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.