and Evolution of the Amphidromus in Nusa Tenggara
by Richard L. Goldberg
& Mike Severns
The eminent malacologist and
land snail aficionado William Clench once claimed that if he
were transported blindfolded to a tropical Pacific island, he
could determine his location based on the color forms of land
snails that he would encounter. If, as Clench proclaimed, an
island's unique land snails can act as indicators of geographical
location, then, ostensibly, a snail sleuth familiar with variations
of insular species could also ably track his way through any
tropical archipelago of the world.
Variable color and pattern are a hallmark of many tropical land
mollusks. Yet this variability has caused countless wrong turns
on the road map of taxonomy. The polymorphic nature of these
snails confounded early taxonomists who often described many
color forms as new, without any consideration of their relationship
to adjacent populations.
Studying landshell variation
can be a fascinating pursuit for any shell collector. When laid
out in collection drawers, a well organized and documented series
of landshells can provide a conchologist with the opportunity
to observe an evolutionary moment in time of a species. The
varied and aesthetically pleasing series of snail shells can
also reveal information from which to learn more about the similarities
and differences among neighboring species.
A land snail species can develop
a shell with myriad color forms within one population (intra-population
variability), or may vary from the norm only in isolated populations
within its range (inter-population variability). Occasionally
a species can exhibit both phenomena. A number of biological
concepts may be useful in explaining why a species of snail
can vary locally or over its entire range.
The external appearance of a
shell -- color and pattern, for instance -- are likely to be
influenced by natural selection when determining the genetic
make up of a snail. Extreme color polymorphism in a population
perhaps indicates that looking different from your neighbor
is advantageous, making it more difficult for predators to develop
a search image, such as color and contrast, to locate their
prey. The unique color form can then spread rapidly in a population.
This phenomenon, known as frequency dependent selection, may
explain why some intra-populations of snails in the genus Amphidromus
from eastern Indonesia often have unpatterned or strikingly
different colored shells mixed in among the predominantly multi-colored
or strongly patterned shells. Natural selection is typically
considered to favor one form over another, thereby shifting
the bell curve, yet frequency dependent selection flattens that
bell curve and spreads it out.
One explanation for inter-population
variability may be a phenomenon called genetic drift. To illustrate
this, let's say a population gets split in two, one on island
A, and one on island B. Both populations might start off identical,
with chances at 50/50 for a particular trait. It is unlikely
that the offspring will be 50/50. They may be 47/53, and the
subsequent generation may be 43/57. Over many generations, the
percentages will fluctuate up and down at random, and they can
end up being very different from where they started. Yet, in
the real world, a new mutation doesn't start at 50/50. It might
be only 1/10,000 or 1/100,000. By chance it can have more surviving
offspring than average, and slowly increase in the population.
Smaller populations will drift faster than large populations.
But, if by chance that first mutant does not have any surviving
offspring, that trait disappears. This is a more likely scenario.
So, genetic drift tends to eliminate variation within a population
and increases differences between populations. Over a period
of time, the populations from islands A and B will end up looking
might also be explained by the founder effect. The snails that
colonize an island may represent only a small fraction of the
parent population's genetic diversity. If a rare mutant color
form is mixed in among the new colony, a colony that was 1 in
1000 in the parent population, it may instantly be 1 in 10 in
the new population. Given a brand new set of environmental influences,
the population may evolve in whole new directions from its parent
Environmental factors such as
the geology, climate, flora, fauna, and food are all suspected
contributors to the rise of a unique shell appearance. To reduce
the competition with other species which overlap in some aspect
of their niches, a phenomenon called character displacement
occurs. Shell, behavioral, anatomical or biochemical characters
eventually deviate from the ancestral form to allow the species
to survive with its neighbors. Character displacement might
also manifest itself in the color, pattern, and even the form
of the shell. Changes brought on by one, or a combination of,
these biological concepts can eventually render an isolated
population unable to interbreed with the parent population,
producing a situation which makes speciation possible.
Isolation is the key ingredient
for a species to evolve, and natural barriers provide the necessary
separation to allow speciation to take place. Whether the barrier
is a series of mountain ridges separated by deep valleys, as
in the case of the Hawaiian Achatinella tree snails, or islands
separated by wide channels as in the case of some species of
Indonesian Amphidromus, isolation allows a species to evolve
independently from its ancestral form.
