Workshop: Biology of Ampullariidae
Conchological variation in Pomacea canaliculata and other South
American Ampullariidae (Caenogastropoda, Architaenioglossa)
ALEJANDRA L. ESTEBENET✝, PABLO R. MARTÍN AND SILVANA BURELA
Universidad Nacional del Sur, Departamento de Biología, Bioquímica y Farmacia. San Juan 670, 8000 Bahía Blanca, Argentina
Key words: shell, operculum, periostracum, banding pattern, apple snail
2006, 30(2): 329-335
ISSN 0327 - 9545
PRINTED IN ARGENTINA
Pomacea canaliculata (Lamarck, 1822) is a fresh-
water snail belonging to the family Ampullariidae, a
taxon that includes Asian, African and American spe-
cies collectively known as apple snails. P. canaliculata
ranges from the Amazonas Basin (Brazil) to Tandilia
and Ventania Mountain systems (Buenos Aires prov-
ince, Argentina) and is the most widely studied snail in
Argentina, being the object of different morphological,
anatomical, ecological, embryological and taxonomi-
cal studies (Catalán et al., 2002; Castro-Vazquez et al.,
2002; Estebenet and Martín, 2002; Cazzaniga, 2002;
Heras and Pollero, 2002). International concern about
P. canaliculata rose enormously when it became estab-
lished as a serious rice pest in Asia (Cowie, 2002). Asian
pest managers were very much troubled with the iden-
tification of the invading apple snails (Wada, 1997), a
task seriously complicated by the multiple origin of in-
vaders and their great morphological and ecophysiologi-
cal variability (Cowie, 2002; Estebenet and Martín,
2002, 2003; Martín and Estebenet, 2002). Although al-
ready recognized by d’Orbigny (1835-1846), the wide
conchological variation of P. canaliculata has been sel-
dom quantitatively studied and most studies on the sub-
ject were performed in a restricted area (Buenos Aires
province) in the southern area of its natural distribution
(Cazzaniga, 1990; Estebenet, 1998; Estebenet and
The shell of P. canaliculata has been described as
globose to subglobose, with a low spire and an oval ap-
erture; the color of the shell is brown-green, showing
several dark spiral bands of variable width and trans-
verse growth lines (Castellanos and Fernández, 1976).
The operculum is corneous with concentric growth lines
around an excentric nucleus. Most of these traits, and
many others, show a great influence of ontogenetic,
sexual, genetic and ecophenotypic components, which
give place to an important intra- and interpopulation
variation. The aims of our study are to describe and ana-
lyze the variation and the origin of P. canaliculata shell
traits, and to compare them with the information avail-
able for other Neotropical Ampullariids, focusing mainly
on quantitative or experimental studies.
✝ Deceased October 1st 2005
Address correspondence to: Universidad Nacional del Sur,
Departamento de Biología, Bioquímica y Farmacia. San Juan
670, (8000) Bahía Blanca, ARGENTINA.
E-mail: firstname.lastname@example.org / email@example.com
Received on March 31, 2005. Accepted on October 31, 2005.
ALEJANDRA L. ESTEBENET et al. 330
Periostracal hairs disposed in spiral series are com-
mon in P . canaliculata and have been recorded in other
apple snail species (Berthold, 1991). In P . canaliculata
we observed that the protoconch and the pre-hatching
teleoconch are devoid of periostracal hairs and that spi-
ral rows of thin triangular flakes appear early in the post-
hatching teleoconch (Fig. 1). Periostracal hairs are also
quite evident in newly deposited portions of the shell of
post-hibernating adult field snails and apparently abraded
in older portions. Extremely hirsute juvenile snails, prob-
ably P . scalaris (d’Orbigny, 1835) are occasionally ob-
served (Cazzaniga, pers. comm.). Both embryonic and
juvenile periostracal hairs are well developed in the sis-
ter family Viviparidae, showing important interspecies
and intergenus differences (Jokinen, 1984). Periostracal
hairs, though lost in later stages, can be used in identifi-
cation of newborn or juvenile viviparids (Ribi and Oertli,
2000), but the information on the fine morphology of
these structures in Ampullariidae is insufficient yet to
know if it could be used for this purpose. Berthold (1991)
suggested that periostracal hairs could play a role in the
homogenization of intracapsular fluid in Ampullariids,
though this is not the case at least in P . canaliculata, since
the shell of intracapsular stages is notably smooth.
