Yellow-tipped Oahu tree snail

Hawaii’s first scientifically described snail

The yellow-tipped Oahu tree snail Achatinella apexfulva is not only the first scientifically described snail of the Hawaiian archipelago but also the first officially declared extinct. The species was first described in 1789 by British sailor and explorer George Dixon, who acquired a traditional necklace made of snail shells during his visit to the Hawaiian Islands between 1786 and 1787. The jewelry was made entirely from the yellow-tipped Oahu tree snail and Achatinella decora; the latter species became extinct around 1900.

The extinction of the yellow-tipped Oahu tree snail is well documented, as the likely last specimen of the species lived in a terrarium at the University of Hawaii. Named George, after the Galápagos giant tortoise Lonesome George, who was also the last of his kind – a sole survivor – he died on New Year’s Day 2019 at the age of 14.

George was born in 2004 or 2005 in a lab at the University of Hawaii in Manoa. As it became clear that the numbers of yellow-tipped Oahu tree snails were dwindling, scientists collected George’s parents and all remaining members of the species from trees near the Poamoho Trail on the Hawaiian island of O’ahu in 1997, according to Christie Wilcox in a 2019 National Geographic article. Their plan was to breed the animals in captivity to save the species.

However, all offspring bred under laboratory conditions died for unknown reasons – except for one: George. At the time of his birth, there were about 20 individuals in captivity, but by the mid-2000s, only George remained. He became sexually mature in 2012, and during this time, snail researchers repeatedly tried to find more of his species in the wild, unfortunately without success.

In recent years, the tree snails of O’ahu have received a lot of attention; they are particularly endangered, and George, as a representative of the Achatinella genus, demonstrated the extinction of a species. In 2001, zoologist Robert H. Cowie estimated in Can Snails Ever Be Effective and Safe Biocontrol Agents? that the extinction rate for these tree snails was between 75 and 90 percent, with most species surviving only on a single ridge or in a single valley.

Yellow-tipped O’ahu tree snail – Fact sheet

Alternative name	Hawaiian yellow-tipped tree snail, dusky-apex Oahu tree snail
Scientific name	Achatinella apexfulva, Achatinella pica, Achatinella lugubris, Achatinella vespertina, Achatinella apicata var. alba, Achatinella apicata, Achatinella swiftii, Achatinella aptycha, Helix Avex Fulva, Turbo apexfulva, Turbo lugubris, Monodonta seminigra, Bulimus seminiger, Apex gulickii, Apex lilaceus
Original range	Oahu (Hawaii)
Date of extinction	2019
Causes of extinction	animals and plants introduced to the island, snail collectors, habitat loss

A thumbnail-sized spiral in dark brown and brown

The genus Achatinella, belonging to the family Achatinellidae, consists of viviparous, nocturnal tree snails that are endemic exclusively to the island of O’ahu in Hawaii. These are terrestrial pulmonate gastropods, which breathe through a lung-like cavity. The genus includes 41 species: all are endangered or threatened with extinction, and at least 16 have already gone extinct (such as Achatinella buddii).

These mollusks once inhabited the forests of the Koolau and Waianae mountain ranges on O’ahu. Today, according to the U.S. Fish & Wildlife Service (USFWS) in its 1993 Recovery Plan for the Oahu tree snails of the genus Achatinella, they are only found in regions above 400 meters in the remaining dry to moist forests and bushlands of both mountain ranges. The yellow-tipped Oahu tree snail was ultimately only found in a small area of the Koolau ridges.

Oahu tree snails typically live on trees, where they feed on fungi growing on leaves. They have a partly glossy, right- or left-coiled shell. Julia Jacobs described George in a 2019 New York Times article as a “thumbnail-sized spiral in dark brown and brown.” Indeed, the elongated to oval-shaped snail shells within the genus display a wide range of shapes, colors, and pattern variations, which have long fascinated evolutionary biologists, malacologists, and amateur collectors.

Although George was given a male name, he was actually a hermaphrodite. Land lung snails are bisexual, meaning individuals of the species possess both male and female gametes and reproductive organs. George was unable to reproduce without a partner.

Why did the yellow-tipped Oahu tree snail, Achatinella apexfulva, go extinct?

Due to their slow reproductive rate and slow growth, the tree snails of O’ahu are sensitive to disturbances. Achatinella snails can produce only a few offspring each year and take about six years to reach sexual maturity. Previously, these snails were very common, which is why they are mentioned in many Hawaiian songs and tales, but today the opposite is true.

Destruction of natural habitat

The destruction of forests and the introduction of invasive plant species began with the arrival of Polynesians on the Hawaiian Archipelago over 1,000 years ago; the situation worsened with the arrival of Europeans in 1778. The US Fish and Wildlife Service (USFWS) notes that with the settlement of the islands, landscapes at lower elevations, which were previously inhabited by tree snails, were used for grazing, agriculture (particularly sugarcane and pineapple), and housing.

Forests not cleared for agriculture were destroyed by feral livestock such as goats, horses, and pigs, all introduced by settlers. These animals disrupted the balance between endemic plant species: by grazing, they damaged the underbrush and prevented the regrowth of native plants. This, in turn, paved the way for non-native plants and their seeds. Changes in the underbrush also altered moisture and humidity levels in the forests, rendering them unsuitable as habitats for snails.

