How extremely stupid not to have thought of that!

December 10, 2014 • 11:32 am

by Matthew Cobb

The title of this post – “How extremely stupid not to have thought of that!” – was Thomas Huxley’s response when reading Darwin’s idea of evolution by natural selection. It was also my response, and Jerry’s, on seeing this amazing video of sponges eating.

Every year, I teach students that sponges are animals, and that they eat by wafting water currents through their bodies, with a flagellum situated in structures called collar cells. The flagellum captures tiny organic particles in the water, which are then absorbed into the sponge’s body and used for both metabolism and to make more sponge. Whenever I’ve said this in a lecture, there’s been a tiny voice in my head saying ‘But how strong is that current? Can they really get enough particles to grow? Am I actually telling the truth?”.

Now I know the answer – the current creating by those hundreds of thousands of whipping flagella is amazingly strong, as shown by this simple experiment: you release a dye around a sponge, and see what happens to it. How extremely stupid of me not to have thought of that!

62 thoughts on “How extremely stupid not to have thought of that!

  1. Don’t recall seeing this demonstrated with a sponge, but we used same procedure with live tunicates in an invertebrate zoology lab at University of Washington — many decades ago!

  2. Wow, that’s cool.
    I’ve gone snorkeling all over the reefs of the Florida Keys hundreds of times and I knew that sponges were filter feeders and I even knew that they moved water through their bodies via tiny flagellum, but I had absolutely no idea that they were capable of generating that much force. I wonder how many calories, in say one day, this type of filter feeding would yield? As it would appear to require some significant expenditure of energy for tiny flagellum to move that volume of water.

  3. That is awesome. I’d like to try it the next time I dive but I don’t think I can get hold of a nontoxic dye. 🙁

  4. Does this way of feeding also explain the overall shape of the sponge? (If it does, my thought now is: “How extremely stupid not to have even wondered about that!”)

    1. No, not really related to shape. Most sponges feed this way, but shapes of sponges vary a lot: flat crusts, blobs, tubes, spheres.

      1. Hm, now I’m suspiciously reluctant to give up on my idea.

        Are the bigger sponges mostly that (or some other hollow-with-a-hole) shape?

        1. Sponges don’t have a skeleton as such. Many have individual spines of silica or calcium carbonate, but these are rarely preserved closely interlinked, so it’s difficult to get an idea of their form from the fossil record. However from the Early Cambrian (525 Myr ago) there was a similar group of sessile (fixed to the substrate) filter-feeding organisms which formed the earliest reefs, and they show much the same range of forms as modern sponges. Look up the Archaeocyatha.
          There is a substantial range in shape, but the need for support, surface area and an excurrent (out-flowing current) port does somewhat constrain the overall form.

            1. I’ve never seen a fossil of one – not even in a museum. And I have kept my eyes peeled when in rocks of appropriate age. Not the best-known of fossicks.

      1. I wouldnt call it the opposite orifice either since the plankton is retained. It would be closer to being like a fish’s gills.

    1. The outlet hole is the ‘osculum’. There are different forms of sponges. The ones in the video have a pretty simple form with one or a small # of big outlets. Others are more of a multiplex, with lots of smaller outlets. Those include the kinds favored as bath sponges.

        1. ‘But… that means I am ingesting some of your sponge pee’.
          ‘Well, that means I am also ingesting your sponge pee’.

          1. Swansea’s favourite son, Dylan Thomas, is reputed to have declined to drink water – preferring whiskey – on the grounds that “fish pee in it”.

  5. Wow. Wow. My first thought (with the music) was that New Age people might start getting attracted to sponges and putting a variety of dye colors and see ‘sponge power’ develop much like they think ‘crystal power’ works.

  6. I appreciate straightforward experiments like this that illustrate what a seemingly inert animal is actually doing. Neat-o

        1. In your world, maybe. I’m from an alternative universe that’s converged with yours, so (contrary to the usual many-worlds expectation) the future of this present has multiple pasts.

  7. New toy, not sure if I’ve got the magic words set up.
    That is a wonderfully cool demonstraton.
    Some technical points.
    Matthew’s scuba/ snorkelling hours need buildng up. within the first few hours of drifting above the reefs of Nyuni, I was used to seeing the incurrent sweepnig up the (shell fragment) sand I stirred up. Hours with a snorkel and very shallow water … means some serious sun burn. Really! Painful. On my return job I took opaque tee-shirts and still had some bad-leg days.
    Dyes. I do wonder exactly what they use. I would START from food colouring. (Principle : if humans can eat it, then it’s probably not a big problem.
    ‘d also look carefully at using a lamp that leaks a lot of UV. That video does have a slightly “glare” quality” that makes me think “UV” lamp. Or maybe they have a UV lamp next to the main lamp, and both are offset by 10s of degrees from the optical axis. In general, moving the lamps away from the optical axis is good for dealing with back-scatter light. Which is why cars put their forward fog lights down near the road.

