How to train your sponge: Imprinting material memories in marine sponge tissue

Sponges, though relatively simple in structure and function, are living animals and thus operate out of thermodynamic equilibrium. Often, they appear in aquatic environments with a moving current. It is natural to assume that they can adapt, actively or passively, to such an external drive. Here, we hypothesize that sponge tissue encodes material memories from cyclic driving that are similar to what they experience from moving current in vivo. We simulate externally driven current by way of oscillatory shear strain applied by a rheometer and show that sponge tissue remembers aspects of its previous loading history. In particular, we uncover that sponges, both living and decellularized dead skeletons, can form memories of two different kinds: memory of the largest strain, as well as memories of the training amplitude. We find that sponges with siliceous spicules embedded in their tissue have a higher capacity for both kinds of memory when compared to a species with no spicules, but surprisingly, a synthetic cleaning sponge also outperformed the species without spicules. We also shed light on so-called orthogonal memory effects. Our results broaden the class of disordered systems that form memories to include spongy materials.