How Intricate Venus-flower-baskets Manipulate the Flow of Seawater

A Venus’s-flower-basket isn’t all show. This stunning deep-sea sponge can also alter the flow of seawater in surprising ways. A lacy, barrel-shaped chamber forms the sponge’s glassy skeleton. Flow simulations reveal how this intricate structure alters the way water moves around and through the sponge, helping it endure unforgiving ocean currents and perhaps feed and reproduce. Previous studies have found that the gridlike construction of a Venus’s-flower-basket (Euplectella aspergillum) is strong and flexible. But no one has ever tried to see if these beautiful structures have fluid-dynamic properties.

Harnessing supercomputers, scientists simulated how water flows around and through the sponge’s body, with and without different skeletal components such as the sponge’s myriad pores. If the sponge were a solid cylinder, water flowing past would form a turbulent wake immediately downstream that could jostle the creature. Instead, water flows through and around the highly porous Venus’s-flower-basket and forms a gentle zone of water that flanks the sponge and displaces turbulence downstream, the team found. That way, the sponge’s body endures less stress. Ridges that spiral around the outside of the sponge’s skeleton also somehow cause water to slow and swirl inside the structure, the simulations showed. As a result, food and reproductive cells that drift into the sponge would become trapped for up to twice as long as in the same sponge without ridges. That lingering could help the filter feeders catch more plankton. And because Venus’s-flower-baskets can reproduce sexually, it could also enhance the chances that free-floating sperm encounter eggs, the researchers say. This simulation shows how water flows around and through a Venus’s-flower-basket (gray). Ridges that spiral across the outside of the sponge cause water inside to somehow slow and swirl, forming particle-trapping vortices. And the sponge’s shape creates a gentle zone of slower water that forms immediately downstream, buffering the creature against turbulence. Vertical cross sections contrast the flow activity of the calm zone (nearer the sponge) and the turbulent zone (downstream).

CITATIONS:

G. Falcucci et al. Extreme flow simulations reveal skeletal adaptations of deep-sea sponges. Nature. Published online July 21, 2021. doi: 10.1038/s41586-021-03658-1.