Sinistral coiling of the shell in P. canaliculata is
extremely rare: only one female has been reported from
an artificial pond in La Plata city, that showed also an
inverted body organization (Cazzaniga and Estebenet,
1990). After fifteen years of intensive field and labora-
tory work with thousands of specimens of this species
only one new sinistral specimen has been found: an adult
male with the same inverted body organization, retrieved
in La Corina stream, a small watercourse in Buenos
Aires province. Both specimens were unable to copu-
late even when grouped with normal dextral snails of
the opposite sex, probably due to the inability of the
males to find the gonopore of the female partner when
located in the other side of the body. However, copula-
tion between individuals with opposite chirality is pos-
sible among those pulmonate snails in which the indi-
vidual playing the male role mounts the shell of the one
playing the female role (Asami et al., 1998). If inverted
body organization has a genetic basis as in other snails,
then this would preclude the transmission or conserva-
tion of “inverting” alleles and could explain the very
low frequencies of sinistral shells. Sinistral coiling is
also exceptional in the family Ampullariidae as a whole
(Cazzaniga and Estebenet, 1990).
FIGURE 1. Scanning electron micrographs of critical-point dried shells of newly hatched
Pomacea canaliculata (less than three days old): a) dorsal view (1: protoconch, 2: pre-
hatching teleoconch, 3: post-hatching teleoconch, arrowhead: hatching mark; scale bar:
200 μm); b) detail of periostracal hairs near the apertural lip (scale bar: 40 μm).
331 CONCHOLOGICAL VARIATION IN Pomacea canaliculata
The banding pattern is highly variable among indi-
viduals from the same population of P. canaliculata.
This variation involves the color, the intensity, the num-
ber (up to 30 bands) and the width of bands. In some
populations from Southern Buenos Aires province,
unbanded individuals are not infrequent in any given
large sample of snails. However, apparently unbanded
individuals that appeared in our laboratory stocks from
one of these populations showed a very weak banding
pattern (both in intensity and number of bands), visible
only after a close examination of the empty shells un-
der proper illumination.
The presence of bands is under the control of a
single locus gene in the giant ramshorn snail Marisa
cornuarietis (Linné, 1758), with the unbanded condi-
tion (“golden”) recessive; the inheritance of banding is
independent of that of body color, being the “golden”
snails indistinguishable from the wild phenotypes in all
other respects (Dillon, 2003), although this variant has
not been recorded in field populations. The bands are
absent also in albino P. canaliculata snails (“yellow”)
that lack dark pigments in the skin, the eyes and the
shell, a recessive condition with simple Mendelian in-
heritance (Yusa, 2004). Unbanded and albino strains of
Pomacea spp. generated and maintained in the aquarium
trade are common in Europe, North America and Asia
(Perera and Walls, 1996; Raut and Aditya, 1999) and
probably have been the source of “golden” apple snail
variants that now thrive in the wild in some of these
regions (Dillon, 2003).
Bands darker than the background are a frequent
feature in ampullariid shells. According to some authors
the pigment of the bands is deposited in the periostracum
(Castellanos and Fernández, 1976; Cazzaniga and
Estebenet, 1990) although this is not the common rule
among gastropods, in which the pigments are produced
by specialized cells in the mantle margin and deposited
in the outer calcareous layer or ostracum. In P.
canaliculata at least, the bands are included within the
calcareous shell matrix, as can be proved by acid disso-
lution of the shell, which leaves only a homogeneously
brownish-colored periostracum (pers. obs.); on the other
hand the chemical digestion of the periostracum only
fades the general shell coloration, leaving the bands
The banding pattern variation shows ontogenetic and
ecophenotypic components: the color intensity of the
bands in P . canaliculata increases during posthatching
development, and the shells of hatchlings have no bands.