An excellent illustration of
how isolation influences independent evolution is seen in the
species Amphidromus inconstans Fulton, 1898, which inhabits
a limited group of islands in Nusa Tenggara, eastern Indonesia.
Nusa Tenggara has served as a natural laboratory of evolution
for Amphidromus and other biota, because of the region's long
isolation from the surrounding Indonesian islands, and from
Australia. Many endemic species are found here, including the
exotic Komodo dragons.
Amphidromus inconstans has also
caused a great deal of taxonomic confusion due to the complexity
of the inter-relationship among neighboring species. A. inconstans,
as it is traditionally understood and revised by Laidlaw &
Solem in 1961, exhibits a broadly differing array of colors,
patterns and, at times, shell form, throughout its range. A
perfunctory examination of these color forms is deceiving, and
gives a false impression of one widely ranging species. Recent
field collecting throughout the range of A. inconstans has provided
a much clearer picture of this intriguing group of snails. Initial
observations of the comprehensive series of shells from this
field work, and comparisons with neighboring species has shown
that A. inconstans has a much narrower range than originally
Taxonomically, Amphidromus inconstans
is placed in the subgenus Syndromus, a group of sinistral species
noted for their brightly colored and variably patterned shells
(see Am.Conch 21(1): 5, "Variation in the Amphidromus").
Older collections with limited habitat documentation made it
difficult to comprehend the full extent of the variability of
A. inconstans, and its associations with species from adjacent
islands. Also, early collectors often sorted out various color
forms for aesthetic reasons, giving a false impression of inter-
and intra-population variability.
For instance, many species of
Syndromus contain a solid yellow intra-population color form.
These yellow shells were often separated by field collectors
because of their desirability. Since the shells had limited
habitat data, by the time they reached museum collections, they
were assumed to have come from different localities. It is now
known that, in most cases, intra-population variability is commonplace
in this group. Even with this in mind, A. inconstans poses a
dilemma for taxonomists since there is strong evidence to indicate
that we are dealing with more than one species.
Determining whether the color
forms traditionally thought of as A. inconstans actually represent
more than one species requires an understanding of Laidlaw &
Solem's view of its distribution. As currently understood, the
string of islands north of Timor between eastern Flores and
Romang are home to the Amphidromus inconstans complex, a seemingly
whimsical group that displays an enormous range of color and
pattern. Each island in the geographical range of A. inconstans,
starting in the west on Pantar and moving east to Romang, has
populations that take on a unique appearance. Yet a traceable
pattern to the whimsical nature of these extreme variations
exists, and clearly offers visible evidence that populations
of A. inconstans in the eastern and western portions of the
range have evolved from different ancestral species.
The palette of colors and patterns
exhibited by these snails becomes a paradigm for the evolutionary
changes that are influenced by isolation. In this case, sea
water provides a formidable barrier for isolating these insular
snails. The bright, but mostly unicolored forms on the islands
in the western part of its range are in stark contrast to the
extremely intricate patterns of the eastern island forms.
Basically, the western islands
of Pantar and Pura are home to the solid yellow color form described
as A. gracilis von Martens, 1899 (Fig.1: 1 - 2). On the small
island of Pura, the species begins to exhibit faint banding
around the periphery of the body whorl (Fig.1: 4 - center).
Moving east to Alor, three distinct
A. inconstans forms are found. The solid yellow A. i. gracilis
form previously seen on Pantar and Pura lives side-by-side with
an intricately patterned form considered typical A. inconstans
(Fig.1: 3). This nominate form typically has four spiral brown
bands on the body whorl, the upper two broken into irregular
brown squares. It has a base color of yellow and white. This
pattern varies considerably and, in a series, shows a remarkable
gradation into the unicolored gracilis form (Fig.1: 4 - left
Also on Alor, a thin shelled
form, Amphidromus i. oscitans von Martens, 1899 (Fig.2: 5),
with its wavy radial streaks and more convex outline, lives
at higher elevations. A solid yellow form of A. i. oscitans,
in the past also referred to as A. i. gracilis, can also be
found (Fig.2: 5 - right). The appearance of this distinct form
on Alor, isolated from the lower elevation A. inconstans and
A. i. gracilis forms, is one piece of evidence that A. i. oscitans
may in fact be a distinct species. Early accounts report finding
it only on Pura and Alor. Recently a few specimens were also
found at higher elevations on Atauro (Fig.2: 6), the next island
in the chain.
