Achatinella mustelina

Geographic Range

Achatinella mustelina is an arboreal pulmonate gastropod that is endemic to the island of O’ahu in the Hawaiian archipelago. This endangered terrestrial snail has a fairly large distribution from the southern edge of the island to the northern boundary of the Wai’anae Mountains.

• Biogeographic Regions
• oceanic islands oceanic islands
  • native native

• Other Geographic Terms
• island endemic island endemic

Habitat

Achatinella mustelina inhabits elevated forests that are dry, wet, or mesic. Generally, A. mustelina attaches itself to leaves at the tops of native trees and shrubs such as Metrosideros polymorpha , Dubautia plantanginea , Myrsine lessertiana , Pisonia sandwicensis , Antidesma platyphyllum and Nestegis sandwicensis . Individuals may live on a single tree for their entire lifetime.

• Habitat Regions
• tropical tropical
• terrestrial terrestrial

• Terrestrial Biomes
• forest forest
• mountains mountains

Physical Description

Achatinella mustelina from different locations vary in size, shape, and color, but no specific patterns in these differences have been described. In general, adults range 19 – 24 mm in length, with an average length of 21.4 mm. The shells have a shiny finish and are usually brown with light bands that circle the suture convex, or they are white with transverse black or brown lines. Shells of A. mustelina can be either dextral or sinistral and consist of five to seven convex whirls. The shell is relatively high-spiraled and oblong to ovate with an aperture that is also oblong and ovate. The columella of A. mustelina is short, stout, and slightly twisted, and it has a well-developed spiral lamella, or ridge. A callus on the columella closes the umbilicus. The lip of the organism does not have any ridges, ribs, or folds.

• Other Physical Features
• ectothermic ectothermic
• heterothermic heterothermic
• bilateral symmetry bilateral symmetry

Development

Achatinella mustelina develops from an intrauterine embryo, and its growth in utero is thought to be logarithmic. Young are born live and are approximately 4.5 mm in length. The snail grows at a relatively slow rate of 2 mm per year, and over the course of four to five years, it will only grow to be five times its birth length. Growth in A. mustelina is determinate and individuals reach their maximum size before becoming reproductively mature. A unique characteristic of development in A. mustelina is that individuals of different sizes grow at the same rate.

Reproduction

Achatinella mustelina is hermaphroditic, and may self-fertilize.

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

Achatinella mustelina has a lengthy gestation period and gives birth to large, live young that mature late and have a low fecundity. Although reproductive maturity occurs after the maximum size is reached, reproductive maturity may be dependent on age, not size. Reported age at maturity varies, ranging from 3 to 6.9 years. Similarly, reports of fecundity vary, but rates as low as 0.4 offspring/adult/year have been observed, as well as rates as high as 7 offspring/adult/year.

Little information about reproductive behavior in Achatinella mustelina is known, but this species is hermaphroditic and thought to breed year-round. Some species of the genus Partulina , sister to Achatinellidae , are thought to self-fertilize, so A. mustelina may self-fertilize as well. Partulina redfieldii can reproduce even in long-term absence of mates. The mechanism of this form of reproduction is not clear, however. Possibilities include parthenogenesis and long-term sperm storage, in addition to self-fertilization.

Genetic evidence suggests interchiral mating takes place for A. mustelina . Other terrestrial gastropods with high-spired shells like A. mustelina align themselves parallel and mount the other snail’s shell when mating. One snail inserts its penis in to the other and releases spermatophores. Embryos are intrauterine, and no more than two large embryos are contained in the uterus; usually there is only one. The large size of newborn A. mustelina , approximately 4.5 mm in length, limits the size of the maternal snail.

• Key Reproductive Features
• year-round breeding year-round breeding
• simultaneous hermaphrodite
• sexual sexual
• fertilization fertilization
  • internal internal
• ovoviviparous ovoviviparous
• sperm-storing sperm-storing

Although the literature does not include specific information about parental investment in Achatinella mustelina , in general, terrestrial gastropods do not provide parental care after birth. The most common form of parental investment for terrestrial gastropods comes in the form of calcium carbonate and other nutrients given to the developing egg.

