Samoan Snail Field Guide

INTRODUCTION

The islands of the Samoan archipelago lie about 14 degrees south of the equator in the central Pacific Ocean (Fig. 1). Politically, they are divided into two groups: Samoa (formerly Western Samoa), an independent state; and American Samoa, a territory of the USA (Fig. 2). The islands have been formed as a result of complex volcanic activity; they are geologically young. As for most islands of the Pacific, many of the plants and animals of the Samoan Islands are found nowhere else on earth.

Figure 1. The location of the Samoan archipelago in relation to the Hawaiian Islands in the central Pacific.

Figure 2. The Samoan archipelago.

This field guide is intended to permit identification of most of the terrestrial snails and slugs found in the Samoan Islands. It is intended to be accessible to the amateur naturalist but also to provide the experienced scientist with a compact resource of information. The guide covers both the native species and the non-native, introduced species, which are generally more frequently encountered, especially at low elevations and in habitats disturbed by human activities.

Much of our scientific knowledge of the native land snail fauna of the Samoan Islands is founded on the work of nineteenth century European and American naturalists, who were the first Western scientists to describe, name and illustrate the many different species. Their work was published in the scattered technical literature of the time. Not only are these publications difficult to understand without intensive technical training and experience, but also they are only readily available in the libraries of major universities and natural history museums. The non-native snails and slugs come from all over the world; literature dealing with them is therefore also scattered and not easily accessible. Snails are an important part of the fauna of the Samoan Islands, and so it seemed worthwhile to produce this manual, with the aim of bringing this information together in a form accessible both to the interested but inexperienced amateur and to professional scientists, conservationists and land managers working in the Pacific.

The basic objective of the guide is to allow identification of terrestrial snails and slugs encountered in the Samoan Islands. An additional objective is to stimulate interest and appreciation of these important animals, to enhance understanding of their biology and habitats, and to increase awareness of the importance of conservation.

WHAT IS A SNAIL?

Snails, and their shell-less or semi-shelled counterparts, slugs, 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 and slugs (known scientifically as "gastropods") form the biggest subdivision of the mollusks.

Snail Biology

Snails live on land and in the water. The familiar sea-shells found on the beach are mostly the empty shells of snails that live in the ocean. But more than half the world’s snails live on land or in freshwater (lakes, rivers, ponds, streams). Land snails are found in almost every habitat, from the baking deserts of the Middle East to the steamy jungles of Southeast Asia, from the mountaintops of volcanic Pacific islands to the hedgerows and woodlands of Western Europe. Slugs are almost as widespread, but are not found in such extreme hot and dry environments where their lack of a protective shell would make them vulnerable to desiccation.

Snails are an important part of many ecosystems, constituting a major portion of the total animal biomass. They are food for other animals; some snails are predators themselves; many consume dead and dying plant material and therefore are important in the cycling of nutrients through the ecosystem. A few species are important to humans. We eat escargot. We use shells as ornaments and jewelry. Other species of snails carry diseases, such as schistosomiasis, that infect humans, and some species have become crop pests, especially when they have been introduced by people outside their natural range. There are many examples of the importance of snails and slugs to humans.

Most snails hatch from eggs laid by the parent, but some are born live. All have a shell, known as the protoconch, that is formed inside the egg or inside the parent. The protoconch becomes the apex of the shell, which then grows by successively adding new shell material at the shell opening, the mouth or aperture, thus forming an ever-increasing spiral. The shell is contructed from a number of layers, the outermost of which is called the periostracum. The periostracum is thin and often translucent; it may give the shell a shiny appearance. As snails get older, the periostracum often wears off; and in dead shells that have been lying around, exposed to rain and sunshine for a long time, it has usually disappeared altogether.

Shells and Identification Features

Most snails can be identified by looking at features of the shell: what shape it is; how big it is; what color it is; and so on. The main features of a snail shell are illustrated in Fig. 3. Technical terms used here and throughout this book are explained in the glossary that is provided towards the end of the book.

Figure 3. Features of a snail shell. A-height, B-width, C-apex, D-suture, E-whorl, F-spire, G-columella, H-aperture margin or lip, I-position of umbilicus if present, J-body whorl (last whorl). This is a partulid shell. Like some other snails, partulids develop a "lip" around their aperture at sexual maturity.

Shells coil to the right (dextral) or the left (sinistral). If you hold a shell with the apex upwards and the aperture (the shell opening) facing you, the aperture of a dextral shell is on the right and that of a sinistral shell is on the left. Most species only coil in one direction. But in a few species, both sinsistral and dextral individuals may be found.

