Giant African
Snail
The Giant African Snail, Achatina fulica, has been widely
introduced to Asia, to Pacific and Indian Oceans islands, and to
the West Indies. It has also been intercepted widely by quarantine
officials and incipient invasions have been eradicated, for
instance in the mainland USA. It is a major agricultural and garden
pest, and a general nuisance. It is also a vector (as are many
snail species) of several human pathogens and parasites. Often its
introduction leads to the subsequent introduction of predatory
snails and, more recently, flatworms as putative biological control
agents that can have devastating effects on native land snail
diversity.
Giant African
Snail, Achatina fulica
Description
A very large snail, adults usually around 7-8 cm tall, but can
reach 20 cm or more. The shell has a rounded conical shape, being
about twice as high as it is broad. The shell is generally brown in
color with irregular darker streaks running transversely across the
whorls.
Habitat
The Giant African Snail is native to East Africa, especially Kenya
and Tanzania, but also include most regions of the humid tropics,
including many Pacific islands, southern and eastern Asia, and the
Caribbean. The Giant African Snail can now be found in agricultural
areas, coastland, disturbed areas, natural forest, planted forests,
riparian zones, scrub/shrublands, urban areas, and wetlands.
Impact
The Giant African Snail has been widely introduced throughout the
tropics and subtropics]]. After introduction, its population
increases dramatically, perhaps following a variable lag time, as
seen in many other introductions of non-native species. The snails
frequently reach such enormous numbers, at least locally, that they
become not only agricultural and garden pests but also a major
public nuisance, causing road hazards and making it difficult to
avoid them while walking. The snail has been considered the most
important snail pest in the tropics and subtropics and perhaps the
world.
However, some argue that its agricultural impacts may have been
exaggerated, the nuisance factor perhaps being more important. By
reaching such enormous numbers and invading native ecosystems they
also pose a serious conservation problem. Not only may they eat
native plants, modifying their habitat, but they probably also
out-compete existing native snails.
Disease
The Giant African Snailis also a vector of Angiostrongylus cantonensis (Nematoda;
Protostrongylidae), the roundworm responsible for eosinophilic
meningo-encephalitis in humans and the spread of the disease has
been correlated with the spread of the snail. However, many other
introduced snails in the tropics are also vectors of this parasite
and the spread of the disease has not definitively been related to
the spread of the Giant African Snail.
The more insidious conservation problem they cause is that they
tempt agricultural officials to initiate a number of putative
biological control measures. The most well publicised of these
measures is the introduction of predatory snails, most notably
Euglandina rosea. The
first attempts at such biological control were made in Hawaii.
Fifteen carnivorous snail species were deliberately introduced. Of
these, nine did not become established; the fate of three is
unknown but they are certainly not common and do not appear to be
causing serious problems. However, three have become established:
Euglandina rosea, Gonaxis
kibweziensis, Gonaxis quadrilateralis. There is no
scientific evidence that the predatory snails are the reason for
the observed decline in numbers of the Giant African Snail,
however.
Similar ill-conceived attempts at biological control involving
Euglandina rosea in particular have been implemented in French
Polynesia, American Samoa, Guam, and a number of other places in
the Pacific and Indian Oceans. In addition to the deliberate
introduction of predatory snails, the predatory flatworm
Platydemus manokwari has
also been introduced, although as yet less widely. It is reported
that this flatworm can indeed cause populations of the Giant
African Snail to decline, but the evidence is only correlative, not
convincingly causative. However, the flatworm has also been
implicated in the decline of native species on Guam. It cannot be
stressed enough that these introductions of putative biological
control agents against the Giant African Snail are extremely
dangerous from the perspective of the conservation of native snail
species, and in any case, there is no good evidence that they can
indeed control Giant African Snail populations.
Population control
Suggested preventative measures must include strict quarantine
measures to prevent introduction and further spread. Many methods
have been tried to eradicate the Giant African Snail. Generally,
none of them have been effective except where implemented at the
first sign of infestation. Methods include hand collecting, use of
molluscicides, flame-throwers, and the failed attempts at
biological control discussed above. In some regions, an effort has
been made to promote use of the Giant African Snail as a food
resource, collecting the snails for food being seen as a method of
controlling them. However, promoting a pest, for whatever seemingly
positive reason, seems fundamentally counterproductive as it will
likely encourage the further deliberate spread of the snails. It
seems axiomatic that a pest species should not be promoted.
Nutrition
The Giant African Snail is a macrophytophagous herbivore.
Reproduction
The Giant African Snail is a cross-fertilizing, egg-laying
hermaphrodite. The number of eggs per clutch averages around 200. A
snail would lay 5-6 clutches year with a hatching viability of
about 90%.
Lifecycle
Adult size is reached in about four months, but growth may continue
slowly beyond this up to at least one and a half years.
Issue #355
