The relationship between the scyphozoan Nausitoe punctata and the horny sponges Cacospongia scalans, Dysidea avara and D. fragdis was analyzed. Evidence was found for utilization of the thecae of N. punctata as a substitute for skeletal fibres. Consequently some metabolic costs associated with skeleton-buildng may be reduced for the sponge. Further potential benefits for the scyphozoan, e.g. protection against predation and mechanical disturbance, and trophic advantages, are discussed This association is suggested to be non-parasitic and mutualistic, and widely distributed among 6 orders and 8 families of the Demospongiae which occur throughout the Northwestern Mediterranean.This study was supported by Project CICYT MAR91-0528 (Marine Resources and Aquaculture National Program) and a 'Generalitat de Catalunya' fellowship.Peer reviewed