Integrative approaches to the trophic ecology of freshwater ostracods (Crustacea) in a Neotropical floodplain.

Abstract

Freshwater ostracods (Crustacea) are small, bivalved crustaceans widely distributed across lakes, rivers, and reservoirs. They are often abundant and associated with aquatic macrophytes, playing an important role in aquatic food webs by linking primary producers and detrital resources to higher trophic levels. Ostracods are considered herbivores, detritivores, omnivores, and occasionally carnivores, while also representing an important food source for fish and other consumers. However, the trophic ecology of freshwater ostracods remains poorly understood, particularly in tropical regions. A major challenge in studying ostracod diets is the preservation of ingested food items, since they are fixed in ethanol, which can damage delicate resources such as algal cells, impairing their identification. In addition, the relationship between mouthpart morphology and trophic specialization remains poorly documented for most freshwater species. Thus, we tested the effectiveness of four fixatives (ethanol, formalin, Lugol, and Transeau) in maintaining the faecal pellet contents of the algivorous species Cypricercus alfredo Almeida et al. 2021. A total of 28 algal taxa were recorded, with green algae as the most frequent resource. All four fixatives were suitable for preserving algal cells for quantitative analyses, but Lugol and Transeau provided better short-term estimates of algal richness, indicating that preservative choice may influence dietary interpretations. The trophic diversity of ostracods was examined in 10 species representing four families through analysis of faecal pellets combined with comparative morphology of mouthpart appendages. The results showed that freshwater ostracods are not uniformly generalist feeders, but include species with distinct trophic guilds, such as algivorous, detritivorous, fungivorous, omnivorous, and opportunistic feeders. Mouthpart morphology was strongly associated with feeding strategies, allowing the recognition of five functional feeding groups, including scraper, predator/scraper, scraper/filter, gatherer, and gatherer/filter. Although different families exhibited contrasting body plans, species from unrelated groups could occupy similar trophic guilds, indicating functional convergence. In contrast, closely related species of Cyprididae with similar mouthpart morphology often exploit different food resources, revealing high ecological plasticity. This trophic versatility may help explain the ecological success and diversification of this family in freshwater ecosystems. Overall, the results advance the trophic ecology of freshwater ostracods by improving methodological protocols for dietary studies and demonstrating clear links between morphology and feeding function. These findings expand the ecological understanding of an abundant group of aquatic small crustaceans and provide a basis for future studies on ecosystem functioning and food-web dynamics in the Neotropical region.