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versión impresa ISSN 0327-9545

Biocell v.30 n.2 Mendoza mayo/ago. 2006


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.
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: /

Key words: Shell; Operculum; Periostracum;Banding pattern; Apple snail.

Pomacea canaliculata (Lamarck, 1822) is a freshwater snail belonging to the family Ampullariidae, a taxon that includes Asian, African and American species collectively known as apple snails. P. canaliculata ranges from the Amazonas Basin ( Brazil ) to Tandilia and Ventania Mountain systems ( Buenos Aires province, Argentina ) and is the most widely studied snail in Argentina , being the object of different morphological, anatomical, ecological, embryological and taxonomical 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 established as a serious rice pest in Asia (Cowie, 2002). Asian pest managers were very much troubled with the identification of the invading apple snails (Wada, 1997), a task seriously complicated by the multiple origin of invaders and their great morphological and ecophysiological variability (Cowie, 2002; Estebenet and Martín, 2002, 2003; Martín and Estebenet, 2002). Although already recognized by d'Orbigny (1835-1846), the wide conchological variation of P. canaliculata has been seldom quantitatively studied and most studies on the subject were performed in a restricted area ( Buenos Aires province) in the southern area of its natural distribution (Cazzaniga, 1990; Estebenet, 1998; Estebenet and Martín, 2003).
The shell of P. canaliculata has been described as globose to subglobose, with a low spire and an oval aperture; the color of the shell is brown-green, showing several dark spiral bands of variable width and transverse 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 analyze the variation and the origin of P. canaliculata shell traits, and to compare them with the information available for other Neotropical Ampullariids, focusing mainly on quantitative or experimental studies.

Shell periostracum

Periostracal hairs disposed in spiral series are common 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 spiral rows of thin triangular flakes appear early in the posthatching 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, probably P. scalaris (d'Orbigny, 1835) are occasionally observed (Cazzaniga, pers. comm.). Both embryonic and juvenile periostracal hairs are well developed in the sister family Viviparidae, showing important interspecies and intergenus differences (Jokinen, 1984). Periostracal hairs, though lost in later stages, can be used in identification 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.

Scanning electron micrographs of critical-point dried shells of newly hatched Pomacea canaliculata (less than three days old): a) dorsal view (1: protoconch, 2: prehatching 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).

Shell chirality

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 laboratory 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 copulate 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, copulation between individuals with opposite chirality is possible among those pulmonate snails in which the individual 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 conservation 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).

Shell Color

The banding pattern is highly variable among individuals from the same population of P. canaliculata . This variation involves the color, the intensity, the number (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 under 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 condition (“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 inheritance (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 dissolution 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 unaltered.
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 suddenly appears after a distinct shell growth mark. Perhaps the intensity of the bands is directly related to shell thickness.

Shell shape

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 allometric growth in many dimensions: shell width, aperture 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 tendency.
A sexual component of intrapopulation morphological variation of P. canaliculata has also been described 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 aperture 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 development 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. Probably, 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 (unpub. data).
The morphological variation shows a strong inter-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 different populations located in the same basin in Southern Buenos Aires province showed significant shape differences 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 environmental influences on shell growth patterns. The field snails seem to show a greater interpopulation morphological variation than their descendants reared in the laboratory, 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 example, 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 responsible of the variation among populations remained elusive. An ongoing study on many populations of P. canaliculata from a wider spectrum of waterbodies belonging to different basins and distributed over all Southern Buenos Aires provided some information on the environmental factors that could affect shell shape. Discriminant analysis based on six lineal dimensions of the shell adjusted by size were performed to detect significant 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 contained 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 suggests that water flow and consistency of substrates affect the growth patterns of the shell, resulting in widely overlapping ecophenotypic morphs appearing as an almost continuous variation.

Frequency histogram of scores and standardized coefficients of the canonical function of the discriminant 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%: percentage of correctly classified cases; ***: p<0.001).

Shell size and weight

Sexual dimorphism in shell length has been recorded in populations inhabiting a small stream tributary of the La Plata river (Martín, 1984) and an artificial 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 Southwestern 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 vary in its expression degree among snails from different 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 female 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 temperate 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 eggshell, and also in the perivitelline fluid (Turner and McCabe, 1990), is responsible of this difference. However, as in the case of shell length, the intersex differences 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 drainage 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). However, 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 running 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, probably 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 sandy bottoms; female shells were also larger in the latter. The same pattern was observed among lotic waterbodies: males and females from sand-muddy bottoms 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.

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).

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 relatively 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 Southern Brazil .
The growth rates of the length and specially of the width of the operculum are higher in males than in females (Estebenet, 1998) probably resulting in the sexually 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 entire lifespan, while that of males becomes progressively convex in the labral fringe during maturity and thereafter, remaining concave in the rest of the surface.

Final comments

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 subject 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 restricted geographical region. The knowledge is even more limited for other species of Pomacea or other genera of apple snails, preventing the development of a comparative approach in conchological aspects at generic 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 CONICET.


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Received on March 31, 2005.
Accepted on October 31, 2005.

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