Today, goats and pigs still pose a serious threat to Hawaii’s forests, according to the USFWS. Human activities also promote the spread of exotic vegetation through hunting, hiking, military maneuvers, clearing for illegal marijuana plantations, and the construction of roads, trails, and helicopter landing sites.

Areas where forests were originally cleared and later reforested have proven uninhabitable for snails because non-native plants such as eucalyptus, ironwood, and Norfolk pine were typically planted. Furthermore, wildfires – often caused by slash-and-burn clearing – have devastating effects on snail populations, not necessarily because the fire directly kills the mollusks, but because invasive plants quickly spread in the burned areas. Exotic plant species, such as Miconia calvescens and Dicranopteris linearis, also prevent the regrowth of trees that serve as food sources for the tree snails.

Rats, flatworms, and snails: Predation by invasive species

The US Fish and Wildlife Service (USFWS) identifies the rosy wolfsnail (Euglandina rosea), a predatory land snail native to tropical North America, and black rats (Rattus rattus) as the primary predators of the Achatinella snail genus. Other potential invasive predators that may consume Oahu tree snails include the flatworm Endeavouria septemlineata, the garlic glass-snail (Oxychilus alliarius), the brown rat (Rattus norvegicus), and the Pacific rat (Rattus exulans). Introduced parasites and diseases could also contribute to declining tree snail populations, although this has not been proven in the Achatinella genus, according to the USFWS.

The rosy wolfsnail was introduced by the Hawaii State Department of Agriculture on O’ahu in 1955 and 1956 to control the giant African land snail (Achatina fulica), considered a pest in agriculture. As a snail that eats other snails, the rosy wolfsnail does not selectively feed and consumes both giant African land snails and, primarily, tree snails. It follows the slime trails of other snails and climbs trees and bushes to catch its prey.

Since its introduction, the rosy wolfsnail has spread to both lower and higher elevations in the Koolau and Waianae Ranges. Scientists also suspect, as noted by Wilcox, that climate change, with increased rainfall and higher temperatures, has allowed the rosy wolfsnail to advance into higher elevations of O’ahu Island, invading the last refuges of the tree snails. As an invasive species, it is also linked to the disappearance of all eight tree snail species of the genus Partula on the island of Moorea in French Polynesia.

Historical snail collecting

Up until the mid to late 19th century, snail collectors gathered 10,000 or more mollusks in a single day, according to Wilcox. Locals collected the snails for their beautiful shells to make traditional Polynesian neck or head ornaments (lei). By 1914, biologist Henry A. Pilsbry and malacologist Charles M. Cooke noted in Achatinellidae. Manual of Conchology that various Achatinella species had become rare. Although collecting activities declined around 1940, most species were already greatly diminished.

The US Fish & Wildlife Service (USFWS) points out that currently, collecting two or three adult tree snails could mean the loss of a large part of a reproductive population in a bush or tree. In the past, snail collectors undoubtedly contributed significantly to the decline of tree snail species. However, most scientists today see the predation by the rosy wolfsnail as the primary cause of the disappearance of Achatinella snails.

The US Fish & Wildlife Service summarizes past and present threats to the Oahu tree snails as follows: While historically, the primary reasons for the decline in tree snail populations were solely the destruction of natural forest habitats and the introduction of predators like rats, today they include predation by the rosy wolfsnail and rats, as well as habitat loss due to the spread of invasive plant species into higher-elevation forests. Excessive collecting of snails for their shells and introduced diseases are also considered by the USFWS as factors threatening the genus.

Why there are so many tree snails in Hawaii

On the Hawaiian Islands, located 3,800 kilometers from the nearest mainland, some animal groups have diversified into many different species over time due to the lack of competitors. To minimize interspecific competition, these different species occupy various ecological niches. Thus, they live sympatrically within a small geographic area.

Thanks to the biological phenomenon of adaptive radiation, the Hawaiian Islands have produced not only countless flowering plants and about 400 species of fruit flies but also approximately 750 different snail species, which are divided into two families of land snails: the Achatinellidae and the Amastridae.

For many different Achatinella species to have evolved on O’ahu, there must have been an “original snail” from which these species developed. Ecologists Brenden S. Holland and Michael G. Hadfield explored how this original snail could have traveled 3,800 kilometers across the ocean to O’ahu in their 2004 study, Origin and Diversification of the endemic Hawaiian Tree Snails. One of their theories suggests that a very small ancestor of the snail was dropped on the island by a bird, as the dispersal of other snail species by birds is documented. Additionally, it is possible that the original snail floated to the island on an object, such as a tree. A combination of both theories and island hopping could also have brought the original snail to O’ahu. The exact origin of this original snail remains unknown.

The most famous example of adaptive radiation is the Hawaiian honeycreepers; there were once probably 57 species, differing not only morphologically but also in their specializations: some were nectar feeders, others seed or insect eaters, and some fed on seabird eggs. Many honeycreeper species, such as the Ula-ai-hawane and the Black Mamo, have already gone extinct.