Fast growing laboratory snails develop thinner
(Estebenet and Martín, 2003) and at the same time
weaker-banded shells than their source field populations.
Moreover, in many field snails the banding pattern sud-
denly appears after a distinct shell growth mark. Per-
haps the intensity of the bands is directly related to shell
Ontogenetic growth patterns of the shell of P.
canaliculata have been studied only for a population
from Paseo del Bosque pond, La Plata city (Estebenet,
1998). The shell shows a gentle though definitely allo-
metric growth in many dimensions: shell width, aper-
ture width and spire length grow faster than total length
while aperture length grows slower; the overall shape
of the shell becomes more globose and the aperture
wider during ontogeny. These patterns are valid for snails
larger than 9.0 mm of shell length, which are oblong.
However, newborn are almost isodiametric (Hylton-
Scott, 1958; Estebenet and Cazzaniga, 1993) so the early
post-hatching growth patterns must show the opposite
A sexual component of intrapopulation morpho-
logical variation of P. canaliculata has also been de-
scribed although the sexual dimorphism involves only
the aperture shape; as in Pomacea urceus (Müller, 1774)
(Burky, 1974), the main proportions of the shell are not
different. The specific growth rate of aperture width
increases in males during maturity, so the male aper-
ture growth pattern diverges from that of females, which
is almost continuous with that of juveniles (Estebenet,
1998), resulting in a wider male aperture (Cazzaniga,
1990). A similar but somewhat more pronounced sexual
dimorphism was described in the planispiral shell of
M. cornuarietis (Demian and Ibrahim, 1972). The de-
velopment of the penis sheath complex, located in the
mantle margin, may result in this sexual differentiation
of the aperture growth pattern (Demian and Ibrahim,
1972; Cazzaniga, 1990; Estebenet, 1998), and hence
would be a feature of other ampullariid species. Prob-
ably, the scarcity of reports of sexual shape dimorphism
is only the result of the few specific studies aimed on
the subject and the subtleness of the differences, rarely
perceptible to an inexperienced eye (Perera and Walls,
1996). The degree of sexual shape dimorphism in P.
canaliculata shows a wide interpopulation variation
The morphological variation shows a strong inter-
ALEJANDRA L. ESTEBENET et al. 332
population component in P. canaliculata (Cazzaniga,
1987, 2002), that is the result of environmental factors
and genetic differentiation (Estebenet and Martín, 2003).
Snails reared from egg masses collected at three differ-
ent populations located in the same basin in Southern
Buenos Aires province showed significant shape differ-
ences when reared under homogeneous conditions in
the laboratory, indicating a genetic basis for this shell
shape variation. On the other hand, adult snails collected
in these same populations showed different shell shapes
than their laboratory counterparts, suggesting environ-
mental influences on shell growth patterns. The field
snails seem to show a greater interpopulation morpho-
logical variation than their descendants reared in the labo-
ratory, probably due to the interaction of ecophenotypic,
genetic and also allometric components, since great size
differences among populations exist.
The genetically based differences in shell shape
among lentic and lotic populations do not seem to be
the result of local adaptation to different flow regimes
but a collateral outcome of adaptive differences in some
life history traits (Martín and Estebenet, 2002). For ex-
ample, the higher oviposition rate and clutch sizes in
one of the populations imply longer egg-laying periods
out of water, during which the shape of mantle margin
is altered and consequently the deposition of new shell
material along the reproductive life.
