The EDO mission's main objective was to estimate the densities of brittle-stars (Ophiocomina nigra and Ophiothrix fragilis- Echinoderm), using a drop-down video frame (HD camera) combined with laser altimeter and quadrant (0.05m²). This system provides accurate estimations of the density levels of organisms on the seafloor (for species which are quite visible on the substrate) and the different assemblage modes and size structures of brittle-stars depending on sites and species, in relation with their environment (hydrodynamism, substrate, food source and competition with other benthic invertebrates). Better understanding the dynamics of species' development and better identifying the favourable environmental conditions for the development of species, especially for Ophiocomina nigra whose presence is very dense at the tip of Brittany (Brest bay and Douarnenez bay). Concurrently, brittle-star specimens were sampled using a Van Veen grab and dredge to perform a series of measurements on them. The organisms were to be measured on board and a sub-sample frozen for analysis in the laboratory. The cruise is related to the CRAPO action linked to the HABITAT project.