Shell shape as a potential predictor of age class in the invasive snail Lissachatina fulica

Abstract

Understanding the life-history traits of invasive species can increase the effectiveness of eradication efforts and decrease costs. In the giant African snail Lissachatina fulica, one of the 100 worst invasive species on the planet, age structure can be an important indicator of invasiveness. Here, we associated variation in snail size with variation in weight and shell shape, in an effort to better understand the growth pattern in this species. Our study involved two components: we estimated the allometric relationship between weight and length in a natural population and monitored developmental variation in snail shape, weight, length and nutritional condition (condition factor) in a lab population. The weight and length growth patterns of the lab population were estimated independently using exponential regressions based on weekly measurements; shell shape was quantified using geometric morphometrics. In both populations, we found negative allometry between weight and length, suggesting that snail shells tend to elongate during growth. Geometric morphometrics confirmed this pattern and revealed that the change from a rounded to an elongated shell shape does not occur at a constant pace but during a specific growth interval (i.e. at a shell length of 55–65 mm), which is described in the literature as the size when the animal becomes hermaphroditic. Future studies should test whether the shape change reported here corresponds to the transition to the hermaphrodite age class, and with this in mind, we provide two simple morphological indexes that allow us to characterize shell shape as rounded or elongated. If shell size and shape are indicative of the transition to the hermaphroditism, then our findings can help to place individuals of L. fulica into appropriate age classes, and this will represent a useful baseline for development of eradication policies.