There is growing awareness that to improve the understanding of the biological control of silicon (Si) cycling in the oceans, the biogeochemical models need to incorporate Si users other than diatoms. In the last decades, siliceous sponges are coming into sight as important Si users, but the scarce quantitative information on how they use Si is hindering the assessment of their role. We are here investigating Si consumption kinetics in two demosponge species (Tethya citrina and Hymeniacidon perlevis) that have contrasting biological features while inhabiting at the same sublittoral habitat. In laboratory experiments, we have determined that both species share some common traits when incorporating Si from seawater: (1) saturable Michaelis-Menten kinetics; (2) maximum velocity of Si consumption occurring at high silicic acid (DSi) concentrations ( 150 lM) that are not available in shallow waters of the modern oceans; (3) the ability to increase consumption rates rapidly and predictably in response to increasing DSi availability; and (4) halfsaturation constants that indicate an affinity for DSi lower than those of diatom systems. Across the four sponge species investigated to date, the affinity for DSi varies about 4.5 times. Our results also suggest that at least part of that between-species variability reflects the skeletonization level of the species. Within a given species, there are also between-individual differences in the DSi demand, which appear to reflect the particular physiological condition of each individual (i.e., body size, reproductive vs. non-reproductive stage).