A comprehensive survey of cryptic molluscs associated with autonomous reef monitoring structures (ARMS) in Trindade Island (South Atlantic)

Cryptic biodiversity in marine ecosystems remains largely underestimated due to the inherent difficulty of sampling organisms inhabiting reef microhabitats without causing significant environmental disturbance. This knowledge gap is particularly evident on highly isolated Brazilian oceanic islands, where cryptic invertebrates are still poorly documented. Here, we present a comprehensive taxonomic checklist of cryptic molluscs collected with Autonomous Reef Monitoring Structures (ARMS) deployed around Trindade Island, the easternmost Brazilian territory in the South Atlantic. Three ARMS were installed in the subtidal zone in each of the four studied sites around the island in August 2022 and retrieved after 26 months. Motile organisms larger than 2 mm and sessile molluscs were collected, counted and identified based on morphology, photographed and, when possible, complemented with DNA barcoding using the CO1 gene. A total of 654 molluscan specimens were recorded, corresponding to 45.3% of all motile organisms sampled. We identified 38 species distributed across four classes, with Gastropoda and Bivalvia being the most diverse and abundant groups. Columbellidae and Limidae dominated in abundance, largely due to the prevalence of Mitrella ocellata (Gmelin) (whitespotted dovesnail) and Lima caribaea A. d’Orbigny (file clam). Four species (9.97%) are endemic to Trindade Island, and several taxa represent new occurrences for the island, extending their known geographic distributions. One species (Chama congregata Conrad) was identified as cryptogenic. Our results highlight the effectiveness of ARMS in revealing overlooked components of reef biodiversity and underscore the importance of cryptic fauna assessments for improving biogeographic knowledge and conservation strategies on oceanic islands.