28/05/2020

Invertebrate of the Month: Polynesian Tree Snail

This month our star invertebrate is the Polynesian tree snail!

Hello, I’m a Polynesian tree snail called Page and I’ve spent all four years of my life so far in bug world at Bristol Zoo Gardens. 

Polynesian tree snails are molluscs which belong to a genus named Partula, consisting of 125 snail species — what a variety!

Unfortunately, fifty species of Partula snails are extinct in the wild.  My species is mirabilis which makes my scientific name Partula mirabilis. Sounds pretty fancy, huh? I live at Bristol Zoo Gardens with four other species of Partula snail: Partula navigatoria, Partula garretti, Partula suturalis strigosa and Partula hebe bella.

Polynesian tree snails were first discovered in 1769 by Joseph Banks and have had a rough existence ever since — making us extremely rare. During the 1960s, giant African land snails were brought to French Polynesia in the South Pacific to breed and eat. They escaped and ate the local’s crops. To eradicate them, the rosy wolf snail was introduced, but it didn’t just find the giant African land snails tasty — it found us enticing, too. As a result, many Polynesian tree snail species are now critically endangered, extinct in the wild or completely extinct.

As a result of our rarity, we are isolated in our own room at Bristol Zoo Gardens to avoid harmful microorganisms from other animals, especially other snails. I’m categorised as Extinct In The Wild on the IUCN Red List of Threatened SpeciesTM and my room mates, Partula navigatoria, Partula garretti, Partula suturalis strigosa and Partula hebe bella are also Extinct In The Wild.

Our geographic range is wide and we are native to many volcanic islands such as the East Philippines and Marquesas Islands. We can grow up to 20mm in length and weigh less than one gram when fully grown – that’s less than fifteen grains of uncooked rice. We are detritivores and love to snack on decomposing plant material but, at the Zoo, we get fed a special diet that has been designed to replicate our natural diet. Our keepers feed us a mixture of grass pellets, trout pellets, porridge oats and vitamin supplement, all mixed up into a paste. We also enjoy cuttlebone which is important for extra calcium.  

We take about three months to a year to mature; once matured, we live up to ten years. When it comes to our young, we are pretty unique as, unlike most snails, we give birth to live young. However, unlike mammals, we give birth without a placenta — scientists call this phenomenon ovoviviparity. Our gestation period is three months and when we enter the world for the first time, we are a teeny tiny size of 1 to 2mm in length.

What has a foot but no legs? Me! We have one foot which has a gland at the front that produces slime so that we can move over all surfaces easily. I move using waves of muscular activity in my foot. (I’m good friends with most surfaces as I’m always WAVING at them!) Our favourite times to be active are at night and just after it has rained. We lose water very quickly so we avoid the sun and dry weather in order to stay happy.

In 1986, the International Partulid Conservation Programme was started, including the International Partula Breeding Programme, which is working hard to help us thrive. They started by collecting some of my few remaining ancestors and other Polynesian tree snails that had managed to survive thus far. They were then distributed to an international group of zoos (including Bristol Zoo Gardens, my home), where they are currently being preserved and bred. Since 2013, Polynesian tree snails have been sent back to French Polynesia to support the wild populations so that we can thrive once again!

To assist our survival, little reserves were set up on Moorea and Tahiti (islands in French Polynesia). They have been effective at keeping the rosy wolf snails out. Conservationists are still conducting regular surveys and investigating new potential release sites, so maybe my children or I will get to go home soon! So far, the programme has been super helpful and over 10,000 snails from ten different species have been sent home. We really appreciate all their hard work!

We are also very grateful to members of the public for all the hard work you do to fundraise. In 2016, a young boy called William fundraised through cake sales, wildlife crafts and colouring competitions – he raised £30 to help us little snails thrive!

A big thank you to all of you and to Bristol Zoo Gardens; please pay us a visit - you’ll say hi to some extremely rare snails! 

To help support the continued success of the Zoo, and all of the society's breeding programmes, visit: https://uk.virginmoneygiving.com/fund/covid19appeal

Blog post written by Laura Thomas, placement student in Bug World. Image taken by Nicola Cooke.

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