Numerical model of the flow past and through the sponge Euplectella aspergillum

In a recent study, Falcucci et al. [Nature, 595: 357-541, 2021] examined the
hydrodynamics of the deep-sea sponge Euplectella aspergillum using a model
of just the skeletal motifs of the sponge. The skeleton was described as a
porous cylindrical structure decorated with helical ridges. Their simulations
revealed that the flow passes through the skeleton from the upstream to
the downstream hence providing strong internal recirculation enhancing
feeding and sexual production. However they neglected the sponge tissue
including the outside porous surface, the internal canal systems, and the
choanocytes --- the pumping units of the sponge. This approximation will likely
influence the flow of the living sponge cf. Leys et al.
[Nature, in press, 2022].

In the present study we include a realistic model for the outside porous
surface, while neglecting the skeleton and internal canal system. We impose
the pumping rate explicitly through specification of the exhalent flow.
Simulations with an oncoming flow speed of 5 cm/s and an exhalent flow of
3 cm/s predict a pressure drop across the outer porous surface of approximately
8 Pa in good agreement with the expected pressure delivered by the choanocytes.

Moreover, the porosity is found to prevent cross flow and strong internal recirculation, highlighting the importance of including the sponge tissue.