• Parental Investment
• pre-fertilization
  • provisioning

Lifespan/Longevity

Achatinella mustelina has a relatively long lifespan compared to other terrestrial gastropods. Longevity is estimated to be 10 years, but individuals may live up to 15 or 20 years.

Behavior

The Oahu tree snail is a terrestrial snail that is primarily found in native Hawaiian trees, such as Osmanthus sandwicensis , Gouldia sp. , Metrosideros polymorpha , and large bushes. Generally, this snail attaches to leaves at the tops of native trees and shrubs, and is nocturnal. Individuals of A. mustelina enter a stage of inactivity during the numerous dry periods of its environment, curtailing feeding and growth. Individuals are not highly motile, resulting in many small, relatively isolated populations of A. mustelina .

• Key Behaviors
• arboreal arboreal
• nocturnal nocturnal
• motile motile
• sedentary sedentary

Home Range

The home range of A. mustelina is often limited to a single tree and the nearby surrounding shrubbery. Individuals rarely move between trees.

Communication and Perception

Achatinella mustelina , like most terrestrial gastropods, communicates through both touch and chemical signaling, using the lower set of head tentacles. Individuals transmit pheromone signals both through direct contact and through the mucous trail left during locomotion. This includes warning of predator presence.

Achatinella mustelina has simple eyes for visual perception, albeit limited to larger objects. However, vision is not a primarily means of searching or foraging, given the snail’s nocturnal habits.

• Communication Channels
• tactile tactile
• chemical chemical

• Other Communication Modes
• pheromones pheromones

• Perception Channels
• visual visual
• tactile tactile
• chemical chemical

Food Habits

Achatinella mustelina feeds primarily at night. This species is a mycophage, grazing upon epiphytic fungi growing on the bark or leaves of native plant species, such as Osmanthus sandwicensis , Gouldia sp. or Metrosideros polymorpha . Few imported species of plant are suitable for the fungi eaten by this snail species. In captivity, A. mustelina has been grown on cornstarch or sooty mold, with a supplement of cuttlebone providing the calcium necessary for shell growth.

• Primary Diet
• mycophage mycophage

• Other Foods
• fungus

Predation

The primary predator of Achatinella mustelina is the introduced carnivorous snail, Euglandina rosea , and A. mustelina has no defense mechanisms for this species. Several rat species eat A. mustelina , particularly the larger individual snails. Other foreign species that prey upon A. mustelina include the terrestrial flatworms Geoplana septemlineata and Platydemis manokwari . The Hawaiian Thrush ( Phaerornis obscura ) eats A. mustelina , although it isn't the bird's primary food source.

Ecosystem Roles

Achatinella mustelina consumes fungi from plant bark and leaves. Historically, A. mustelina had no natural predators that relied upon it as a food source. With the introduction of the predatory snail Euglandina rosea , A. mustelina , along with other sympatric members of the genus Achetinella have become prey for this species.

Economic Importance for Humans: Positive

Due to their vibrant coloration, Achatinella mustelina shells were collected by human inhabitants of the island of Oahu to craft traditional leis and other ornaments. Shells from the genus Achatinella are still collected and sold as ornaments today as part of Hawaii’s tourist trade.

Economic Importance for Humans: Negative

There are no known negative effects of Achatinella mustelina on humans.

Conservation Status

Achatinella mustelina is the currently the most abundant species of the O’ahu tree snails and has been studied considerably for conservation efforts. The slow growth, long pre-reproductive life, and low fertility of A. mustelina , in conjunction with its relatively sedentary lifestyle and small geographic range, make A. mustelina populations very vulnerable to disturbances, either from predation, human collection, or habitat destruction. Lowland habitat destruction by human inhabitants for the purposes of farming and logging have reduced the geographic range of A. mustelina to only high elevation mountainous forests.

However, the introduction of the invasive predatory snail Euglandina rosea from North America led to the rapid and widespread decline and destruction of A. mustelina populations. Euglandina rosea predates opportunistically on a number of species of terrestrial snails. Only a few hundred individuals of A. mustelina are estimated to remain in the wild.

Additional Links

Encyclopedia of Life

Contributors

Peter Bicescu (author), The College of New Jersey, Colleen Stalter (author), The College of New Jersey, Keith Pecor (editor), The College of New Jersey, Renee Mulcrone (editor), Special Projects.

References

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