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Samoan Land Snails (Sisi Vao)

July 4, 2011

Photo of Pythia scarabaeus (Native Samoan Land Snail).

Snails belong to the second largest group of animals on earth, the mollusks. Only the arthropods (insects, crustaceans and their relatives) have more known species. Snails are found just about everywhere – the ocean, streams, lakes, and on land.

Over 90 native species of land snails (sisi vao) occur in the Samoan Archipelago. Of these, 64 occur in western Samoa and 47 in American Samoa (many species are found in both places). Many are found only on our islands – that is, they are endemic to the Samoan Archipelago. Some of them even occur only on a single island, so they are endemic to that island. Slugs, which are snail-like mollusks that have no shell, also occur locally, but none is a native species -all are recent introductions to our islands.

Snail shells come in all shapes and sizes – flat, tall, rounded or spiraled. Some live in trees, where they may eat dying leaves. Others live on the ground and probably feed on dead leaves. Together with fungi and other microorganisms that help to decompose the leaf debris, snails contribute to the cycling of nutrients through the ecosystem.

 We do not know much about the basic biology of these land snails. Some species have separate males and females, but others are hermaphrodites, whereby each snail is both male and female. However, most hermaphroditic snails still reproduce by mating with another individual – each snail can act simultaneously as a male and as a female, or in some species the snails take turns being males and females. Most snails lay eggs, but some give birth to live young – miniature snails that simply crawl away. The snails that produce live young tend to grow and reproduce very slowly – some of the tree snails may take over a year to reach full size and may live as long as 5-10 years, producing only 10-20 young per year. This contrasts with egg-laying species that probably grow much quicker, produce many eggs, but do not live as long.

 There are three possible ways that land snails might have crossed the ocean to get to our remote islands. First, they might have been carried over the ocean from a distant continent – or from another island – on rafts of driftwood or fallen logs. But salt kills land snails, so this seems unlikely. Perhaps they were carried here by birds. Sometimes snails get caught up in the feathers of birds, especially if they are really small snails. And third, they might have been blown by the wind. Scientists have found that very small snails can indeed be blown long distances by strong winds. A small snail attached to a leaf, caught up in a cyclone, could be blown hundreds, perhaps thousands of miles across the ocean. Through evolutionary time (millions of years), these seemingly unlikely events had only to happen very occasionally in order for a few land snails to eventually colonize our islands.

 Once the land snails arrived and managed to survive, they began to evolve to local conditions. Some species changed and became so different from their ancestors that scientists now identify them as different species. Others evolved into more than one species. This is how the Samoan islands came to have many land snail species found nowhere else on earth – they evolved after they arrived here.

 Many of the local snail species have attractively colored shells and have often been used in the making of ula or lei  and for other ornamental purposes. For instance, the hanging light fixtures in the old lobby of the Rainmaker Hotel in Pago Pago contained 10,000 or more shells of tree snails that used to be abundant in the forests of Tutuila.

 

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