By Justin Gerlach, 4th February 2026
Endemic to many Polynesian islands, and known for their characteristically striped shells, Polynesian Partula tree snails are closely linked to the cultural identities of the islands they inhabit. To the Polynesians, Partula snails had been a familiar part of their environment and were culturally significant. But in 1974, an ill-advised biological control programme introduced the predatory rosy wolf snail Euglandina to Tahiti in an attempt to suppress agricultural pests. This proved catastrophic, with the predators ignoring the target species, and rapidly causing the extinction of 42 of the 65 Partula species. This tragic tale, however, initiated conservation efforts for the few remaining species and a drive to save the Partula snail, inspiring my own interest in them.
A rosy wolf snail searching for prey on Moorea island. Photo: Justin Gerlach.
Fortunately, there was some hope for a small number of species. The remarkably varied colour patterns of their shells had attracted early geneticists, and species from the island of Moorea had been breeding in laboratories since 1962. Noticing the sudden extinction of Partula in their field sites, the geneticists switched their experimental lab colonies to conservation breeding populations. In so doing, they saved most of the Moorean species from extinction.
Having been inspired by their work, in the early 1990s I studied the rosy wolf snail for my doctoral research. On Raiatea island, which had only recently been invaded by the predators, I was able to see the devastation at first hand: the ground was covered in shells. In one valley the shells were still wet, having been discarded by the predator just hours before, and only a single survivor remained. In valleys that the rosy wolf snail had not yet invaded, Partula were still abundant. It was from these valleys that I collected individuals and brought them back to London Zoo. By this time, the original research colonies had formed an international conservation breeding programme with multiple zoos across Europe and the USA. The programme was breeding most of the Moorean species, as well as a few others brought in by rescue missions for any remaining individuals, including mine from Raiatea.
Caring for snails in the Partula room at London Zoo. Photo: Jackie Ossowski-Mackie.
When we rescued those last wild survivors, we were preventing their immediate extinction, and had only the faintest hope of ever being able to return them to the wild. By 2015, the rosy wolf snail had declined enough that re-establishing wild populations of Partula seemed feasible. This would be very experimental—we did not know if snails that had been breeding in artificial conditions for generations could even survive in the wild. The first release in 2015 showed that the snails could survive, and this encouraged further attempts in subsequent years. These reintroductions had some success; released snails were marked with enamel paint so that we could recognise released animals, and with unmarked juveniles of Partula tohiveana being found, we knew there were some wild-born animals being produced.
Newly released Partula tohiveana. Photo: Justin Gerlach.
Annual releases took place until 2020, when the pandemic halted all field work. During the pandemic, our invaluable Tahiti-based field biologist Trevor Coote died. Whilst this was a major blow, his legacy was an exceptionally good collaboration with the French Polynesian government’s environment agency. Without this, it is unlikely that Partula conservation could have continued. With this foundation in place, I was able to take over the organisation of the releases and to revise our approaches. We had found wild-born juveniles and shells of adults, but I decided that successful establishment could only be declared if we had photographic evidence of at least one live wild-born adult, with several such animals needed to demonstrate a population.
In 2023 we released thousands of snails. It was still hard to tell if the programme was succeeding; we were not finding vast numbers of dead shells, but nor were we finding re-established populations (although there were shells of wild-born Partula tohiveana). Frustratingly, it was impossible to tell if these were from a temporary population or an established one hiding out of sight.
The release site, finding snails in this is a three-dimensional puzzle. Photo: Paul Pearce-Kelly.
Nearly five thousand more snails were released on Moorea in 2024. With these numbers, there was little time for searching for wild-bred snails on the release day. We returned the following day to monitor immediate survival. The results were good and, encouragingly, I found a live adult Partula tohiveana released in September the year before. During a break for lunch I spotted another snail on a leaf, and on close examination found it to be an unmarked wild-born adult. That first evidence of success was backed up over the next two days, when we found more such snails. This discovery of a re-established wild population made Partula tohiveana the 13th Extinct in the Wild animal species to be successfully returned to the wild, and the first invertebrate.
The first wild-born Partula tohiveana adult. Photo: Paul Pearce-Kelly.
This year we returned to the site and were able to confirm that the population persists one year on, and has even expanded its range by a few metres. This is wonderful success, albeit at a snail’s pace!
The article ‘Establishment of wild Partula tohiveana tree snails in French Polynesia’ is available open access in Oryx—The International Journal of Conservation.
