First results of field monitoring of oviposition patterns in the Iguazú snail (Pomacea americanista) at the Iguazú National Park (Argentina)

Abstract

Pomacea americanista is a freshwater snail, belonging to the Ampullariidae family, endemic to the Iguazú and Upper Paraná rivers. It has been categorized as Endangered at the regional level due to the reduction in its area of occupancy, extent of occurrence and habitat suitability. The objective of this work was to carry out field monitoring and analysis of the egg masses of this vulnerable species. To this end, egg masses laid on visible emergent substrates along the upper and lower trails of Iguazú Falls at Iguazú National Park, Argentina, were monitored. For 15 consecutive days, data were recorded on date, time, weather, developmental stage of the egg masses (fresh, advanced, very advanced, hatched, fully hatched) and substrate type. Egg masses were photographed daily, and some were collected to estimate the average egg size. Clutch size and distance from the waterline were measured using scaled photographs. Additionally, substrate coverage and egg mass counts by substrate type (woody, herbaceous, artificial and basalt rock) were recorded to assess oviposition preferences. A total of 75 egg masses were initially recorded, 58.9% of which were at an advanced developmental stage. By the end of the study, 101 egg masses had been documented, due to the addition of new clutches at the selected sites, where in total 51.5% were in the hatched stages. Eighteen egg masses were already hatched at the beginning of monitoring, 26 never hatched, 28 hatched during the first week, 23 during the second week, and 11 completed development (from fresh to hatched) in 11.55 days on average. The average egg size was 3.18 mm (± 0.09 mm). Among fresh egg masses, 30.3% (33) were located less than 10 cm from the water, considering the lowest egg. The average egg mass length was 4.76 cm (± 1.95 cm). No significant correlation was found between clutch size and its distance from the waterline. Substrate preference data suggests a positive tendency towards woody substrates, with no apparent preference among the remaining types. The Iguazú snail showed a clear oviposition preference for woody substrates, a novel finding for the species. Hatching success was relatively high, with many clutches completing development within two weeks, in line with lab data. The total number of egg masses observed was notably higher than in previous years, suggesting favorable reproductive conditions. A second sampling will complement the preliminary substrate use patterns observed in this first stage.