Marine Sponge Tissue Displays Both Dynamic and Diverse Mechanics

Sponges are animals that utilize active and passive filtration to consume bacteria from the surrounding water and eject wastes and recycled carbon. They have evolved to inhabit nearly every type of flow regime, and are an ancient phylum, apparently predating the evolution of nerves and muscles. Sponge connective tissue is composed of a diverse set of collagens with an embedded scaffolding of siliceous spicules. It has long been suspected that there is a feedback between the mechanics of this tissue in environmental flows and the sponge’s body form. However, the physical behavior of sponge tissue has not previously been explored in any detail. Here we perform rheological experiments on a diverse set of sponge species and fit the data using hyperelastic constitutive models. Our results show that living sponge tissue displays anisotropic elasticity. The degree and direction of the anisotropy are both correlated with a given species’ overall growth form. The ability of this soft material system to grow tall in strong currents can be explained by the presence of silica-spicule-reinforced collagen fibers that allow dynamic strengthening along an axis aligned with the net force on the animal due to drag and lift in current.