Population Structure and Dynamics of Lymnaea columella (Say, 1817) (Gastropoda: Lymnaeidae) in Wetlands of Northeastern Argentina

We examined the abundance, the population structure and the dynamics of Lymnaea columella, an intermediate host of Fasciola hepatica, in the Ibera Macrosystem, Argentina, and their relationship with biotic and abiotic factors. A 6-month drought in the middle of the study provided an opportunity to analyze population recovery. Seasonal surveys were performed over a 3-year study period (2002-2005) in banados and malezales, 2 typical wetlands of the Macrosystem. Snails of different taxa were collected in each environment and survey. Lymnaea columella (n=7851) was found throughout the study period except during the drought. In humid periods, abundance differed seasonally (p＜0.001), being highest in winter, intermediate in autumn-spring, and lowest in the hot, dry summer (＞58.5%, 1.6%-34.6%, and ＜1.7% of the total collected, respectively). Sexually immature individuals (＜6.5 mm) appeared in autumn and peaked in spring (＞60%); adults (＞6.5 mm) dominated in winter (＞60%), and among these, intermediate-sized snails (6.6-13.1 mm) were the only ones found in summer and immediately after the drought. Intermediate-sized snails, which are resistant to environmental stress and capable of producing viable offspring when conditions become favorable, may allow rapid population recovery, which begins in autumn. Continuous reproductive activity was observed between autumn and spring. The banado showed the highest abundance (410.9 snails/30 min; 62.8% of the total collected; p＜0.001) and probably serves as the aestivation site and as a source of L. columella in the area. Biomphalaria spp. and L. columella showed opposite patterns of seasonal abundances, which may explain the low and high abundances of the latter in winter and summer, respectively. This is the 1st report of L. columella in these wetland types. The successful establishment of this invasive lymnaeid suggests a broad distribution throughout the region.