Although the above mentioned study proved the
existence of an ecophenotypic component on shell
shape, the identity of the environmental factors respon-
sible of the variation among populations remained elu-
sive. An ongoing study on many populations of P.
canaliculata from a wider spectrum of waterbodies be-
longing to different basins and distributed over all South-
ern Buenos Aires provided some information on the
environmental factors that could affect shell shape. Dis-
criminant analysis based on six lineal dimensions of the
shell adjusted by size were performed to detect signifi-
cant shell shape differences between contrasting types
of waterbodies. Shell shape of both males and females
from lotic and lentic habitats differed significantly (Fig.
2). There were also significant differences in shell shape
of both sexes between lakes and reservoirs with hard
bottoms located on hilly terrains and shallow lakes with
sandy bottoms. Among the lotic waterbodies, shell shape
was different between those with sand-muddy bottoms
and those with limestone bottoms for both sexes. Each
of the habitat types used in the precedent analysis con-
tained waterbodies from different drainage basins,
suggesting that the environmentally based variation in
shell shape overrides the genetically based variation that
would result from isolation or genetic drift. This sug-
gests that water flow and consistency of substrates af-
fect the growth patterns of the shell, resulting in widely
overlapping ecophenotypic morphs appearing as an al-
most continuous variation.
Shell size and weight
Sexual dimorphism in shell length has been re-
corded in populations inhabiting a small stream tribu-
tary of the La Plata river (Martín, 1984) and an artifi-
cial pond in the same area (La Plata city, Estebenet and
Cazzaniga, 1998); in both cases the females showed
higher mean shell lengths than males. However, mean
shell lengths were not significantly different between
sexes in one lentic and two lotic populations from South-
western Buenos Aires province, even though females
grew larger than males in the laboratory (Estebenet and
Martín, 2003). Similar dimorphic growth patterns have
been reported in all the experimental cohorts hitherto
studied (Estebenet and Cazzaniga, 1998; Tanaka et al.,
1999; Estoy et al., 2002) but the sexual dimorphism can
FIGURE 2. Frequency histogram of scores and standard-
ized coefficients of the canonical function of the discrimi-
nant analysis between females from lentic and lotic
waterbodies in Southern Buenos Aires province, based on
the following shell ratios: RAP = AP/TL, RAL = AL/TL, RS =
SW/TL, RAW = AW/TL, RBL = AP/TL (TL: total length, AP:
apertural projection, AL: apertural length, SW: spire width,
AW: apertural width, TW: total width, BL: body whorl length).
(Can. Corr.: canonical correlation coefficient; CC%: per-
centage of correctly classified cases; ***: p<0.001).
333CONCHOLOGICAL VARIATION IN Pomacea canaliculata
vary in its expression degree among snails from differ-
ent sources (Estebenet and Martín, 2003). Adult size
dimorphism varied also among egg masses collected
from the same lake (Estebenet and Cazzaniga, 1998),
presumably spawned by different females. Bigger fe-
male sizes have also been reported for other species of
ampullariids (Burky, 1974; Keawjam, 1987; Lum-Kong
and Kenny, 1989; Perera and Walls, 1996).
Shell thickness in Pomacea glauca (Linné, 1758)
and P. canaliculata is inversely related to growth rate
(Zischke et al., 1970; Estebenet and Martín, 2003). The
shell weight-shell length relationship studied in a tem-
perate pond population of P. canaliculata did not adjust
to a simple allometric model due to the ample variation
of shell weights for snails of the same size, probably
resulting from the different growth rates of snails born
in different seasons (Estebenet, 1998).
Cazzaniga (1990) reported that male shells were
significantly heavier than those of females of the same
length for a sample from an artificial pond in Buenos
Aires city. He suggested that female investment in egg-
shell, and also in the perivitelline fluid (Turner and
McCabe, 1990), is responsible of this difference. How-
ever, as in the case of shell length, the intersex differ-
ences in shell weight seem to be less pronounced in
natural populations than in laboratory populations from
the same sources (Estebenet and Martín, 2003).
