0.8 in (20 mm) deep. After excavation, the snail turns around and inserts the anterior head foot in the cavity, and continues to excavate the cavity to a depth of 1.6-2.4 in (40-60 mm).
The snail is hermaphroditic (with both male and female reproductive organs), although it is not self-fertilizing. It needs a mate to reproduce, and the mating process takes about 12 hours. The species begins mating at one year of age. Nest building takes place in September, shortly after mating. The eggs hatch the following June. About 8-21 eggs are laid per nest. Eggs are covered by soil as the snail crawls out of the nest. During dry weather, the snail enters a dormant state, known as aestivation. It aestivates in tree cavities that are sufficiently large for the snail to enter. A significant natural mortality occurs when tree growth decreases the size of the opening and traps aestivating specimens within.
This species is non-territorial (as many as 14 snails have been found in the same tree) but returns to the same tree cavity to aestivate. Individuals may favor certain limbs, bole, or fork in the tree to rest before foraging activities, but the species is mobile and frequently moves to new trees.
This snail is seasonally active. It aestivates during the dry season, usually December to May. In June it becomes active if the wet season has begun; otherwise it returns to aestivation on the surface of large limbs. During the wet season, it is more frequently active at night.
The Stock Island tree snail probably grazes on minute fungi and algae that grow as epiphytes on tree leaves and stems. Bacteria have also been found in the guts of some specimens but it is not know if they are a food source or a contaminant.
The Stock Island tree snail inhabits a wide range of tropical hardwood trees and has adapted to several types of exotic ornamentals. It apparently has no host tree preference and has been found on such native trees as sweet acacia, saffron plum, gumbo-limbo, icaco-coco plum, and mahogany. Non-native trees include Jamaica caper, lead tree, and tamarind. It is most common in mature hammocks but does occur in second-growth and new-growth thickets.
The historic range encompassed Stock Island and Key West in Monroe County, Florida. The snail has since disappeared from Key West.
When the snail was placed on the federal list in 1978, the population was estimated between 200-800 individuals, found at two sites on Stock Island. In 1986, a survey conducted by The Nature Conservancy determined that a major portion of the habitat had been lost to residential development and golf course expansion. Fewer than 100 of the snails were thought to survive on about 5 acres (2 hectares) on Stock Island adjacent to the Key West Municipal Golf Course. In 1987, Fish and Wildlife Service (FWS) biologists visited the site and determined that the snail population was confined to about 20 trees and probably numbered less than 50.
The population seems to be falling still more sharply. By the mid 1990s, surveys were unsuccessful in finding any Stock Island tree snails within its historical range. With the cooperation of the Florida Game and Fresh Water Fish Commission, Stock Island Golf Course, Key West Botanical Gardens, and U.S. Navy, the FWS is taking recovery actions, including reintroductions, to reestablish this unique mollusk into part of its historical range.
The causes of the species' decline are not certain, but fragmentation of hardwood hammocks, changes in microclimate, and increased predation have been suggested as possible reasons for its loss. The snail's remaining range is so restricted, almost any natural or man-made disaster could render the species extinct. Much of the ground area near inhabited trees has been paved to accommodate a county parking lot. County workers, in a misguided effort to protect snail-inhabited trees, surrounded the bases of the trees with gravel. This gravel deprived the snail of access to leaf litter and humus in which to lay its eggs. In 1988, the FWS successfully negotiated with the county to remove the gravel and replace the natural soil and organic litter.
Conservation and Recovery
The precipitous decline of the snail population has forced FWS personnel to consider translocating snails and eggs to a more protected location. The population has declined to such a low level, however, that translocation might actually exterminate the snail. A second problem with translocation is that no suitable habitat appears to remain in the Florida Keys. Biologists have little material to work with and no room for error. The FWS and Florida chapters of the Audubon Society and The Nature Conservancy have unsuccessfully surveyed the region for other populations of the snail. The FWS South Florida Ecosystem Office in Vero Beach was moving forward with efforts to reintroduce the Stock Island tree snail starting in 1996.
U.S. Fish and Wildlife Service
Division of Endangered Species
1875 Century Blvd, Ste 200
Atlanta, Georgia 30345
U.S. Fish and Wildlife Service. 1982. "Stock Island Tree Snail Recovery Plan." U.S. Fish and Wildlife Service, Atlanta.