Sexual dimorphism in shell shape: is Pomacea canaliculata an exception or an example among Neotropical apple snails?

Sexual dimorphism has often been recorded in apple snails (Caenogastropoda Ampullariidae), but reports are concentrated in a few species, either invasive or with biocontrol potential, which sugests some taxonomic bias. To find out evolutionary and ecological correlates of sexual dimorphism it is necessary to detect and quantify it but also is important to detect its absence. Our aims were to confirm or not the existence of sexual dimorphism in shell shape of Felipponea neritiniformis and Asolene platae, using Pomacea canaliculata as reference and applying the same methodology (landmark-based geometric morphometrics) and statistical power. Significant intersexual differences were only found in P. canaliculata and, in a lesser degree, in F. neritiniformis: males have larger apertures relative to body whorl and more rounded apertural outer edges than females. Female shells are larger in F. neritiniformis and P. canaliculata, but not in A. platae. Using comparable methodologies and statistical power, sexual dimorphism in shell shape is detectable in some apple snails but not in others. Interspecific variation in sexual dimorphism in the Ampullariidae is not only due to taxonomic bias and deserves more research to establish the main patterns and possible causes.

Established but not spreading: the tropical invasive snail Melanoides tuberculata in a geothermally warmed channel in temperate Southern Pampas.

Melanoides tuberculata is a freshwater snail native to Old World tropical areas but has invaded tropical and subtropical regions around the world. In Argentina, populations established in natural environments were reported from northeastern tropical provinces. Here we report for the first time the presence of M. tuberculata in a geothermally warmed channel in temperate Southern Pampas. We mapped its distribution in the channel, searched for its presence in five nearby basins, estimated the risk of establishment and expansion in Argentina with distribution models and analyzed shape variation through geometric morphometrics. Melanoides tuberculata was recorded exclusively in the channel in sites with temperatures between 20 and 40°C, with almost no overlap with other snails. No evidence of M. tuberculata was found in nearby basins. The distribution model predicted that only northernmost areas from Argentina are suitable for this species, where it could impact snail communities and food webs if introduction through the aquarium trade is not prevented. The absence of males indicates parthenogenetic reproduction and probably a recent invasion. Shell shape variation in this population, 15 % of which is attributable to allometry, encompasses the shapes of specimens from other South American populations, suggesting that all belong to the same lineage.

Dealing with a hyper-successful neighbor: effects of the invasive apple snail Pomacea canaliculata on exotic and native snails in South America.

Pomacea canaliculata is a successful invader and also a competitor and predator of other snails and may play a key role in structuring freshwater snail communities both in its native and invaded range. In the present study we evaluated the contact and distant effects of P. canaliculata in its native range on exotic (Melanoides tuberculata and Physa acuta) and native snails (Heleobia parchappii, Biomphalaria peregrina, and Chilina parchappii). Habitat use was affected in P. acuta, H. parchappii, and B. peregrina by contact effects of P. canaliculata, whereas survival was only affected in P. acuta through combined contact and distant effects. Fecundity was reduced in P. acuta and B. peregrina by combined contact and distant effects; evidence of egg mass predation was also observed in both species. Melanoides tuberculata was not affected at all by P. canaliculata. The snail species with higher withdrawal responses to contacts with P. canaliculata were those that suffered less mortality by corporal contact, whereas snails with high crawling away responses suffered from higher mortality. The effects of P. canaliculata seem to be highly negative to small nonoperculate snails that lay gelatinous egg masses, whereas large operculate ovoviviparous snails are not affected in their survival and reproduction. This apple snail may exert biotic resistance against P. acuta but could favor the establishment of M. tuberculata and other functionally similar species in new habitats in South America.

Temperature-induced plasticity in morphology and relative shell weight in the invasive apple snail Pomacea canaliculata.

Temperature has a great influence on the life-history traits of freshwater snails. In this study we investigated the long term effects of a range of temperatures on shell morphology of the apple snail Pomacea canaliculata, a highly invasive species and an important pest of rice. Analysis of shells using geometric morphometrics showed that the main source of morphological variation was allometry, which was detected in males but not in females. This intersexual divergence in allometric trajectories generates much of the morphological variation evidenced. In females, the monotonic relationship with temperature produced narrower shells in the snails reared at lower temperatures, and more expanded apertures, relatively bigger than the body whorl, at higher temperatures. We also found an inverse relationship between relative shell weight, a proxy for shell thickness, and temperature. The differences in shape and relative shell weight are attributable to the different growth rates associated with different temperatures. Temperature fluctuation around a mean of 23.2 °C seemed to have no influence in shell shape and relative weight when is compared with a constant temperature of 25 °C. Information on the influence of temperature on freshwater snails is important for understanding and predicting changes in the face of global climatic change, especially in traits exhibiting great plasticity, such as shell shape and thickness. This work showed that higher temperatures could result in a relatively thinner shell, implying a greater significance of corrosion in flowing waters and a lower resistance to crushing by predators, especially in low latitude areas.

Predation risk affects growth and reproduction of an invasive snail and its lethal effect depends on prey size.

