Aim: Understanding how species’ traits mediate environmental responses provides a mechanistic perspective on community assembly processes. Although traits that influence the response of multicellular organisms to environments have been studied, the identification of such traits in unicellular organisms, like ciliates, is underexplored. Since ciliates are dominate animals at coastal sandy sediments and contribute significantly to interstitial community respiration, identifying links between their occurrence and sandy-coast characteristics is crucial for conserving biodiversity and protecting coastal habitats.

Location: Coasts of China.

Methods: We used Hierarchical Modeling of Species Communities, a trait-based joint species distribution model, to analyze ciliate community data collected from 344 sites belonging to 31 beaches across the Chinese coastline.

Results: We found that physical variables play the most important role in determining ciliate occurrence. The beach index which combines the tidal range, sediment grain size and beach slope of beach selected species based on body size as well as feeding type, while sediment grain size constrained different mobility types, with swimming species favoring fine sandy sediments. The significant contribution of phylogeny in explaining the residual variation among species responses indicates that there are phylogenetically conserved but unmeasured traits too that influence species’ responses. Species richness of benthic ciliates was higher in dissipative beaches with high primary productivity.

Main conclusions: Based on our research, ciliate occurrence in coastal beaches is mediated by their traits which are filtered by local environmental variables. The beach index is essential in predicting distribution patterns and species richness of benthic ciliates. Nevertheless, further research is needed to identify additional traits that will enhance the accuracy of predicting ciliate niches. Our study provides insights into the mechanisms driving ciliate community assembly processes and has implications for the conservation of biodiversity in sandy coastal habitats.