Scaling in Filter-Feeding Sponges

Allometric equations for pumping rate of filter-feeders are widely used in physiological, ecological and modeling studies. The dependence of dry weight-specific filtration rate on size, for example for the blue mussel, is well known and has been modeled, but no similar scaling results exist for the morphologically and functionally more complex sponges.

Here, existing results on volume-specific pumping rate versus size show conflicting trends, while data on pumping rate per basic pump-unit, the choanocyte, are sparse although of crucial interest for our understanding and description of these basic pump units and the resulting sponge performance. Sponge pumping rate determines feeding rate, hence grazing impact, but may be difficult to measure.

Based on sponge modeling, we present results that show how the volume-specific pumping rate changes with morphological size-change for the case of a leucon-type demosponge and a sycon-type calcareous sponge. This requires: (i) a known morphology and pumping rate of a ‘reference sponge’, and (ii) knowledge of the characteristics of both the choanocyte flagella-pump and the aquiferous canal system, i.e. the relations between volume flow rate and pressure gains/losses.

Such scaling is governed by mechanistic flow phenomena but other factors may be at play. Sponges may comprise several modules of different size and age, each with a certain choanocyte density, and more data is needed for the undisturbed structure and functioning of the choanocyte filter-pump in the different types of sponges, which may represent a variety of pump principles yet to be discovered.