    1. They say in the film what they use for dye. I think he says “fluorescein”, which is indeed a dye that looks like that.

      1. Is that the fluorescein of FITC fame?

        It does actually look like it’s a fluorescent dye. Makes the experiment extra clever because it converts the predominantly blue light under water into yellow/green, making the colour stand out more than a normal dye would.

        A large part of my PhD depended on FITC, as I was looking at proteins inside nuclei using immunofluorescence 😊. Never occurred to me to stick it into a life sponge though 😃.

      2. It does look very like fluorescein in colour (but getting good colour register is not easy – pet peeve of mine, and why I carry colour reference charts in my kit bag to and from the rig). Since it’s an intense dye, you can get away with not using a lot, which is the other approach to take. I know that we routinely use it dissolved in seawater for pressure testing lines and also for hydraulic transmission fluid, so that we can spot leaks easily. So we include it on our PON-14 (or is it PON-15? Mud-man’s job, not mine.) declaration of intended chemical usage which might be discharged to the sea, and it’s a permitted discharge. But the faff-factor of getting that permit would be a discouraging factor to me in a hobby setting – I’d start from the food dye end of things – it would be a much easier argument to convince a bureaucrat of.
        You might be able to get fluorescein as a foodstuff. I can’t quite imagine why, but that’s folk for you.

  8. What a beautiful experiment. The KISS principle at its most telling and powerful demonstration of the explanatory power of science.

  9. I used to play with live sponges when we would go diving in Mexico. Many of the live sponges were surprisingly heavy, like chunks of raw liver. But you learned to be careful b/c there could be something nasty hiding inside of them like a ‘fireworm’, which is a big segmented worm with stinging spines.

  10. Next demonstration — show the sponges feeding at night by pushing waves of water at them. When one sees cascades of sparks screaming up the sides of esp. those huge barrel sponges, and also know that one is “dusting” the sponge in the process (i.e., unplugging clogged holes so it can eat more efficiently), it’s all good.

  11. I used to teach an organismal biology lab, and I would demonstrate this for my students each semester. It’s quite fun!

  12. With that big sponge at the end that you could see down into, I wanted to see the tracer injected at different heights to get an idea of the diffusion patterns through the sponge.

    Perhaps a very small gadget that forms a gentle seal on the outside of the sponge that restricts the dye intake to a very precise locus?

    Also seems likely that flotsam – say, an algae leaf — could possibly get stuck by suction to the outside of the sponge, and that it’s gotta have some mechanism for clearing away such debris. Does it reverse the flow?

    As always, answering one question only means dozens more to be asked….


    1. Good questions, although I think that the suction force would be relatively low (as suction forces tend to be).

      Just think of the difference between the force behind a jet engine and in front of it, or behind a large fan ventilator as opposed to in front of it. The sponge dilutes the effect further, it seems, by having a much larger “intake area” than outlet.

  13. I got landed with demonstrating in a lab practical on sponges in Hong Kong in 1966. I recall a film or a series of stills that showed the water circulation but have no idea where I saw it. I have also seen something more recently – in colour but again I cannot remember where.

  14. Prefer the term cilium versus flagellum.Amazingly, both have the 9-2 cross-sectional structure as do our cilia and flagella. What is so biomechanically “precious” about these micro-tubullar arrangements?

  15. If you are a diver – notice he is using an old timey double hose. Great for photographers – no bubbles in front of you. But a real dog to breathe from. His regulator is behind his head – well above his lungs. The water pressure is at the higher point is sufficiently lower than at lung level to require a lot of heavy suculation ™ to get air.
    Conversely – roll over and get way more air than you want.
    Bill the Cat(fish)

  16. It wouldn’t be hard at all to feed a sponge a smorgasbord of microorganisms and find out what it filters (eats?) by inspecting what comes out.

  17. When I was a kid, my dad used a similar looking dye to trace illegal sewage systems. He’d flush it down people’s toilets and see if it popped out in a nearby stream or lake. Maybe the same dye?

    One surprising thing I noticed in the video is that the dye seemed to be coming up through the middle of the tube, not close to the sides where cells would be taking up nutrients. Any thoughts about why this would be?

    1. One surprising thing I noticed in the video is that the dye seemed to be coming up through the middle of the tube, not close to the sides where cells would be taking up nutrients. Any thoughts about why this would be?

      He’s releasing the dye near the base of the sponge. The sponge is presumably filtering water through the whole length, meaning the water coming through the upper portions is pushing inwards the water coming from the lower portions.

      That’s why I’d love to see dye released from various elevations of the sponge….


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