Let’s do an experiment to test the hypothesis that a growing sponge adopts a form that minimises the amount of exhaled water re-entering its incurrent pores. First, we dissociate the cells of the demosponge Clathria and allow the reconstituting sponges to grow on microscope slides.Experiments and results from Warburton 1960 Science 132: 89.

NOTE the genus, formerly Microciona, includes several species of small marine demosponges, some found in the low intertidal region

Do our results support our research hypothesis, namely, that a sponge adopts a form that minimises the amount of exhaled water re-entering its incurrent pores? Think about which answer may be correct from the choice below and CLICK on it for explanations.

Yes 

No

What other explanations might there be for our results? – ones perhaps unrelated to what we initially had hoped to show? Think about this for a moment and compare your idea(s) to the two listed here.

1. What if the actual force of the current is bending the chimneys, through pressure drag or frictional drag? Thus, the new growth of the chimneys may be simply be a streamlining tendency on the part of the sponge and not related to water re-entering the sponge’s incurrent pores. This is good “lateral” thinking, something that is lacking in the original publication.

2. If your idea is that bacterial food taken in on the upstream side of the sponge and the subsequent growth from that side may have caused the chimneys to bend in the downstream direction, then you are thinking creatively. So, the bending may have had nothing to do with our original hypothesis about used water re-entering the incurrent pores. 

NOTE  these two forces, pressure drag and frictional drag, come into play when any object moves through a fluid medium, whether air or water, or when fluid passes by an object. Pressure drag comes from the mass of fluid being displaced as it passes around an object, in this case, the sponge’s chimney. The displacement creates an area of negative pressure behind the object and the pressure of the fluid pushes the object into it. The second force, frictional drag, is the frictional resistance of molecules of the fluid moving past the object – the resistance pulls the object in the direction of the moving fluid. Confused? Think of walking into a wind. The two drag forces combine to impede your forward movement, with the frictional part being visually manifested by rippling of your clothing. If you are swimming, the forces are much greater because of greater density of the fluid. Here, pressure drag comes into play with the mass of water that has to be displaced with each swimming stroke, and frictional drag with the resistance to water molecules passing along the surface of your skin. Nothing can be done about pressure drag when swimming, but frictional drag can be lessened by streamlining...such as a full-body Speedo seamless suit, or removal of body hair