Habitats & ecology
   
  Included in this section is the topic of chimneys, while GROWTH FORMS and GLASS-SPONGE REEFS are considered elsewhere.
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  Chimneys
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photograph of a subtidal demosponge Polymastia pachymastia

Chimneys are a distinctive feature of sponges.  It has long been recognised that their function (hence, their name) is to vent waste water from the sponge. The water leaving the chimneys has had its food and oxygen removed, and carries digestive and metabolic wastes – the latter in the form of ammonia and carbon dioxide. It is important that this waste water be eliminated in such a manner as to not contaminate the incurrent streams entering the sponge. This seems obvious, but has it been rigorously tested?

 

 

A subtidal demosponge Polymastia pachymastia
with multiple chimneys 5X

 
Research study 1
 

drawing of 4 demosponges Clathria sp. in preparation for an experiment on chimney functionLet’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 (3419): 89.

 

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

 

 

 

 

 

 

 

 

 

After a couple of days we move half the slides to a bowl in which a current is created by a bubbler (red arrow), and half to a bowl in which the water is still (with just a light drip to freshen the water).

  drawing of a demosponge Clathria sp. placed in a beaker with static water conditions
drawing of a demosponge Clathria sp. placed in a beaker with a unidirectional water flow
  Conditions for growth are good in each treatment and, within a week, the sponges grow larger with the chimneys growing in the orientations shown. The chimneys in the current treatments have grown in the direction of the current, while the chimneys in the still-water treatments have grown straight up.   drawing of a demosponge Clathria sp. placed in a beaker with static water conditions showing that the osculum grows straight up
drawing of demosponge Clathria sp. in a current showing bending of the oscululm in the direction of the flow
  We now move half the slides from each treatment to the other treatment. A few days later, we see that the growth in the transplants is consistent with the patterns observed initially in the two treatments.   drawing of a demosponge Clathria sp. originally from a beaker with a unidirectional water flow now placed in a beaker with static water conditions showing that the osculum now grows straight upSpecimens from the current treatment now grow with their chimneys straight up
drawing of a demosponge Clathria sp. originally from a beaker with static water conditions now placed in a beaker with unidirectional flow showing that the osculum grows bent overSpecimens from the still-water treatment now grow with their chimneys bent in the direction of the current
 

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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

 

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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-type seamless suit, or removal of body hair

   
 

Ideas that the chimneys might bend over in response to force of the current or to food being provided to the upstream side of the chimneys causing them to grow more on that side should be easily testable. It might even be possible to go out in the field, check for tilted growth of chimneys, and from the direction of the tilt predict current direction.

Another consideration relates to multi-chimneyed species, such as the purple sponge Haliclona permollis featured in real life in the Left-hand photo, and in manipulated view in the Right-hand one.  If the sponge were to grow in a current and its chimneys were to adopt a bent-over shape (Right photo), what conceivable functional advantage would the bent form have over the unbent form in terms of venting away wastes? Each chimney would potentially contaminate the ones downstream. With respect to a multi-chimneyed Haliclona, it might be better for the sponge if its chimneys didn't bend over at all!

NOTE the problem would be that currents, especially in shallow water, are rarely unidirectional

photograph of a purple sponge Haliclona permollis with stright-up oscula photograph of a purple sponge Haliclona permollis with bent-over oscula
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