Nanoindenation studies were carried out on single crystal calcite and on sea urchin spines from Heterocentrotus mammillatus, Phyllacanthus imperialis, and Prinocidaris baculosa. Unlike dense calcite single crystals resin embedded porous sea urchin spine segments showed a strong dependence of the indentation modulus, but not the indentation hardness, on the local porosity. This implies that the sampled volume for the indentation modulus in nanoindentation with forces down to 15 mN is not nanoscopic but extends approximately 50 mu m around the indentation spot. Only for indentation depths a parts per thousand(a)100 nm more or less mount-unaffected values of the indentation modulus could be found. The Voigt model for composite materials (calcite/resin) was found to be applicable for the dependency of the indentation modulus on the porosity. This is attributed to the network type of porosity and opens strategies for the control of stiffness in porous networks.