The Brazilian semiarid region is characterized by high temperatures, irregular rainfall, and two distinct seasons: dry and rainy. These environmental conditions shape aquatic habitats and influence the population dynamics of freshwater organisms, reflecting adaptations to the semiarid climate. Among these organisms, two snail species stand out: the native Biomphalaria straminea (Dunker 1848), a host for the parasite causing schistosomiasis, and the invasive, parthenogenetic Melanoides tuberculata (M & uuml;ller 1774), renowned for its resilience to adverse conditions. Despite their ecological and epidemiological importance, the population dynamics of these species at local and temporal scales remain poorly understood, known as the Prestonian shortfall. To address this, we conducted monthly monitoring from June 2017 to March 2020 in three reservoirs in the municipality of S & atilde;o Juli & atilde;o, Piau & iacute;, Brazil. We evaluated the dynamics of both species in response to semiarid seasonality. A total of 72,677 individuals were collected, comprising 5555 B. straminea and 67,122 M. tuberculata. B. straminea exhibited higher abundances, particularly among juveniles during the dry season, indicating sensitivity to seasonal variations. In contrast, M. tuberculata demonstrated consistent dominance throughout the year, regardless of environmental fluctuations. Analyses revealed that B. straminea was negatively impacted by the rainy season and rising temperatures, while M. tuberculata displayed remarkable tolerance and minimal seasonal adjustments. These findings underscore the enduring impact of seasonal patterns on freshwater organisms, even in artificially regulated environments such as reservoirs. They contribute to management strategies, epidemiological surveillance, and conservation efforts for aquatic biodiversity in semiarid regions.