The cytoskeleton in cells is composed of semiflexible, inextensible actin filaments, dynamic cross linkers, and molecular motors. It can adapt its shape, thereby allowing the cell as a whole to change its mechanical properties. In this talk, I will discuss our novel numerical method for simulating the hydrodynamics of passively cross-linked actin networks and what our simulations tell us about the feedback between rheology, morphology, and the role of hydrodynamic interactions in these networks. In our numerical method, the fibers are represented by continuum Chebyshev interpolants, the nonlocal hydrodynamic interactions are computed by slender body theory, and fiber inextensibility is enforced by strictly confining the velocity to the space of inextensible motions. To enable rapid simulation of transient cross linkers, we coarse-grain their dynamics into a rate at which each end binds and unbinds from the filaments. To model polymerization, we turnover the filaments with a characteristic time constant. Our simulations show that the steady state morphology can vary from a homogeneous meshwork (for fast turnover times) to a clustered bundle state (for slow turnover times), and that the mechanical behavior and role of hydrodynamic interactions are functions of the underlying morphology and timescale on which we look at the network. Our simulations also demonstrate that Brownian motion speeds up the bundling process more in the initial stages than in the latter. Speaker(s): Ondrej Maxian (New York University)