Kāhuli Tree Snails

Description

The kāhuli tree snail (Achatinella mustelina), also called the Oʻahu tree snail, is a small, jewel-like land snail that lives its entire life in the trees of Oʻahu’s Waiʻanae Mountains. Its name can cause confusion: “kāhuli” is a Hawaiian word for land snails, and “kāhuli tree snail” and “Oʻahu tree snail” are used for all 41 described species in the genus Achatinella, and often more loosely for related tree snails on other Hawaiian islands. This entry covers Achatinella mustelina alone, the most widespread and best-studied member of the genus.

The shell is glossy, oblong to egg-shaped, and drawn up into a fairly tall spire of five to seven rounded whorls. Adults measure about 0.7 to 0.9 inch (18 to 23 millimeters) long, about as long as a nickel is wide. Every individual is both male and female, so there are no differences in size or color between sexes. Most shells are brown with a pale band along the seam between whorls, or white to buff with dark spiral lines and bands, in shades running from tan to deep brown. When a snail reaches full size, the outer lip of the shell thickens, which is how biologists tell adults from juveniles in the field.

Some shells coil to the right and others to the left, often within the same population. Within its range, the kāhuli tree snail is easily separated from the much smaller native Auriculella snails that share its host trees. It is an enduring symbol of how much variety evolution can pack into a single mountain range.

Range and Habitat

The kāhuli tree snail lives only on the island of Oʻahu, in the Waiʻanae Mountains, a ridge system about 15 miles (24 kilometers) long on the island’s west side. Its range runs from the northern Waiʻanae around Mākua and Kahanahāiki south to Palikea, giving it the largest natural range of any Achatinella species. Snails once occurred more or less continuously along these ridges. Today they survive in scattered, isolated patches, many of them inside fenced enclosures.

Kāhuli tree snails barely move between trees, let alone between mountain ridges. As a result, populations separated by steep valleys have stayed apart long enough to become genetically distinct. Researchers divide the species into six or seven isolated genetic groups, and managers treat each one almost like a separate species.

This snail is a forest specialist. It lives mainly in native mesic (moderately moist) and wet forest above about 2,000 feet (610 meters), reaching about 3,800 feet (1,160 meters) near Mount Kaʻala, the highest point on Oʻahu. It spends nearly its whole life on the leaves and branches of native trees and shrubs, including ʻōhiʻa lehua, olopua, pāpala kēpau, kōlea and hame. It needs a cool, humid canopy that keeps leaf surfaces damp enough to grow the microbes it eats. Snails are sometimes found on non-native trees such as strawberry guava, but it is not known whether those plants can support healthy populations over the long term.

Diet and Foraging

Unlike garden snails, the kāhuli tree snail does not eat leaves. Instead, it feeds on the thin film of fungi, bacteria and other microorganisms that grow on the surfaces of leaves and bark. It scrapes this film off with its radula, a ribbon-like tongue lined with tiny teeth, and does most of its feeding at night.

The snail is not a picky eater. DNA studies of its droppings show that it grazes whatever microbes happen to grow on the plant it is sitting on, so its diet differs from place to place across the mountains.

Because natural microbial ecosystems are difficult to replicate in a controlled setting, captive snails depend on native leaves collected from the wild. The Hawaiʻi Department of Land and Natural Resources Snail Extinction Prevention Program (SEPP) is working on a manufactured diet because collecting enough native vegetation has become a bottleneck to repopulation efforts.

Behavior and Adaptations

Kāhuli tree snails are nocturnal and arboreal. By day they rest tucked against the undersides of leaves and at night they crawl over the foliage to feed. They are homebodies: many individuals spend their entire lives in a single tree, which helps explain why separate ridges hold such genetically distinct populations.

The snails evolved on islands with very few snail-eating predators, and they have no effective defense against the rats, predatory snails and chameleons that people later brought to Hawaiʻi. Because they grow and breed so slowly, even modest predation can wipe out a population that took decades to build.

Ecological Role

These snails are part of a three-way system of snail, leaf microbes and host plant. Its droppings may also shape the microbe communities on leaves. Historically, kāhuli were a common part of the native forest invertebrate community.

Life History

Each kāhuli tree snail is a hermaphrodite, meaning they have both male and female reproductive organs. Snails usually mate with one another, but captive snails kept alone have produced young, which suggests they can also fertilize their own eggs. Breeding happens year-round. The embryo develops inside the parent, and the young are born live and fully shelled, one at a time. A typical adult produces only about one to four young a year, though some estimates run as high as seven.

Newborn snails are about 0.18 inch (4.5 millimeters) long. They grow slowly, adding roughly 0.08 inch (2 millimeters) of shell length a year, and must reach full size before they can breed. Estimates of age at maturity range from about four to seven years. Most young never reach adulthood, and juveniles rarely move far from the tree where they were born.

