Decapod Crustaceans in a Transitional River System: Insights from the Ribeira de Iguape Ecoregion, Brazil

Freshwater ecosystems are essential for biodiversity conservation but face increasing threats from human activities. This study assessed the species richness, diversity, and spatial and temporal distribution of decapod crustaceans (Caridea and Brachyura) in the lower Ribeira de Iguape River, southeastern Brazil, a globally recognized freshwater ecoregion. Monthly sampling took place over one year at three sites using both active and passive capture methods. Environmental parameters were measured in the field, and multivariate analyses (correspondence analysis, PERMANOVA, and multiple regression) were applied to explore species-environment relationships and community structure. A total of 42,897 individuals were recorded, including eight Caridea and four Brachyura species. Carideans accounted for over 97 percent of the total, with <i>Potimirim</i> spp. and <i>Macrobrachium acanthurus</i> as the most abundant. The site nearest the river mouth showed the highest species richness and diversity, likely due to its transitional characteristics and moderate salinity variation. Environmental variables such as dissolved oxygen and salinity influenced species occurrence, although biological interactions may also contribute. Seasonal changes in abundance appeared most evident during winter and spring, driven by shifts in dominant taxa. The presence of the non-native <i>Macrobrachium rosenbergii</i> suggests early establishment and reinforces the need for long-term ecological monitoring in transitional freshwater environments.