Evaluation of diversity patterns of echinoderms with cryptic habits using Autonomous Reef Monitoring Structures (ARMS) in two reefs of Gulf of Mexico and Mexican Caribbean Sea

Echinoderms are a conspicuous group on coral reefs. Juvenile and small-sized species often hide in living or inert substrates to protect themselves from predation. This cryptic behavior makes it difficult to count, identify, and capture them during fieldwork, resulting in a poor understanding of their ecology and an underestimation of their abundance. Autonomous Reef Monitoring Structures (ARMS) provide a protected environment that mimics natural habitats and is capable of recruiting echinoderm cryptofauna. They also serve as a standard structural measurement unit, allowing comparisons between samples and detection of spatiotemporal patterns of compositional change. To explore these patterns, we deployed and recovered 50 ARMS on two reefs in the Yucatan Peninsula from 2018 to 2020. A total of 976 individuals belonging to 36 species and five morphotypes were collected. The dominant class was Ophiuroidea, which accounted for 92.4% of the abundance. Bajo de Diez Reef (B10) hosted approximately 92% of the total abundance. Both reefs shared only around 37% of their species. Although species abundance and composition changed over time, no species with pioneering or late-settlement tendencies were identified. October 2019 saw the lowest recruitment abundances on both reefs. This decrease may be linked to the effects of the positive phase of ENSO, in synergy with the massive flux of Sargassum spp. during summer and autumn of 2019. Since cryptic echinoderm abundance and composition seem to be influenced more by resource availability and environmental conditions than by community succession, future studies should compare environmental factors and variations in benthic groups that serve as resources for echinoderms to test our conclusions.