Feature

SAMOA’S UNIQUE SNAILS FACE MODERN THREATS

Dr Robert Cowie
PAGO PAGO, American Samoa, (Samoa News, April 4, 2010) - 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.

But now many of our local snail species are disappearing.

At least 7 species have become extremely rare and one of them, Diastole matafaoi from Tutuila, is now extinct. Others are undoubtedly in trouble but have simply not been fully evaluated yet. The two main reasons for their decline are the same reasons that biodiversity is vanishing all over the world.

The first reason is habitat destruction or modification. As native forest is cut down for timber or cleared for agriculture or urban development, the habitat of those snail species that depend on the forest disappears and so the snails disappear too. Second is the introduction to the islands of alien species— species that have been brought to the islands by humans, either on purpose or accidentally.

These aliens include pigs that degrade the forest by rooting for food and creating wallows, plants that grow and reproduce more strongly than the native species and replace them in the forest, birds that spread alien plants by carrying their seeds into native forest, rats and ants that prey on snails, and many other species that people have introduced to the islands.

These alien species also include snails and slugs that come from elsewhere in the world. Most people are likely to see only these aliens unless they hike into the most pristine native forest.

Perhaps the most commonly seen are the giant African snail (sisi aferika, Achatina fulica) and the large black or brown slugs. But there are many other species that are not so obvious but can be found easily just by turning over some leaves on the ground almost anywhere in the islands. The giant African snail is a pest of agriculture and in gardens but some of these other less well-known species may also be causing problems for the native snail species. Two of the alien species (Subulina octona, Paropeas achatinaceum) are now the most abundant snails in the leaf debris of native forest throughout the islands. It is quite possible that they are impacting native snails by competing them for the resources they depend upon.

Another alien snail, the predatory snail known as the ‘cannibal snail’ or ‘rosy wolf snail’ (Euglandina rosea), was introduced in an ill-conceived attempt to control the giant African snail by eating it. It was introduced even though there is no scientific evidence that it would reduce populations of the African snail.

The problem is that it attacks native snail species that had evolved in the absence of such aggressive predators. Some of our local species reproduce at a very slow rate and this means that their populations are highly vulnerable to sustained predation. Euglandina rosea is thought to have been introduced only to Tutuila and Ta’u. In Western Samoa it was introduced to Upolu in the 1990s.

An even more voracious predator of snails, the flatworm Platydemus manokwari, was unfortunately introduced to Upolu in 2003 to control the African snail. This free-living flatworm was also found on Tutuila and Ta’u islands in about 2004. It is black or dark brown, with a single line down its back, and it can grow to almost 3 inches long and 1/4 inch wide. This flatworm is believed to have decimated snail populations on other Pacific islands.

Dr. Robert Cowie is a research professor at the University of Hawaii’s Center for Conservation Research and Training.

Samoa News: www.samoanews.com
Copyright © 2010 Samoa News. All Rights Reserved