The evolution of marine demosponges has led to two basic life strategies: one involving close associations with large and diverse communities of microorganisms, termed high microbial abundance (HMA) species, and one that is essentially devoid of associated microorganisms, termed low microbial abundance (LMA) species. This dichotomy has previously been suggested to correlate with morphological differences, with HMA species having a denser mesohyl and a more complex aquiferous systems composed of longer and narrower water canals that should necessitate slower seawater filtration rates. We measured mesohyl density for a variety of HMA and LMA sponges in the Florida Keys, and seawater pumping rates for a select group of these sponges using an in situ dye technique. HMA sponges were substantially denser than LMA species, and had per unit volume pumping rates 52-94% slower than the LMA sponges. These density and pumping rate differences suggest that evolutionary differences between HMA and LMA species may have resulted in profound morphological and physiological differences between the two groups. The LMA sponge body plan moves large quantities of water through their porous tissues allowing them to rapidly acquire the small particulate organic matter (POM) that supplies the majority of their nutritional needs. In contrast, the HMA sponge body plan is suited to host large and tightly packed communities of microorganisms and has an aquiferous system that increases contact time between seawater and the sponge/microbial consortium that feeds on POM, dissolved organic matter and the raw inorganic materials for chemolithotrophic sponge symbionts. The two evolutionary patterns represent different, but equally successful patterns and illustrate how associated microorganisms can potentially have substantial effects on host evolution.