Kāhuli tree snails are long-lived for their size. They often live 10 years or more, and in the absence of predators may live 15 years or longer. Captive snails have been raised since 1989, but how long they can live in captivity compared with the wild is not yet well established.

Conservation History

Invasive species were and are the greatest threat to the kāhuli tree snail. The most damaging is the rosy wolfsnail, a predatory snail released in Hawaiʻi beginning in the 1950s to control another invasive species, the giant African land snail. It ignored its intended target and instead hunted native snails.

Introduced rats eat tree snails throughout their range, and Jackson’s chameleons, released on Oʻahu from the pet trade in 1972, were later found with kāhuli tree snails in their stomachs. Predatory flatworms, garlic snails and yellow crazy ants added to the pressure. Habitat loss from feral pigs and goats, invasive plants and wildfire shrank the native forest the snails depended on, and models of climate change have projected that most of their climate-suitable habitat could disappear within the century. In the 1800s and early 1900s, shell collectors also took kāhuli tree snails by the thousands.

In 1981, the entire genus Achatinella was listed as endangered under the U.S. Endangered Species Act. The State of Hawaiʻi also lists the genus as endangered, and the species was assessed as Critically Endangered on the IUCN Red List in 1996.

Recovery efforts have relied on two main tools. Beginning in the late 1980s, scientists raised kāhuli tree snails in captivity. Beginning in 1998, land managers built predator-proof enclosures on Waiʻanae ridgetops, ringed with barriers and electrified wire to keep out rats, wolfsnails and chameleons, and moved captive-bred and wild snails inside. By 2024, a federal review counted roughly 2,100 snails in the wild outside enclosures, the lowest tally on record.

For more information on the status of this kāhuli tree snail, please visit NatureServe Explorer.

Cultural Significance

Kāhuli tree snails as a group have played an outsized role in the history of science. In the mid-1800s, Hawaiian-born missionary’s son John Thomas Gulick collected thousands of Achatinella shells and noticed that each valley seemed to hold its own forms. He argued that isolation alone, without any difference in environment, could drive species apart, an idea that fed into early debates over evolution and foreshadowed what biologists now call genetic drift. The same craze for collecting Hawaiian tree snail shells, prized for their colors and patterns, removed snails from the forest by the thousands.

The snails’ story is also a famous cautionary tale. The release of the rosy wolfsnail is often cited around the world as an example of biological control gone wrong, and the decline of Hawaiian tree snails became one of the best-known examples of island extinction. That loss gained a face in “George,” the last known snail of a related kāhuli species, Achatinella apexfulva. George spent his final years alone in a state laboratory, and his death on New Year’s Day 2019 drew international news coverage and marked the extinction of his species.

Fun Facts About Kāhuli Tree Snails

1. Kāhuli tree snails don’t eat leaves. They graze the invisible film of fungi and bacteria that grows on top of them.
2. Shells of right-coiling and left-coiling kāhuli tree snails can turn up on the same branch, and the two forms mate with each other freely.
3. Baby kāhuli tree snails are born live, fully shelled, and only about the size of a grain of rice.

How to Help Kāhuli Tree Snails

• Clean your gear. If you hike on Oʻahu, brush mud and seeds off your boots, clothing and gear before and after each trail so you don’t carry invasive plants, flatworms or ants into the mountains.
• Never release pets. Jackson’s chameleons reached Oʻahu’s forests through the pet trade. Report Jackson’s chameleons seen in the wild on Oʻahu to the Hawaiʻi pest hotline, 643-PEST, and never move chameleons or snails between sites.
• Leave snails and shells where you find them, and keep their locations off social media, which can draw collectors and trampling to fragile sites.
• Support native Hawaiian forests through NWF’s Hawaiʻi affiliate, the Conservation Council for Hawaiʻi.

Resources and More Information

Hawaiʻi’s Endangered Jewels, National Wildlife magazine, 2022

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Select Sources

Chiaverano and Holland (2014), “Impact of an invasive predatory lizard on the endangered Hawaiian tree snail Achatinella mustelina: a threat assessment,” Endangered Species Research

Hadfield and Mountain (1980), “A field study of a vanishing species, Achatinella mustelina (Gastropoda, Pulmonata), in the Waianae Mountains of Oahu,” Pacific Science

Meyer et al. (2017), “Two for one: inadvertent introduction of Euglandina species during failed bio-control efforts in Hawaii,” Biological Invasions

O’Rorke et al. (2015), “Dining local: the microbial diet of a snail that grazes microbial communities is geographically structured,” Environmental Microbiology

Last updated: October 29, 2026