Atauro (also sp. Atuaro) is a
small island in the wide, deep channel between Alor and Wetar,
just north of Timor. It possesses one of the truly beautiful
and unique Amphidromus. This population exhibits a highly developed
color pattern not seen anywhere else in the range of A. inconstans.
Its white base color, with a dark blue/black to pale sky blue
pattern overlaid with bands or flushes of yellow, provides a
bridge to the more intensely colored eastern island forms (Fig.3:
7). Unlike the A. inconstans color forms from other islands,
which tend to exhibit intra-population variability, the Atauro
population is amazingly consistent in form throughout the island.
This form has never been recorded in literature, and is presented
here for the first time.
Continuing northeast to Wetar
and the surrounding islets, one finds A. i. wetaranus Haas,
1912, a form which represents the greatest divergence from the
previous forms (Figure 4: 8 - 9). Until recently, only a few
specimens of A. i. wetaranus were available for study. Haas
originally described the shell as a full species. Later, Laidlaw
and Solem (1961) reclassified it as a subspecies of A. inconstans
based on its similarity to the Romang Island form named A. i.
subporcellanus discussed below. There is a strong argument for
removing A. i. wetaranus from the shadow of A. inconstans because
of the consistently different shell, generally smaller size,
more slender profile, and a unique and intense color pattern.
It also exhibits red bands not seen in any of the A. inconstans
color forms from the islands west of Wetar. The Wetar snails,
like the low elevation Atauro snails, more closely resemble
some forms of A. contrarius (Mueller, 1774) endemic to Timor
and its western satellite islands (Figure 5: 12). Much like
the forms found on Alor and Romang, A. i. wetaranus also exhibits
a great deal of intra-population variability.
Due east of Wetar is the termination
point in the traditional range of the A. inconstans complex,
Romang Island (Roma in older literature) and its satellite islets.
Romang is home to an extraordinarily varied population of snails
(Figure 6: 10 - 11). It is currently known as A. inconstans
rollei Laidlaw & Solem, 1961; Rolle described the population
as A. i. gracilis Rolle, 1903, but that name was preoccupied
by the unicolored westerly forms from Pantar, Pura and Alor.
In that same paper, Rolle also described three distinct color
forms of his extremely variable A. gracilis (=rollei). Shells
with narrow radial bands broken into spots by spiral bands of
ground color were given the form name A. subporcellanus; A.
viridistriata are those shells with a green band above the red
columellar color patch; and A. subsimplex are those unicolored
shells with only a red columellar color patch. These varieties
of A. inconstans rollei intergrade so completely that the names
have no real taxonomic value except as a means for collectors
to single out a color form.
The geology of these islands
plays a significant role in understanding why the western and
eastern forms of A. inconstans probably represent distinct and
separate species (see map). The islands west of Wetar, namely
Alor, Pura, and Pantar, sometimes referred to as the Alor Archipelago,
are geologically part of a string of volcanic islands that extend
west through Flores, Indonesia and belong to the Asian continent.
These islands are covered with lush green rain forests. The
islands are very close together, allowing some genetic communication
among snail populations. East of Alor, the islands of Atauro,
Wetar, Reong, Nyata, and Romang are all part of the Australian
Plate (Timor and Kissar are also grouped here, but are not inhabited
by A. inconstans). These uplifted limestone islands are covered
with dry-land forest and, in some cases, eucalyptus.
It is significant to note that
the forms on these limestone islands of the Australian plate,
long considered to be related to A. inconstans, almost always
have a black dot protoconch, a characteristic that is exhibited
by A. contrarius on Timor (Figure 6: 15). But to the west, on
the volcanic islands of the Alor Archipelago, the forms of A.
inconstans lack the black dot protoconch; instead, they have
either light or white embryonic whorls. The only exception to
this on the western islands is the high elevation A. i. oscitans
of Pura and Alor, which does exhibit the black dot protoconch
of the eastern populations.
