Oahu Tree Snails

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Oahu Tree Snails

Achatinella spp.

Status	Endangered
Listed	January 13, 1981
Family	Achatinellidae (Hawaiian tree snails)
Description	Brightly colored spherical, conical, or egg-shaped shell.
Habitat	Undisturbed native forests.
Food	Fungi, algae.
Reproduction	Hermaphroditic; young born alive.
Threats	Deforestation, collectors, predation.
Range	Hawaii

Description

The Achatinella are divided into three subgenera according to shell shape, which is either spherical, conical, or egg-shaped. Adult shell length is 0.2-1 in (0.5-2.5 cm), depending on the species. The number of whorls varies between five and seven. All species are brightly colored and distinctively patterned. Most species have glossy shells, but some have a sculptured surface.

Some species of Achatinella, such as A. juncea, A. buddii, and A. papyracea, were rare as long ago as the 1930s. Other extremely rare species, such as A. lehuiensis, A. thaanumi, and A. spaldingi, became extinct soon after being discovered. It is believed that only 19 of 41 documented species of the snail still exist. All surviving species are federally listed as Endangered.

Those species still living, but in danger of extinction, are A. apexfulva, A. bellula, A. bulimoides, A. byronii, A. concavospira, A. curta, A. decipiens, A. fulgens, A. fuscobasis, A. leucorraphe, A. lila, A. lorata, A. mustelina, A. pulcherrima, A. pupukanioe, A. sowerbyana, A. swiftii, A. taeniolata, and A. turgida.

Behavior

Oahu tree snails live singly or in small clusters in the crevices of tree bark, on the undersides of branches and foliage, or in deep leaf litter around the base of the trunk. Most species are nocturnal, grazing on leaf and bark surfaces for microscopic algae, fungi, liverworts, and detritis, but apparently do not feed upon the leaves or bark themselves. Individual snails are sedentary and may spend their entire life on a single tree.

As a group, the Achatinella are hermaphroditic (with both male and female reproductive organs) but not self-fertilizing. They breed year round and usually bear one live young, born complete with shell. Studied species were found to grow at a rate of about 0.08 in (2 mm) in length per year and to reach sexual maturity in six or seven years.

Habitat

The island of Oahu encompasses some 607 sq mi (1,433 sq km). Over 35% of this land area is devoted to urban areas or agricultural crops, 45% to grazing land, and less than 20% to forest lands, mostly at high elevations. Only a small portion of this forest (2-3%) is still in relatively pristine condition.

Oahu tree snails are dependent upon the remnants of undisturbed forests and upon native trees. A single species of snail is often associated with a specific tree species. Rarely do any of these species cross over to trees that have been introduced from outside the island.

The snails occur primarily in subtropical moist or tropical wet evergreen forest land. The Oahu tree snails usually occur at elevations above 1,000 ft (305 m) where native species predominate. In mountain bogs, the snails may also inhabit forested wetlands.

Distribution

Much of the island of Oahu, Hawaii, was once heavily forested, and tree snails were dispersed throughout. It is estimated that the original extent of forested land has been reduced by about 85% by agriculture and human settlement. Native trees have declined in large numbers, losing ground to introduced, non-native trees.

Tree snails survive in scattered enclaves throughout portions of the historic range, typically at higher elevations along the crests of ridges where the forests are least disturbed. Only two such areas on Oahu remain—the Waianae Range, and the Koolau Range. Current population levels are low for all of the tree snails, perhaps only five to ten percent of 1960 levels.

The occurrence of snails on the Makua Military Reservation and Schofield Barracks Military Reservation, as well as its possible occurrence at the Mauna Kapu Communication Station, was reported in detail in the 1970s and 1980s. Snails also occur (or occurred) at the site of a now abandoned Nike Missile Facility near Peacock Flats and in the vicinity of the access road to the Air Force/Federal Aviation Administration facility on Mt. Ka'ala. The snails' occurrence on the Lualualei Naval Reservation is probably restricted. Virtually all snail populations occur on lands zoned by the State as Conservation lands and as Forest Reserves, affording the snails and their habitat considerable protection. However, their ranges are greatly reduced and observations show their numbers to be rapidly declining.

