Sponges are animals that feed by filtering water through their perforated body. We examined the in situ diel dynamics of sponge metabolism by continuously measuring the oxygen concentrations in the water inhaled and exhaled by undisturbed sponges. A clear daily pattern of oxygen removal was evident for six of the seven species we studied with their nocturnal oxygen removal being almost double the diurnal values (+ 86 ± 57%). Oxygenic photosynthesis by the sponge's symbiotic or endolithic phototrophic microbes may explain some of the diel difference, but significant day–night differences were also observed in three sponge species for which no evidence of photosynthetic activity (tested with imaging pulse-amplitude-modulation Fluorometry) was found. Mean oxygen removal (± 95% confidence interval for the mean) per species ranged from 1.7 ± 1 μmol O2 per liter (hereafter: μmol O2 L−1) for the low microbial abundance (LMA) sponge Callyspongia siphonella to 30.5 ± 10.5 μmol O2 L−1 for the high microbial abundance HMA) sponge Theonella swinhoei with considerable variation in oxygen removal across all scales (minutes to hours, within and among specimens). Events of high oxygen removal (> 50 μmol L−1) were regularly observed for five of the seven species and were predominantly nocturnal, occasionally lasting several hours. The high variability in oxygen removal stresses the need for long-term in-situ measurements of benthic suspension feeders metabolism.