As the shell grows, each successive coil is known as a whorl. The junction between successive whorls is known as the suture. As the whorls coil, they form a central column known as the columella. The columella may be solid or hollow. If hollow, the hollow center is generally open on the underside of the shell and forms a cavity known as the umbilicus. But in some cases it is almost or completely closed off.

Some species continue growth throughout life. Others, however, stop growing when they become adult. At this point the shell often forms a thickened lip around the aperture. There may also be structures, usually known as lamellae or teeth, within the shell aperture that restrict the opening.

The surface of the shell may be smooth or sculptured. The sculpture may take the form of ridges or grooves that run either along the whorls in the direction of growth, or across the whorls. As the shell grows, the laying down of successive shell material at the aperture may produce fine transverse growth lines. Sculpture of the protoconch may differ from that of the rest of the shell. Sculpture may be worn down in old individuals. Sculpture varies greatly among species. Some species have little or no sculpture but bear smooth, glossy shells.

Some species bear thick, heavy shells, while others bear thin almost translucent shells. Shell color is very variable among species and, because it may also vary considerably within species, it is not always a good identification feature. However, once the snails inside them have died and the shells have lain exposed to rain and sunlight for some time, almost all shells turn opaque white. Even shells that were very thin, translucent and glossy often turn opaque and white.

Slugs are species that have lost their shells through the course of evolution. Some slugs retain a small vestige of a shell, which might be visible externally, or in some cases is completely internal. Semi-slugs have a larger shell but are unable to retract the body fully inside it.

Snails’ bodies vary little externally and features of the body are not as useful as those of the shells for field identification purposes. However, the internal anatomy of snails is important in their classification; some species can only be distinguished on the basis of differences in internal anatomy that can only be revealed by dissection. Most species in this field guide are distinguishable without dissection.

The main visible features of a snail’s body (Fig. 4) are its head, with one or two pairs of tentacles, its foot and its tail. Some snails, known as operculates, carry a horny or calcareous structure, the operculum on their tails. When the snail withdraws into its shell, the operculum acts like a trap-door and seals the shell aperture. Operculate snails have only one pair of tentacles and the eyes are at the base of the tentacles. In those land snails with two pairs of tentacles (belonging to the group known as pulmonates), the eyes are at the tips of the upper, longer pair of tentacles. Snails’ bodies may vary considerably in color. Color is not a good identification feature. Slugs may also be very variable in color.

Figure 4. Features of a snail body. A-shell, B-head, C-eye, D-tentacles (two pairs; only the top pair has eyes), E-mouth, F-genital pore, G-sole, H-foot. This is Succinea crocata (Succineidae) from 'Upolu. Its flat shell with little coiling is characteristic of the family.

Names of Snails

None of the native snails of the Samoan Islands has a common English name. Only a few have Samoan names. This guide therefore uses the scientific names. Scientific names of animals are always in Latin and always formed from two words. The first is the generic name, which always begins with a capital letter and which places the species in a genus of closely related species. The second is the specific name, not capitalized, which defines the species within the genus. Both names are always italicized. Sometimes, names are given to subspecies, which are geographic subdivisions of a species, but these are not included in this guide. When a species is mentioned for the first time in a book or article, the name of the person who first published its description and gave it its name, as well as the year in which the description was published, are often also given, following the species name. If someone at some time has decided that the original author placed the species in the wrong genus, the specific name is now associated with the new, correct genus, and the original author’s name is placed in parentheses. Often, if it is clear which species is being discussed, the name is abbreviated to the first letter of the generic name, followed by the species name.

For example, the Samoan tree snail Eua zebrina (Gould, 1847) was described and named by the American naturalist Augustus Addison Gould in a work published in 1847. However, Gould named the species "Partula zebrina", placing it in the genus Partula. Subsequent workers decided that it should be placed in the genus Eua. Hence Gould’s name and the date of the description are placed in parentheses. Often the name can be abbreviated simply to E. zebrina.

