Photosynthetic properties of two symbiotic demosponges were compared using Clark-type oxygen microsensors. The putatively distinct sponge species, Cliona viridis (Schmidt, 1862) and Cliona nigricans (Schmidt, 1862) were discriminated by their mean megasclere lengths of 296 and 387 lm, respectively. Photosynthetic behavior was used to generate additional taxonomic information. Sponge-dinoflagellate symbioses were well adapted to low light due to the hosts' endolithic lifestyle. Both sponges reached light compensation and saturation at similar light levels with means close to 10 and 30 lmol photons . m À 2 . s À 1 , respectively. The gross photosynthetic activity was closely related to symbiont cell density in the sponge surface tissue. Mean symbiont densities, chl a content, and gross photosynthesis were about six times higher in C. viridis than in C. nigricans, with respective values of 3000 and 440 symbiont . mm À 2 , 1.3 and 0.2 lg chl a . g À 1 , and 5.4 and 1.0 lmol O 2 . cm À 3 . s À 1 gross photosynthesis. Net photosynthesis and respiration could not be calculated accurately from the oxygen gradients, because significant gas exchange occurs through the pumping activity. Thus, assumptions of diffusional oxygen exchange via the surface do not hold for sponges. Combined data of this study indicate that the metabolic activity of C. viridis depends on photosynthetic activity of its symbionts, whereas C. nigricans appears to have a higher pumping intensity and is more actively filter feeding. The difference in photosynthetic activities is not caused by different light adaptations but provides new evidence against the conspecifity of C. viridis and C. nigricans.