Limnetica 35

The gastropod Heleobia parchappii is distributed in lentic and lotic environments in the Pampas region of Argentina and is a host for at least 24 species of digeneans. Because ecoparasitological investigations of snail-digenean associations frequently lack knowledge of the structure and dynamics of the snail host population, the purpose of this study is a long-term analysis of the basic aspects of the H. parchappii life cycle as a preliminary step of an environmental parasitology study. Monthly collections were made in Nahuel Rucá Lagoon from August 2010 to February 2013, and a total of 24 931 individuals of H. parchappii were measured and size-frequency distributions were constructed. The von Bertalanffy growth equation and the von Bertalanffy equation with seasonal oscillation growth equations were fit to shell length data. The size-frequency distributions of H. parchappii fit a polymodal distribution, and 14 age classes were identified. The breeding periods of H. parchappii in 2011 were in summer (February and March), autumn (June) and spring (October). However, in 2012 the periods of recruitment were in autumn (May), winter (July) and spring (October). The von Bertalanffy models best fit the data, and the likelihood ratio test comparison showed significant differences in the estimated parameters between cohorts. This is the first study in which three cohorts of H. parchappii were followed from recruitment until death, and a life cycle of approximately 20.33 months was estimated. The observed differences in the recruitment periods and growth parameters could be a strategy by the snail to prevent the negative effects on reproductive success caused by digeneans. We suggest that long-term studies that include a larger number of environmental and biological factors are needed to fully understand the dynamics of the recruitment and growth of juveniles of this species across its distribution range.