First record of biogenic silica in the stomach content of freshwater turtles

Aquatic species such as fish and turtles consume sponges and plants, which are sources of biogenic silica. Sponges belong to the phylum Porifera, and species in the class Demospongiae produce siliceous spicules, while plants form phytoliths. Some turtle species adjust their diet according to ontogenetic stage, with most being predominantly omnivorous and predators of silicified organisms. Studies on the ingestion of biogenic silica by turtles remain scarce in freshwater environments. This study aimed to analyze the diversity of biogenic silica bodies in the stomachs of the species Phrynops geoffroanus. Specimens were captured in Iguaçu National Park (PNI), Paraná, a vital remnant of the Atlantic Forest. Individuals underwent biometric measurements, photographic documentation, and euthanasia using Thiopental (93 mg/kg), following strict ethical protocols. Biological material was sent to the State University of Maringá, where stomachs were extracted. Stomach contents were processed at the Laboratory of Paleoenvironmental Studies (LEPAFE) at the State University of Paraná (UNESPAR), treated with HNO₃ on a heating plate, and the resulting material was mounted on slides for analysis. Three specimens of Phrynops geoffroanus at different ontogenetic stages were examined. Stomach analysis revealed the presence of biogenic silica, including phytoliths, diatom frustules, and sponge spicules, with the highest concentration found in young individuals and the lowest in juveniles. The predominant phytolith types suggest interactions with grasses and Podostemaceae. Gemmuloscleres of Oncosclera navicella were identified in young and adult individuals, confirming predation on freshwater sponges. This study highlights the trophic ecology of Phrynops geoffroanus and its interaction with silicified organisms.