ORIGINS AND EVOLUTION

Most islands in the central Pacific have been formed as a result of isolated volcanic activity. They have never been connected to continental land masses. All the plants and animals that occur naturally on these islands arrived by chance dispersal or evolved on the islands from species that arrived by chance. Dispersal could have been by wind, perhaps blown high into the atmosphere during hurricanes and typhoons, and perhaps attached to leaves or other small bits of vegetation. Some organisms could have dispersed attached to floating logs and other debris. Yet others could have been carried accidentally by birds. Small juveniles, or even eggs, might have been more likely to have been dispersed than larger adults. Successful colonization of a new island would have been a rare event, but over the millions of years of geological time, would have been sufficiently frequent to allow many kinds of organisms to reach newly formed islands. Once there, many of these plants and animals evolved into diverse forms, taking advantage of the new ecological opportunities. Many of them, having evolved on a particular island, are unique to that island. Most original colonization of Pacific islands was in a west to east direction. Thus, many Pacific plants and animals are related to species in eastern Asia and Australasia. They have dispersed from these major continental sources of diversity, first to the closest islands of Melanesia and Micronesia. Then, with new species evolving as new islands were colonized, they have continued to disperse far out into Polynesia, reaching the most distant islands of Hawaii and the Marquesas. The chances of dispersing to these most distant islands are extremely low; some kinds of organisms have never reached them. However, those that did reach them frequently radiated evolutionarily into huge numbers of different but closely related species unique to particular islands. Less isolated islands tend to have fewer unique species.

Many of the islands of the Pacific have been formed as the Pacific tectonic plate moves northwestward over stationary "hot spots" in the earth’s mantle. Every so often lava from the hot spot rises and breaks through the earth’s crust, forming an island. As the plate moves, carrying this new island with it, newer islands continue to be formed over the hot spot. Thus, a chain of islands is created. As the older islands in a chain are eroded away they eventually decline to become underwater seamounts. Newer islands may have been colonized not only by plants and animals dispersing directly from continental regions but also by colonizers from earlier islands that are now sumberged. The Hawaiian Islands are a clear example. The Samoan Islands are more complex. They have also been formed largely through hot spot activity at the eastern end of the chain, but complicated by plate edge volcanism associated with the interaction of the Pacific plate and the Tonga trench to the south west.

Land snails on islands in the Pacific are classic examples of the kinds of patterns expected as organisms disperse to new islands and then radiate to produce many new species. The Hawaiian fauna comprises over 750 species, all but a handful of which are unique to the extremely isolated Hawaiian Islands. In the less isolated Samoan archipelago, there are only about 90 native species, of which about a third are also found elsewhere, generally in the neighboring islands of Tonga and Fiji.

CONSERVATION

Declines

Like the native plants and animals of most tropical Pacific islands, those of The Samoan Islands are disappearing. The land snails have declined dramatically. Seven species are listed by the World Conservation Union in the 1996 IUCN Red List of Threatened Animals; one of them, Diastole matafaoi, is listed as extinct. The United States Fish and Wildlife Service, with jurisdiction in American Samoa, lists two species as "candidates" for endangered or threatened status, and six as "species of concern". Others should undoubtedly be listed, but have simply not yet been evaluated.

Most of the species of the Samaon Islands were originally described for Western science by nineteenth century Europeans and Americans who did not visit the Islands. Their work was based on collections made by exploring expeditions and by a small number of amateur naturalists and collectors. The locations from which these specimens were collected are not well recorded; generally only the island where they were found is known. Major surveys were undertaken by the Bishop Museum (Honolulu) in 1926, 1928 and 1937 in American Samoa and 1967 in [Western] Samoa. Surveys of [Western] Samoa in 1965 and Tutuila (American Samoa) in 1975 were undertaken under the auspices of the Field Museum (Chicago). In the 1990s, survey work in both American Samoa and [Western] Samoa by Bishop Museum and other scientists has been carried out. All this twentieth century material has accurate and detailed locality information associated with it. It is thus possible to document the decline of the native land snail fauna during the twentieth century and it is clear that while it is now often difficult to find native snails, they were once extremely abundant. It is also clear that the number and abundance of non-native species is increasing. Most of these introduced species are common species, widely distributed in the tropics. As they increase and the native species decrease, the faunas of the Samoan Islands and other tropical islands around the world are being homogenized.

Threats

The decline in the native land snails has resulted from a number of causes, most of which remain significant threats.

Collection of shells for ornament, jewelry, and simply as a hobby may have had some impact. For instance, collection of the 10,000 or so shells of the Tutuila tree snail Eua zebrina that make up the chandeliers in the lobby of the Rainmaker Hotel in Pago Pago (Fig. 5) surely must have reduced at least some populations of that species. But most of the native snails are small and relatively inconspicuous, and of little interest for these purposes.

Figure 5. Chandelier constructed of Eua zebrina shells.

