Revealing polychaete (Annelida) diversity using fish-specific ARMS in the Paranaguá Estuarine Complex, southern Brazil

Autonomous Reef Monitoring Structures (ARMS) are standardized and efficient tools for assessing cryptic biodiversity in marine ecosystems, enabling the study of often-overlooked organisms such as polychaete worms (Annelida). By mimicking the structural complexity of natural reef habitats, these structures support the colonization of a wide range of sessile and mobile organisms and are widely used in biomonitoring, including studies on invasive species, spatial distribution, and temporal dynamics of marine communities. Our study was conducted in the Paranaguá Estuarine Complex in southern Brazil. Five fish-specific ARMS (FARMS) were installed in three estuarine beaches within the estuary 's mixing zone below a meter depth, namely Amparo, São Miguel, and Nova Brasília. At the laboratory, polychaetes were sorted from the retained sediment and fouling organisms were scraped from the FARMS structures. A total of 24 species, 16 genera, and 13 families were recorded, with higher richness in Sedentaria compared to Errantia. The most representative families were Nereididae, Sabellidae, and Terebellidae, and the most abundant species included Alitta succinea and Loimia brasiliensis. Species richness and Shannon diversity varied among areas, with São Miguel showing higher richness and variability, Brasília lower values, and Amparo intermediate but with no replication. Beta diversity was high and mainly driven by species turnover, indicating that differences among areas result primarily from species replacement rather than nestedness. PERMANOVA detected differences in community composition; however, significant heterogeneity in dispersion suggests these results should be interpreted cautiously. NMDS ordination showed a good fit, although low species richness may limit pattern detection. The presence of non-indigenous species highlights the influence of port activities in the region. Overall, results support the hypothesis that polychaete assemblages reflect environmental heterogeneity across the estuary. ARMS proved to be effective tools for detecting fine-scale spatial variation in cryptic benthic communities, reinforcing the role of environmental filtering and spatial processes in structuring biodiversity.