Spatial distribution, sizes and densities of Pinna rudis have been studied in the Cabrera National Park. Densities varied spatially within the park (from low 0-0.16 to high 6.89 ind./100 m ) corresponding to a wide range of sizes. Most pen shells were patchily distributed and mainly concentrated in caves. High densities were observed in two hotspots, and represented the highest densities recorded worldwide, possibly linked to retention processes through high larval accumulation. The population size structure showed a unimodal distribution with individuals ranging from 6.2 to 25.0 cm shell width, with an average shell width of 16.0 ± 3.4 cm. Given the scarce data on this species, the present study provides valuable information for the spatial management and conservation of this threatened species
