Taxonomically standardized iNaturalist records reveal broad-scale patterns in documented polychaete diversity across U.S. coastlines

Citizen-science platforms are increasingly used to document biodiversity, but their value for taxonomically difficult marine invertebrates remains uncertain. Polychaete annelids are diverse, ecologically important, and often difficult to identify from photographs alone, raising questions about how community-science records should be interpreted. Here, we evaluated research-grade iNaturalist observations of Polychaeta across the continental United States. We retrieved 13,775 observations from iNaturalist, standardized names against a World Register of Marine Species-derived taxonomic backbone, assigned records to coastal regions using geographic coordinates, and analyzed observation-based richness, family composition, species overlap, rank-observation structure, and rarefaction patterns. After filtering, 12,539 observations representing 302 species, 54 families, and 4,027 observers were retained. Sampling effort was highly uneven, with the West Coast accounting for most observations, species, families, and observers. Species overlap among coasts was limited, and dominant families differed regionally, although tubiculous families such as the Serpulidae were prominent across all three coasts. Rank-observation curves showed that a small number of frequently documented species dominated each regional record, whereas many taxa were represented by few observations. Observation-based rarefaction indicated continued species accumulation, especially on the West Coast. These results show that taxonomically standardized iNaturalist records can reveal broad patterns in documented polychaete diversity while underscoring the need for expert validation, voucher-based taxonomy, and careful interpretation of observation-derived biodiversity data.