A biologist, who extensively surveyed the snails' forest habitat in the early 1980s, has suggested that the most abundant of these endangered species are A. mustelina and A. sowerbyana, numbering perhaps 400 individuals each. Estimated at about 200 individuals or less were the species A. curta, A. decipiens, and A. lila. The species A. fuscobasis and A. pupukanioe numbered perhaps 100 individuals each. Achatinella concavospira and A. pulcherrima numbered 50 or less individuals each. Achatinella bellula, A. bulimoides, A. byronii, A. fulgens, A. leucorraphe, A. lorata, A. swiftii, A taeniolata, and A. turgida, all numbered less than 20 individuals. There is no current population estimate for the species A. apexfulva, and population numbers for all species have probably declined from the 1980s estimates.

Threats

The ancient Hawaiians believed the tree snails sang, though later it was realized crickets were doing the "singing." But their mystique lived on, and the richly colored shells made Oahu tree snails attractive to collectors. While appreciation for tree snails helped inspire a greater awareness of the diversity and uniqueness of the Hawaiian flora and fauna, collecting also reduced certain localized populations of tree snail species.

Another obvious reason for the precipitous decline of tree snail populations is the loss of native forests on the island. Oahu's lower elevation forests were long ago converted to agriculture, pasture, or for residences. Much of the remaining forests have been degraded by the intermingling of non-native trees and plants, changing the composition of plants; this in turn affects the abundance and dispersal of the snails' chief food sources—the algae and fungi.

Hikers along forest trails often collect the brightly colored snails. But because of late sexual maturity and a slow growth rate (natural replacement of a reproducing adult takes at least six years), tree snail populations can be quickly depleted by even casual collecting. Several of these tree snails have come to the attention of more serious collectors, who seem willing to finance the depletion of these rare snails by purchasing specimens.

Another immediate threat for these tree snails is an exotic, carnivorous snail (Euglandina rosea ), that was introduced from Florida in 1955 to control another snail pest, the giant African snail (Achatina fulica ). This carnivorous snail had little impact on the African snail, but in areas where it has become established, Oahu tree snails are now entirely absent. Other predators, including additional alien land mollusks, a terrestrial flatworm, and ants are also thought to be a continuing threat to the tree snails. The snail's low reproductive rate and the unusually long period required for them to reach sexual maturity is another important factor rendering these snails extremely vulnerable to extinction under unnaturally high predation rates.

Conservation and Recovery

Recovery for these tree snails will depend ultimately upon saving the remaining acreage of native forests on Oahu, stopping collectors, and ridding the habitat of predators. The Fish and Wildlife Service (FWS), the state of Hawaii, and The Nature Conservancy of Hawaii cooperate in an aggressive campaign to acquire and protect forest land. The results of these efforts are slow to materialize, and it remains an open question whether many of these tree snails will survive.

The primary objective of the FWS 1992 Oahu tree snail Recovery Plan is to stabilize populations of tree snails found within essential habitats and to initiate captive propagation of all extant Oahu tree snail species. The plan notes that, before captive-reared snails are placed in the field, the snails should be individually marked so growth and migratory pattern can be followed after they are released. All introduced populations should be monitored to determine their growth or decline. The plan also outlines numerous strategies for species recovery, including surveys to assess the population status; research on population biology to understand life history parameters, reproduction, foraging behavior, etc.; habitat preservation to maintain the natural integrity of protected habitat; forest fire prevention; banning the import of noxious alien plants and animals that may prey on the tree snail or destroy its habitat, and controlling the existing populations of alien predators and parasites; coordinating recovery planning and implementation with the U.S. military (where the species occurs on military property); and enforcement against collection and poaching of the snails for their shells.

Contact

U.S. Fish and Wildlife Service
Division of Endangered Species
Eastside Federal Complex
911 N.E. 11th Avenue
Portland, Oregon 97232
http://pacific.fws.gov/