Abstract One-year time-lapse images acquired via an autonomous photo imaging device positioned at a depth of 20 m in Tethys Bay (Ross Sea, Antarctica) on a rocky bottom colonized by the sponge Mycale (Oxymycale) acerata were analysed. Monthly changes in the abundance and activity of the sea star Odontaster validus and sea urchin Sterechinus neumayeri on the sponge and nearby rocky bottom were compared with respect to environmental variables such as pack-ice presence/absence, temperature, salinity and photosynthetically active radiation. Sea urchins were more abundant on the rocky bottom and sponge during the summer and winter, respectively. Sea stars showed a decrease in the number of individuals on the sponge from January to December. The grazing activity of both species reached its maximum in January–April, when increased sunlight contributed to the phytoplankton bloom. The winter months were critical both for O. validus and S. neumayeri; although the red sea star maintained its pattern of activity on the rocky bottoms in terms of searching for food, the sea urchin reduced its activity. Time-lapse monitoring systems coupled with physicochemical sensors showed potential for revealing species behaviour in polar environments, contributing to the elucidation of future changes in coastal communities facing climate change.