The interpopulation variation in shell size and
weight is very important, even within the same drain-
age basin, and it is basically ecophenotypic in origin
since field differences disappeared when the snails were
reared under homogeneous conditions (Martín and
Estebenet, 2002; Estebenet and Martín, 2003). How-
ever, the precise influence of different environmental
factors on these variables remains unclear. Cazzaniga
(1987) proposed that snails inhabiting lentic habitats
with soft bottoms have thinner and larger shells than
those dwelling in habitats with hard bottoms and run-
ning waters. However, for a set of nineteen populations
from Southern Buenos Aires province, shell length in
both sexes was higher in lentic waterbodies than in lotic
ones (Fig. 3), though no differences in shell weight were
detected. Snails inhabiting lakes seem to be longer-lived
than those from streams in this semiarid region, prob-
ably due to the highly variable hydrological regime of
the latter (Martín and Estebenet, 2002). Females and
males from lakes and reservoirs with hard bottoms
showed lighter shells than those from shallow lakes with
FIGURE 3. Total shell length (TL, mean ± 95%CI) for males and females from lentic and lotic waterbodies in
Southern Buenos Aires province (n ≥ 30 snails, except for five populations where n = 8, 11, 19, 21 and 21).
ALEJANDRA L. ESTEBENET et al. 334
sandy bottoms; female shells were also larger in the lat-
ter. The same pattern was observed among lotic
waterbodies: males and females from sand-muddy bot-
toms showed heavier shells than those from limestone
bottoms, while shell length was higher only for females
from soft bottoms. Male growth rate shows less
ecophenotypic plasticity than female’s (Estoy et al.,
2002) and this would be related to the lesser degree of
male size interhabitat variation.
Operculum shape and weight
The length and width of the operculum grow with
negative and positive allometry, respectively, resulting
in an ontogenetic rounding of the operculum (Estebenet,
1998). At the same time the operculum becomes rela-
tively thicker during growth. There seems to be some
geographic, or at least interpopulation variation in the
operculum growth patterns since, according to Guedes
et al. (1981), the growth in length is isometric relative
to the growth in width in a lentic population from South-
The growth rates of the length and specially of the
width of the operculum are higher in males than in fe-
males (Estebenet, 1998) probably resulting in the sexu-
ally dimorphic operculum shapes reported by Cazzaniga
(1990) in fully grown snails (i.e. masculine wider than
feminine ones). However, the male opercula are lighter
than those of females for a given shell size (Estebenet,
1998), probably due to a lesser thickness in the fore. On
the other hand, Schnorbach (1995) reported that the
opercula of females are concave while those of males
are convex. In fact, we observed that the opercula of
newborn and juveniles of both sexes are concave; the
female operculum conserves this shape during the en-
tire lifespan, while that of males becomes progressively
convex in the labral fringe during maturity and thereaf-
ter, remaining concave in the rest of the surface.
The large conchological variation in P . canaliculata
has been considered a serious hindrance to the study of
several aspects of its biology. However, this apparently
chaotic variation can be split in several biologically
meaningful components, becoming an interesting sub-
ject of research on its own merit. In spite of the fact that
many aspects of the conchological variation have been
already studied, the available information includes, in
most cases, only one or a few populations from a re-
stricted geographical region. The knowledge is even
more limited for other species of Pomacea or other gen-
era of apple snails, preventing the development of a
comparative approach in conchological aspects at ge-
neric and familiar levels.
We want to express our gratitude to Alicia Miravalles
and Patricia Leonardi (Laboratorio de Ficología y
Micología, UNS) for assistance in preparation of SEM
specimens. This work was funded with grants by
CONICET (Consejo Nacional de Investigaciones
Científicas y Técnicas, PEI 6067/04 and PIP 6150/05)
and UNS (Universidad Nacional del Sur, PGI 24/B075
and PGI 24/B108). SB is a predoctoral fellow in
CONICET. Alejandra Estebenet was a researcher in
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