This apparent evidence suggests
that migration has occurred west to east on the volcanic islands,
and southwest to northeast on the limestone islands beginning
with western Timor and including the islands north and east
of Timor. The genus Amphidromus may have originated in Asia
and spread south and east into Indonesia. As the genus reached
the small islands of eastern Nusa Tenggara, populations may
have become more isolated than they were previously in their
range and the distances between habitable islands may have increased.
Those that made the leap between islands may have exploded into
new forms and species with spectacular colors. It was those
colors that immediately attracted naturalists to study these
brightly colored snails, brought back to Europe aboard the ships
of spice traders after they hopped among islands while sailing
through eastern Indonesia.
Low sea levels, in some cases
low enough to provide a land bridge from Singapore to Bali,
provided Amphidromus populations the opportunity to move eastward.
To the east in Nusa Tenggara, migration was most probably by
storms, birds such as eagles, or even fruit bats, which may
have carried young snails between their feeding grounds and
their nesting islands. The result is random dispersal over long
distances. Populations became established on remote islands,
leading to evolutionary isolation and independence from the
ancestral form. With little or no genetic communication with
the parent population, these isolated, island-bound snail populations
could diverge to form new species.
A. i. wetaranus and A. i. rollei,
which inhabit the eastern limestone islands, show a remarkable
likeness to A. contrarius from Timor and its adjacent islands
to the west. The general shell shape, black dot protoconch,
and similarities in color/pattern of A. wetaranus mimic that
of A. contrarius populations from the Niki Niki area of Timor,
due south and west of Wetar Island. The shells of A. wetaranus
tend to be slightly less inflated than A. contrarius, but other
features are very close. These apparent similarities lead us
to believe that A. contrarius, and not A. inconstans as has
been believed, was introduced to Atauro, Wetar and from there
to Romang where isolation and evolution have taken hold.
Other Timor populations of A.
contrarius (Figure 5: 13) also show a close affinity in shape,
size, and dark protoconch to the low level population of Amphidromus
from Atauro, in close proximity to Timor and of similar geological
makeup. Yet the Atauro snails somewhat mimic the shell patterns
of typical A. inconstans from Alor. This anomaly begs the question
A. i. oscitans from Pura, Alor,
and Atauro presents another anomaly. The black dot protoconch
and general shell characteristics exhibited by this species
link it to A. contrarius, A. wetaranus, and A. rollei, yet isolation
has allowed the snail to develop enough distinct shell characters
to warrant specific status. Its habitat at higher elevations,
removed from limestone, may be the reason for its having a thin
shell, a trait often seen in species of snails living at high
This convincing evidence suggests
that A. inconstans, long thought to be one highly variable species,
has, in fact, succumbed to the changes influenced by isolation.
The complex has seemingly evolved into four or five distinct
species stemming from two ancestral forms; A. contrarius from
Timor being the parent population for the Amphidromus inhabiting
the limestone islands; and A. poecilochrous (Figure 7: 14) from
Flores & Sumbawa Islands possibly being the parent population
for the Amphidromus on the volcanic islands. Yet additional
field work is necessary to confirm or deny our speculation that
there is a connection between A. poecilochrous and A. inconstans.
If there is a pattern to this
hypothesis, then the first species, Amphidromus inconstans,
would include the color forms that inhabit the volcanic islands
of Pantar, Pura, Alor (Figure 1: 2 - 4); a second species, A.
oscitans found only at higher elevations on Pura, Alor and Atauro
would also be distinct (Figure 2: 5 - 6); the limestone islands
of Wetar and Romang would be home to the third and fourth species,
A. wetaranus (Figure 4: 8 - 9) and A. rollei (Figure 6: 10)
respectively; and finally the low elevation Amphidromus from
Atauro may, in fact, represent another new species (Figure 3:
7). A paper currently being drafted will delineate the Atauro
Equipped with this empirical
data, not only would William Clench have been assured of which
island he had landed and collected on, but also this data would
have clarified for him what direction he was heading if he had
journeyed in the path of the spice traders through the islands
in eastern Nusa Tenggara. This fascinating complex of Amphidromus
is just one of many in the genus that offers a challenging puzzle
for students of landshells, professional and amateur alike.
the Authors | Glossary