Habitat destruction and modification has probably had a much greater impact. Urban and suburban environments are unsuitable for most of these native snail species, which have evolved in the absence of humans and are closely adapted to the natural environment. Deforestation and clearing of land for agriculture have destroyed native forest that harbored native snails. Equally, the unintentional replacement of native vegetation with aggresively spreading non-native introduced species of plants, to which the snails are not adapted, has reduced available suitable habitat.

Predation by introduced predators has probably been a major cause of the decline of the native snails. The native snails evolved in the absence of many aggressive predators, such as rats. The slow reproductive rate of some of the species of snails (for instance the tree snails in the family Partulidae) meant that their populations were highly susceptible to sustained predation. Others (for instance the ground-dwelling Endodontidae) may have been especially susceptible to predation by introduced ants.

Some of the introduced snails and slugs (there are no native slugs in the Samoan Islands) have probably had an impact on the native snail fauna, through competition or other interactions. Many of the introduced species tend to be found in lowland, often disturbed habitats. Native species probably disappeared from these localities long before the arrival of most of the alien species. So these alien species had little impact on the natives. However, a few alien species, especially species in the family Subulinidae, that are now widespread and abundant, including at high elevations, may be causing declines in native species through competition for resources. But perhaps the single most significant factor in the recent drastic decline of native species has been the deliberate introduction of the predatory snail Euglandina rosea (Fig. 6) in ill-conceived attempts to control another non-native snail, the giant African snail, Achatina fulica. The predatory snail was introduced despite the lack of scientific evidence attesting to its ability to reduce populations of Achatina fulica, and in full knowledge of its likely impact on native snail populations, an impact that now seems to have been realized. Contrary to what some people think, Euglandina rosea will climb into trees in search of its prey. At the time of writing this field guide, this voracious predator is only thought to have been introduced to Tutuila and Ta’u. Recently however, the giant African snail has become a problem on ‘Upolu. Authorities must not be tempted to introduce either Euglandina rosea or other predatory snails. Although a naturally terrestrial species, Euglandina rosea will also go underwater to search for its prey, so freshwater snails are not immune to attack either. The equally or even more voracious and indiscriminate predator, the flatworm Platydemus manokwari (Fig. 7), has unfortunately already been introduced to ‘Upolu. It may rapidly cause the extinction of many of the native snails.

Figure 6. The introduced predatory snail, Euglandina rosea.

Figure 7. The introduced predatory flatworm Platydemus manokwari. Photo: Rob Myers.

Action

The conservation of what is left of the unique native snail fauna of the Samoan Islands depends primarily on there being sufficient interest in conserving it. This depends on education. And it is part of the purpose of this book to inform people about the uniqueness and value of the Samoan snails.

In practical terms, conserving Samoan Island snails will involve establishing reserves, especially reserves that are free of rats and predatory snails. It will involve monitoring these and other areas in order to evaluate the continued well-being of the fauna. And it will involve preventing the introduction of the giant African snail to islands currently free of it, so that authorities and private individuals will not be tempted to introduce predators such as Euglandina rosea or the flatworm Platydemus manokwari in attempts to control it.

You can help by valuing and taking pride in the native fauna and flora of the Samoan Islands; by spreading the word about their special and unique nature; by becoming aware of projects that might impact natural areas, by speaking out against them, and by participating in community and governmental planning processes that shape them.

HOW TO USE THIS ONLINE FIELD GUIDE

If you are unfamiliar with snails and slugs and/or with The Samoan Islands, you are recommended to read the introductory material above. If you want to delve deeper, you will find many sources of additional information in the list of literature page on this site. Use the glossary, also on this site, to understand terms with which you are unfamiliar.

Most people will probably find the image-based interactive identification guide to be the most useful feature of this web site. This is quite appropriate. You begin with a list of all families of land snails included in the Field Guide. Each family is represented by a series of small thumbnail images, representing each genus in the family. Click on any of these images to see a larger version of the same image. When you think you've found the right family, you can click on a family name and see a larger set of images for each species within the Family, arranged by genus. If you click on a particular genus name, you will see a detailed listing of the species within that genus, including information that will help you identify which species you have. If some of the information given for the species you are looking at is clearly contradictory, this will give you a hint that perhaps your identification is incorrect. For instance if you found a snail on Ofu but it is only known to occur on Savai‘i, it would be worth checking your identification--of course you could be correct and be the first person to have recorded that species on Ofu.

Many of the species are large enough to be identified with the naked eye. However, some species are very small, or have features that can only be seen clearly under magnification. A hand lens, magnifying glass, or if you have access to it, a binocular microscope, will be useful for identification of these smaller species.