Select 720p HD for best quality. An ROV dive near Bigbury Bay, Plymouth to ground-truth some grab samples we took resulted in the discovery of this large bed of Ophiothrix (Ophiuroidea) commonly known as brittle-stars. This showed us the immense value of ROV in shallow-water studies where the benthic fauna is known only from grab samples (no brittle stars were recorded in our samples, despite the huge densities obvious in the video footage).
The species in the movie are probably Ophiothrix fragilis, Ophiocomina nigra and Ophiopholis aculeata. They wave their arms in the current to trap food particles held in suspension. The current (based on piloting experience) was probably about 0.5-1 knot during filming. Above 1 knot, it is hard to hold the ROV in position.