The Cauxi sponge, a resident of the Amazon Basin, is a freshwater sponge with impressive adaptability. Belonging to the Demospongiae class, it can be considered a natural mineral-organic composite comprising sub-millimeter spicules embedded in an organic matrix, which acts as an adhesive layer. Two types of spicules are observed in a specimen from the Guaporé River: megascleres (about 150 µm long and 20 µm in diameter) and microscleres (50 µm long and 10 µm in diameter). Electron microscopy reveals that these spicules form a homogeneous, amorphous silica structure. We report the compressive strength of the spicules, obtained from micropillars, their modulus, revealed by nanoindentation, and their fracture toughness, tested using a pre-notch micro cantilever beam. The mesoporous nature of the biogenic silica is evaluated by SAXS data, showing pore sizes around 2.3 nm. Additionally, we revealed the shell structure of Cauxi gemmules, which are reinforced by short silica spicules acting as reinforcing struts. This discovery of mesoporous structures, synthesized under ambient conditions, inspires the design of artificial lightweight protective shell structures comprised of short fibers with disk-like extremities connected by an organic matrix.