The study of neotropical snails has gained importance in recent years because some species, like Succinea costaricana, have become quarantenary or agricultural pests. We studied the yearly body size distribution of this species in an ornamental plant farm at Guápiles, Limón, Costa Rica; S. costaricana originally inhabited the tropical rain forest that was cleared to establish agricultural space in the XIX century. Fixed-time collection was done every other month in randomly selected plants and soil sections. Body size distribution (measured in the shells) indicates a capacity to reproduce year-round with a peak when pluviosity decreases in December. At this time of year the population is dominated by snails under 4 mm in shell length (longest individual: 12.06 mm). However, the yearly rainfall pattern does not correlate with shell length, width or width/length ratio: rainfall alone is not the most important factor affecting population dynamics. Eggs of S. costaricana are covered only with a mucilaginous material that might easily be infected or predated. The reproductive peak in the less humid part of the year may be an advantage if levels of predation or infection are lower.
