Fine-scale aggregations and ecology of high-density Heliometra glacialis (Crinoidea) beds in a Marine Protected Area, Atlantic Canada

Benthic structure-forming species enhance habitat complexity and biodiversity, making them a priority for conservation and monitoring in Marine Protected Areas (MPAs). Here we document the presence of high-density beds of the unstalked crinoid Heliometra glacialis (Owen, 1833 ex Leach MS) within the St. Anns Bank MPA, off the Atlantic coast of Nova Scotia, Canada (45.8°N, 59.3°W). Crinoids were present in five transects within a near-seafloor optical imagery survey conducted in 2023 and 2024 to evaluate benthic community assemblages in the MPA. Within these transects, crinoids were found to occupy much of the area (4811 m ² ; 43.9% of the area covered), at depths of 77 to 119 m, with high-resolution digital still images and seafloor video revealing maximum local densities of 59.1 ind. m ^− 2 and 139.3 ind. m ^− 2 , respectively. Analysis of a subset of still images found relatively little size variation amongst crinoids, with a mean arm length of 19.03 cm (± 2.81 cm). We found that crinoid beds were distributed across variable mixed substrates on or near slope features subject to variable current vectors but were most abundant on cobble substrates. Spatial statistics (including Global and Local Moran’s I and the Gi * hotspot statistic) revealed significant spatial autocorrelation between video observations, resulting in localized clusters and hotspots, highlighting the prevalence of fine-scale patchiness. Collectively, these findings represent the highest recorded densities reported for the region and provide the first extensive documentation of the natural distribution of H. glacialis .