Partula snails  (Partula spp)

There are 79 species of the genus Partula on the IUCN Red List 2004, 50 of which are classified as Extinct (EX), 14 as Extinct in the Wild (EW), and 15 as Critically Endangered (CR) (1). Around 100 species of the genus Partula have been described (2).

In a parallel of Darwin’s finches on the Galapagos Islands, the Partula snails are known for the diversification of species that has occurred across the range of islands upon which they live. They vary in size, shape and colouration, with grey to brown spiral shells sometimes marked with white. Their bodies are pale to dark brown (2).

Found only on islands in the Pacific Ocean, ranging over 8,000 km from Palau to the Society Islands in French Polynesia (3).

Mainly tree-dwelling, Partula snails are found in densely forested regions in a moist habitat (2).

These fascinating molluscs have captivated scientists for centuries, with an interesting reproductive strategy and a particularly impressive diversification of species. Occupying islands that were previously free of both competitors and predators, the Partula snails were able to fill every available niche, evolving many new species. In common with many other snail families, Partula snails are hermaphrodites, meaning that every individual produces both sperm and eggs, and possesses male and female reproductive organs. However, they do not self-fertilise, but instead court another individual by touching tentacles and lips. As they court, the male and female genitals begin to emerge from the skin behind the head, and the snails circle one another to position themselves for copulation. Before they copulate, these snails perform an extraordinary and unexplained behaviour. In a state of excitement, one of the snails expels a long, thin ‘love dart’ made of chalk-like calcium carbonate, pushing it into its partner’s head. Shortly afterwards, the other snail reciprocates, firing a return love dart – named as an analogy to Cupid’s arrow. Copulation follows, and can last for up to eight hours, during which time the mating partners exchange spermatophores (4). Uniquely, both partners give birth to fully formed, shelled offspring two weeks after fertilisation (2).

Partula snails are thought to feed on algae and decaying plant matter and are known to live higher in trees as they mature. They are preyed upon by the introduced carnivorous snail, Euglandina rosea (2).

In a tragic and calamitous story, humans are responsible for the extinction, or extinction in the wild, of 64 species of Partula snails in the recent past. In the 1800s a budding snail farmer introduced the large and edible giant African snail (Achatina fulica) to the islands of the Pacific Ocean in an attempt to profit from providing a new source of food to the islands’ human inhabitants. His venture failed for economic reasons and the snails were released. They became an agricultural pest, prompting a response from the authorities to introduce the predatory carnivorous snail Euglandina rosea to the islands to control giant African snail numbers. A lack of forethought and contained experimentation is evident, as the carnivorous Euglandina did not prey on the giant African snail as expected, instead consuming and decimating the majority of Partula snail species, rapidly driving 50 species to complete extinction. Habitat destruction has compounded the snails’ decline (2).

In a desperate attempt to salvage this genus, many animal collections have joined in efforts to gather sufficient numbers of several Partula species to breed them in captivity. Captive breeding programmes have run at the Shedd Aquarium, Detroit Zoological Park, the Zoological Society of London, and Jersey Zoo amongst others. The Zoological Society of London released captive-bred snails into a protected area on Moorea Island in August 1994. However, extensive release of captive-bred snails is not possible until Euglandina rosea has been exterminated from the islands. This sad loss to global biodiversity serves as a lesson of the ignorance of man and the consequences of the introduction of non-native species. Introduced species remain the second biggest cause of species loss after habitat destruction (2).

For further information snail reproduction see the work of Dr Joris Keone at:
http://www.jkoene.dds.nl/chapter1.html

• Alga: a collection of taxonomically unrelated groups that share some common features but are grouped together for historical reasons and for convenience. They are of simple construction, and are mainly photoautotrophic, obtaining all their energy from light and carbon dioxide, and possess the photosynthetic pigment, chlorophyll A. They range in complexity from microscopic single cells to very complex plant-like forms, such as kelps. Algal groups include blue-green algae (cyanobacteria), red algae (rhodophyta), green algae (chlorophyta), brown algae and diatoms (chromista) as well as euglenophyta.
• Hermaphrodite: possessing both male and female sex organs.
• Spermatophore: gelatinous jelly cone with a sperm cap deposited by a male during courtship and picked up by the cloacal lips of the female.

1. IUCN Red List (June, 2005)
   http://www.redlist.org
2. The Wild Ones (June, 2005)
   http://www.thewildones.org/Animals/partula.html
3. Nutrition Advisory Group to the American Zoo and Aquarium Association (June, 2005)
   http://nagonline.net/Fact%20Sheet%20pdf/AZA%20-%20Partula%20Snail%20Species%20Survival%20Plan.pdf
4. Dr Joris Koene’s website (June, 2005)
   http://www.jkoene.dds.nl/chapter1.html