The behavior of invasive species under predation risk has been studied extensively, but their growth and reproductive responses have rarely been investigated. We conducted experiments with juveniles and adults of the invasive freshwater snail Pomacea canaliculata, and we observed changes in growth and reproduction in response to predation risk from a caged predator (Trachemys scripta elegans). P. canaliculata produced eggs earlier in the presence of predators and injured conspecifics compared with the control group (no risk), although the total number of egg masses laid by per female was exceeded by that of the controls after 15 days. Egg hatching success noticeably decreased under predation risk, and the incubation period was significantly prolonged; however, the oviposition height of the snails was not affected. A lethal effect of predation risk was detected in juvenile snails but not in adults. The growth of juvenile P. canaliculata was inhibited under predation risk, probably due to a reduction in food intake. Adult females exhibited a greater reduction in growth under predation risk than males, which likely resulted in part from the high reproductive investment of females in egg laying. These results indicate that P. canaliculata snails under predation risk face a trade-off between predator avoidance and growth and reproduction, where the lethal effect of predation risk is linked to the size of the prey.

A temperature-dependent physiologically based model for the invasive apple snail Pomacea canaliculata.

In order to set priorities in management of costly and ecosystem-damaging species, policymakers and managers need accurate predictions not only about where a specific invader may establish but also about its potential abundance at different geographical scales. This is because density or biomass per unit area of an invasive species is a key predictor of the magnitude of environmental and economic impact in the invaded habitat. Here, we present a physiologically based demographic model describing and explaining the population dynamics of a widespread freshwater invader, the golden apple snail Pomacea canaliculata, which is causing severe environmental and economic impacts in invaded wetlands and rice fields in Southeastern Asia and has also been introduced to North America and Europe. The model is based on bio-demographic functions for mortality, development and fecundity rates that are driven by water temperature for the aquatic stages (juveniles and adults) and by air temperature for the aerial egg masses. Our model has been validated against data on the current distribution in South America and Japan, and produced consistent and realistic patterns of reproduction, growth, maturation and mortality under different scenarios in accordance to what is known from real P. canaliculata populations in different regions and climates. The model further shows that P. canaliculata will use two different reproductive strategies (semelparity and iteroparity) within the potential area of establishment, a plasticity that may explain the high invasiveness of this species across a wide range of habitats with different climates. Our results also suggest that densities, and thus the magnitude of environmental and agricultural damage, will be largely different in locations with distinct climatic regimes within the potential area of establishment. We suggest that physiologically based demographic modelling of invasive species will become a valuable tool for invasive species managers.

Effects of absolute fasting on reproduction and survival of the invasive apple snail Pomacea canaliculata in its native range.

A South American freshwater gastropod, the apple snail Pomacea canaliculata, has become a driver of ecosystemic changes in wetlands and an important rice pest after its introduction to various parts of the world, mainly Asia. The objective of this study was to study the effect of an abrupt interruption in food availability in the short term (up to 4 weeks) and long term (up to 8 months) on survival and reproductive activity. The main results indicate that short-term fasting mainly affects the survival of males, but only when they are raised together with females, probably due to a greater mate-searching activity that increases mortality in the individuals with lower reserves. The number of copulating snails or egg-laying females shows an abrupt drop when fasting and a rapid recovery after the food supply is restored. The strategy of discontinuing reproductive activity prioritizes energy conservation for the survival of the females. Interpopulation variation in resistance to starvation was observed in adults, which can be explained to some extent by the food availability that they experienced in their natural environment. No interpopulational differences in survival were seen in hatchlings. The mean maximum values of survival under starvation were 52.6 days in hatchlings and the 3.3% of adults survive over than 200 days, which may be a relevant trait in dispersal and establishment in new habitats.

Juvenile growth and survival of the apple snail Pomacea canaliculata (Caenogastropoda: Ampullariidae) reared at different constant temperatures.

BACKGROUND: Pomacea canaliculata is a freshwater snail that cultured under certain conditions could provide interesting rewards in research and aquaculture. P. canaliculata is usually reared at 25°C, though the optimal temperature for culturing this species, that balances growth and survival rates, is so far unknown. In this work we present results of growth and survival of cohorts reared in the laboratory at different constant water temperatures (15, 20, 25, 30 and 35°C) during the pre-reproductive period. FINDINGS: Two different groups were recognized among the five treatments: the two lower temperatures (15 and 20°C) that showed no mortality but with very low growth rates and the treatments of 25, 30 and 35°C in which snails grew faster but displayed a reduction in survival as temperature increases. After 10 weeks, the mean shell lengths attained at 30 and 35°C were only 2-3 mm higher than that of the treatment of 25°C and were not statistically different. CONCLUSIONS: Our results support using water temperatures of 25°C for the rearing of cohorts when the objective is to quickly obtain numerous large snails. Temperatures of 15 and 20°C may be appropriate if the aim is to preserve juveniles for long periods with a very low risk of mortality. The results reported here will be useful to the scheduling of laboratory trials intended for basic research, snail control or mass rearing for different applications of this species.