We suggest that due to the high cost of pumping, current-induced flow is highly beneficial but may occur only in thin walled sponges living in high flow environments. Applying our model to the in situ measurements indicates that even modest pumping rates require an energetic expenditure of at least 28% of the total in situ respiration.
The difference is due to the resistance created by a fine protein mesh that lines the collar, which demosponges also have, but was so far overlooked.
Our model indicates that the head loss across the sponge collar filter is 10 times higher than previously estimated. During short bursts of high ambient current the sponges filtered two-thirds of the total volume of water they processed daily. Studies of sponge filtration have estimated the energetic cost of pumping to be 0.75) with the ambient current velocity. Flow through sponges is thought to be enhanced by ambient current, which induces a pressure gradient across the sponge wall, but the underlying mechanism is still unknown. Sponges are suspension feeders that use flagellated collar-cells (choanocytes) to actively filter a volume of water equivalent to many times their body volume each hour.
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