In the north-east Atlantic, the dominant reef-framework forming coral species, Lophelia pertusa and Madrepora oculata, form a symbiotic association with the polychaete worm Eunice norvegica. The polychaete–coral symbiosis was studied by visually observing and photographing live animals in aquaria over many months and using time-lapse video under infra-red lighting to record activity patterns. The polychaetes act as reef aggregating agents by joining coral colonies and enhancing the development of reef patches in deep water. The symbiosis was investigated using samples collected from a relatively shallow site in a Norwegian fjord and from a deeper open ocean site in the Porcupine Seabight. The potential functional roles of this symbiosis are considered. The reef aggregating behaviour of the polychaete symbiont allied with the ability of the coral host to anastomose its branches, the polychaete tubes and debris falling onto the reef structure will help to shift the balance between reef growth and (bio)erosion in favour of growth.