Orthalicus reses, the Stock Island tree snail or the Florida tree snail, is a species of large tropical air-breathing tree snail, a terrestrial pulmonate gastropod mollusk in the family Orthalicidae. It was first described in 1830 by the American naturalist Thomas Say. The holotype, a specimen probably collected in Key West, was subsequently lost. Over a hundred years later, in 1946, the American biologist Henry Augustus Pilsbry redescribed the species using a specimen from Stock Island, Florida. Orthalicus reses has two subspecies, O. reses reses and O. reses nosodryas. The validity of these two taxa is still being discussed, but some experts argue that considering them as independent units may be important for management purposes.
The Stock Island tree snail has a large conical shell (45–55 mm in length) with variable thickness, generally lighter and more translucent than other species of Orthalicus. It is colored white to buff, with weakly developed spiral bands and several flame-like, purple-brown axial stripes. The subspecies O. reses reses and O. reses nosodryas can be distinguished from one another based on the different color patterns of the apical whorl, columella, and parietal callus.
The nominate subspecies Orthalicus reses reses is threatened; it lives on trees in hardwood hammocks habitat in Southern Florida, USA, specifically the Florida Keys.
Taxonomy Orthalicus reses is a species within the genus Orthalicus, a group of large, arboreal pulmonate snails in the family Orthalicidae. The genus Orthalicus occurs primarily in Central and South America. Two species occur in North America, Orthalicus reses and Orthalicus floridensis Pilsbry, both of which are restricted to South Florida. The Stock Island tree snail (Orthalicus reses) was first described by Thomas Say in 1830, based on a snail which was probably collected in Key West. That specimen was lost, and the species was later redescribed by Henry Augustus Pilsbry in 1946 using a snail collected from Stock Island.
Orthalicus reses has two subspecies: Orthalicus reses reses (Say, 1830) - Stock Island tree snail Orthalicus reses nesodryas Pilsbry, 1946 - Florida Keys treesnail
It is generally thought that the two subspecies of Orthalicus reses do not interbreed due to anatomical incompatibility.U.S. Fish and Wildlife Service. (1982). Stock Island tree snail recovery plan. U.S. Fish and Wildlife Service; Atlanta, Georgia.U.S. Fish and Wildlife Service. (1996). Multispecies Recovery Team/Keys Group meeting, April 1996. However, Emmel and Perry (2004) recommended that if the two Orthalicus reses subspecies prove to be as genetically invariant as was observed in their study, the groups should be considered a single taxon or taxonomic unit. In that case, the nominate form, Orthalicus reses reses, would prevail, and Orthalicus reses nesodryas would be demoted to a synonym instead of a valid subspecies. Emmel and Perry (2004) however asserted that the taxa "should continue to be considered as independent units for management purposes".
Description
Orthalicus reses is a snail with a large, conical shell that is approximately 45 to 55 mm in length. The thickness of the shell varies, but is usually more lightweight and translucent than other species of Orthalicus.Pilsbry, H.A. (1946). Land Mollusca of North America (north of Mexico). Monograph Academy of Natural Science, Philadelphia 3[2(1)]: 1-520. The external ground color of the shell is white to buff, with three poorly developed spiral bands and several flame-like purple-brown axial stripes that stop at the lower of the spiral bands.Deisler J.E. (1982). The Stock Island tree snail Orthalicus reses reses (Say). Pages 8-9. In: Franz R., (ed.) Rare and endangered biota of Florida, volume 6, invertebrates. University Press of Florida; Gainesville, Florida. The axial stripes are typically narrower than their whitish interspaces and do not fork near the upper suture. There are two to three white apical whorls. The last whorl contains two to four darker brown growth-rest varices. The columella and parietal callus are white or faint chestnut brown.
The nominate subspecies Orthalicus reses reses is distinguished from the supposed subspecies Orthalicus reses nesodryas by the lighter color pattern of the apical whorl, columella, and parietal callus. These characteristics are chestnut-brown or darker in Orthalicus reses nesodryas.
Distribution
Henry Augustus Pilsbry suggested, in 1946, that Orthalicus reses reses arrived in Florida from Central America and the Caribbean shortly after the emergence of the Florida peninsula in the late Pleistocene. Snails that were sealed in place on floating tropical trees may have been cast ashore on the Florida peninsula by high winds and hurricanes. This form of dispersal has been suggested for both Orthalicus and Liguus, but the exact origin of these species is still in question. In 1972, Craig suggested that populations of Orthalicus arrived directly across the Gulf of Mexico from Central America, but the mode of transportation and whether dispersal occurred as a single event or multiple events was not known.Craig, A.K. (1972). "Observations on the arboreal snail Orthalicus floridensis". Quarterly Journal of the Florida Academy of Sciences 35(1): 15-20. No one knows when Orthalicus reses reses arrived in the Lower Keys.
