Extreme fluid dynamic simulations reveal environmentally-driven morphological features of Euplectella aspergillum

Abstract Due to its remarkable structural properties as well as its tantalising beauty, silica depth sponge Euplectella aspergillum has attracted the interest of scientists all over the world since its discovery [1, 2]. Its skeletal system, in fact, is composed of amorphous hydrated silica and it is arranged in a highly regular and hierarchical cylindrical lattice, endowing the whole structure with an amazing flexibility and resilience to damage, [3-7]. In contrast with the major interest in the mechanical properties of the skeletal structure of these hexatinellida, the study of the hydrodynamic fields which surround and penetrate the glassy sponge has remained largely unexplored to date, leaving an open question as to the impact of fluid dynamic patterns on Euplectella's environmental physiology. A particularly outstanding question in this respect is whether, besides boosting its tribological characteristics, the structural motifs of Euplectella may also respond to an optimisation design in terms of minimising the hydrodynamic stress experienced by the structure. This is precisely the question addressed in the present work. To this purpose, we resort to extreme fluid dynamic simulations based on the Lattice Boltzmann Method [8], featuring of the order of one hundred billion grid points and spanning four spatial decades, from the micro-scale details of the skeleton, all the way up to the full structure of Euplectella. Such in-silico experiments reproduce the actual living conditions of Euplectella [9-11], and prove that the sponge structural elements, not only reduce the overall hydrodynamic stress experienced by the skeletal structure, but also support coherent internal recirculation patterns, arguably feeding the sponge and its host organisms. The present results open the path towards a new class of numerical investigations at the intersection between fluid mechanics, computational biology and environmental physiology.