Dormant cysts are resistant to acute malathion toxicity, but hatching phenology is altered in Dendrocephalus brasiliensis (Crustacea: Anostraca)

Abstract Dendrocephalus brasiliensis Pesta, 1921 is an anostracan characteristic of temporary aquatic environments whose persistence depends on the viability of dormant cysts accumulated in the sediment. This study tested the hypothesis that nauplii are more sensitive to malathion than cysts and that exposure to the latter may alter hatching dynamics. To test this hypothesis, acute toxicity assays with nauplii and hatching experiments with cysts were conducted using environmentally realistic concentrations of a commercial formulation of the organophosphate insecticide malathion. Malathion exhibited high toxicity to nauplii, whereas cysts maintained high hatching rates even at the highest concentrations tested, indicating greater tolerance to the contaminant and highlighting their role as an essential component of species persistence and resilience. However, this apparent resistance did not imply the absence of effects, as exposure altered hatching phenology by reducing the initial hatching peak and redistributing hatchling emergence over time. These sublethal alterations may impair recruitment and modify population structure and dynamics in temporary aquatic environments, underscoring the importance of incorporating dormant stages into ecotoxicological risk assessments.