Historically, the Stock Island tree snail was believed to have a very limited distribution, being found only in tropical hardwood hammocks on Stock Island and Key West; although it may have been found in other hammock areas in the Lower Keys. The distribution has since been artificially extended by collectors, who have introduced them to Key Largo and the southernmost parts of mainland Florida. Orthalicus reses nesodryas has a broader range, occurring throughout the Florida Keys from Sugarloaf Key north. Orthalicus floridensis is the only Orthalicus species to occur naturally on the mainland, and is also found in the Keys. This species is known to occur in the National Key Deer Refuge.
Behavior Orthalicus reses reses snails are active mainly during the wet season, i.e. May through November, during which time breeding, feeding, and dispersal takes place. Dry periods (December through April) are spent in aestivation, during which time the snail forms a tight sealed barrier between the aperture and a tree trunk or branch. Snails secrete this mucus seal (an epiphragm) that cements their shell to a tree in order to protect them from desiccation during the dry period. Snails may come out of aestivation briefly to feed during dry-season rains, and also may go into aestivation during summer dry spells.
Ecology Habitat Originally, Orthalicus reses occurred exclusively in hardwood hammocks of the Keys. Orthalicus reses survives best in hammocks with smooth-barked native trees that support relatively large amounts of lichens and algae. In the Florida Keys, Orthalicus is limited to the higher portions of the islands that support hammock forests (minimum elevations of 5 to 11 feet).
Lower Keys hammocks consist of thick forests of tropical trees and shrubs which grow in limestone, marl, and calcareous sand soils. Canopy trees include black ironwood (Krugiodendron ferreum), gumbo limbo (Bursera simaruba), Jamaican dogwood (Piscidia piscipula), mahogany (Swietenia mahagoni), pigeon plum (Coccoloba diversifolia), poisonwood (Metopium toxiferum), and strangler fig (Ficus aurea). Hammock understory contains torchwood (Amyris elemifera), milkbark (Drypetes diversifolia), wild coffee (Psychotria nervosa), marlberry (Ardisia escallonioides), stoppers (Eugenia sp.), soldierwood (Colubrina elliptica), crabwood (Gymnanthes lucida), and velvetseed (Guettarda scabra).
Larger trees are more likely to support more Orthalicus reses reses snails than smaller trees, probably because they provide the snails with an increased surface area for foraging.Deisler, J.E. (1987). The ecology of the Stock Island tree snail, Orthalicus reses reses (Say)". Bulletin Florida State Museum Biological Science 31(3): 107-145. It is not known if Orthalicus reses reses prefer certain tree types or species; however, Voss (1976)Voss, R. S. (1976). "Observations on the ecology of the Florida tree snail Liguus fasciatus (Muller)". Nautilus 90: 65-69. suggested tree snails generally prefer trees with smooth bark rather than trees with rough bark, because it would require less energy to crawl over smooth bark. Voss also believed Orthalicus reses reses would prefer smooth bark because it would make it easier for them to form a secure mucous seal when they were aestivating, resulting in lower mortalities from dehydration or accidental dislodgement.
Orthalicus reses reses snails are entirely arboreal except when they move to the forest floor for nesting or traveling. Hammocks that contain well-developed soils or leaf litter are important for nesting activity and dispersal. Essential factors affecting food availability are the light intensity and moisture content of the hammock habitat.
No data are available on minimal hammock size needed to support a viable population of tree snails. Suitable habitat would have to include an area large enough to provide for foraging and nesting requirements, as well as provide for the microclimate (air temperature and humidity) needed by the Orthalicus reses reses. Preferences for edge or interior hammock have been observed in Liguus populations on Lignumvitae Key.Tuskes, P.M. (1981). "Population structure and biology of Liguus tree snails on Lignumvitae Key, Florida". Nautilus 95(4): 162-169. Age-class differences were seen where older individuals were found in the central mature hammock, while younger individuals were found more often along the edges of the hammock. Tuskes (1981) suggested this may be an adaptation of younger snails to move to the edge to escape competition from older snails. It is not known whether Orthalicus reses reses prefer interior or edge hammock areas. Recent surveys of snails in the Key Largo populations have shown higher numbers of snails along the edge of the hammock than in the interior, but this result may be affected by the differences in visibility during surveying.
Feeding habits Little is known about the feeding habits or food preferences of Orthalicus reses reses. They feed on epiphytic growth on hardwood tree trunks, branches, and leaves. Likely food items include a variety of fungi, algae, and lichens found on many of the native hammock trees. Mixobacteria and some small mites may serve as a secondary food source.
The rate at which individuals of Orthalicus reses reses grow is dependent upon the availability of food, and how quickly their food is replenished after being grazed. Regrowth of the epiphytes is affected by the light intensity and moisture (canopy density and climate) of the hammock habitat.
Feeding can occur at any time during the day or night, with peak feeding activity occurring from late afternoon through the night to mid-morning and during or immediately after rainfall. When they are active, Orthalicus reses reses often follow a random twisting path that covers the entire bark surface, but they will move in a straight line if the surface moisture is abundant.
Life cycle