A food web in a fluid droplet (algae, rotifers, and anemone)

In my ongoing quest to explore marine microfauna, I recently observed rotifers under a Foldscope. Like many motile marine invertebrates, rotifers use cilia to maneuver through the water column. They also use these cilia to feed by generating currents. 20151019_184359

The anterior end of the animal is in the top right of the pictures, and there are 2 crowns of cilia on either side (this image in high mag). The cilia on these crowns beat in waves that generate current, allowing locomotion and feeding. Please excuse the psychedelic color changes in the first video, this was due to the light source oversaturating my camera phone’s sensor. I remedied this by using several layers of Scotch brand magic tape to diffuse the light.

High magnification was necessary to even see the cilia beating, as they appeared as a blur in low magnification. For scale, those little spheres of algae are approximatey 4 microns in diameter. I was curious to see whether I could observe the ciliary waves by filming it in high speed. Here’s a video of the cilia beating, filmed at approximately 130 fps and slowed down 8x. Although I wasn’t able to get the cilia entirely in focus, there are a number of curious things happening. First, the algae provide an excellent set of particles to visualize the flow patterns created by the cilia. Second, the rotifer would periodically stop feeding and would retract. I’m wondering if this is because it has caught a particle, or if there is some other reason for this behavior.

BONUS! I was also able to get my hands on a few anemone polyps (seen here in dark field and low mag). These polyps were so tiny that I was able to just barely fit them on a microscope slide, with 2 pieces of double-sided tape used as spacers. 20151020_174022

My challenge was to see whether I could record an anemone capturing and feeding on a rotifer. After a few hours and a dozen trials, success was at hand. Below is an anemone with a rotifer trapped in its mouth (the round shape on the right end), about to be consumed.


In fact, I was able to capture the process of consumption over a 6-minute span.

By expanding and contracting, it looks like the anemone forces the rotifer down using a combination of cilia and peristaltic movement. At the end, you can even see the intact rotifer in the anemone’s gut! Here’s the same video, sped up 24x to get a better sense of the overall ingestion process.

8 Comments Add yours

  1. laksiyer says:

    There are no words to describe these videos. Brilliant, stupendous, remarkable; my limited vocabulary is running out. Looking at the rotifer feed on algae, it struck me that the rotifer is able to sample every individiual. How come they queue in such an orderly fashion in the current? Can we think of a model? The anemone feeding on the rotifer is out of the world. I wonder if we can explore some dyes with this.

    BTW, could you post your slide setup. Liquid samples in the foldscope have always been messy. How do you avoid the mess? I have been using vaseline, but is there something better?

  2. Tom Hata says:

    Laks – Thanks for the kind words! I was seriously blown away by these little guys. I didn’t think I would be able to see half of what I did. My setup is actually very minimal. I contain everything in the smallest droplet possible, and use double-stick tape on either end of the glass cover slip to act as spacers. As long as the droplet doesn’t wander off to the side and wick out, it is fairly stable. I can’t avoid this half the time, but the organisms are able to stay alive for hours in the droplets that do stabilize. It seems to help to make sure the slide is cleaned so that the surface is hydrophobic, I hear rain-x is useful for that.

    Rotifers have very interesting behavior. They’re mostly freshwater and can form dandelion-like colonies https://upload.wikimedia.org/wikipedia/commons/thumb/b/b9/Sinantheria-socialis.jpg/800px-Sinantheria-socialis.jpg
    These ones are marine and free swimming. Although they don’t form colonies, they do like to aggregate and feed near each other, which I find bizarre. The algae I used to feed them is Rhodomonas lens, which I might explore further as flow markers. I think they are transported in such an orderly fashion because flow is likely laminar at that scale.

  3. Manu Prakash says:

    Fantastic videos @Tom. That’s an incredible food web captured in a tiny drop of the ocean.

    I have so many remarks and observations that I am fascinated about – after watching the videos. Incredible piece of work.

    With the crown of cilia being modeled as stokeslets; we can capture the flow structure. But you reveal a fascinating fact of low Re flows ciliary biology in your high speed video – which is the the beads move in a jerky forward backward motion. Since all low Re mechanisms generate some backward flux/force – the bead can be used as a direct sensor. In fact the jerky nature of movement of the bead in high speed is a surrogate to measure the frequency of beating cilia. The fact that it is visible is quiet remarkable. You can also see its a near field phenomena – since the beads farther away from the cilia should have a smooth motion – with viscous dissipation ; the reverse flow field per beat dies out.

    I have a lot more to write here; will pave my commend once I have watched the videos again.

    Great work.. Keep exploring. Poor rotifer got eaten in the end – but did not seem to die. So maybe it will survive 🙂


  4. Saad Bhamla says:

    @Tom – this is an awesome piece of work 🙂
    Your perseverance definitely paid off!

    I’m curious how you managed to squeeze the anemone inside your droplet as well?
    the flow patterns with the cilia are very intriguing too – i have to look up some papers on the flow around beating cilia..

    thanks again for making my day!

  5. Tom Hata says:

    Manu – Great points about the algae! I was focused on directly observing ciliary beating, but the motion of the algae appear to be much more revealing. I’m tempted to use Rhodomonas for other flow visualizations of marine species.

    Saad – I just had a really small anemone 🙂 I can’t imagine they were more than a couple weeks old. They’re actually smaller than the copepod adults.

  6. Cristina says:

    I wonder if you can image nematocysts being discharged from the anemone’ s tentacles by using the high magnification lens.

  7. Manu says:

    @Cristina: Oh, I do have a surprise for you. You just reminded me, I need to post that data. Need to have a machine learning algorithm that makes posts out of all the data stored on my phone.

    I will soon share some surprising results.. The hint is “heavy dose of toxins”..


  8. James Pelletier says:

    Incredible @Tom! WOW! Your movies knocked my socks off!

    After reading the post by Torion from @McKanstry Prep https://microcosmos.foldscope.com/?p=17019 I started to wonder about mechanisms of cooperative feeding.

    In a colony, I wonder if the vortical flows generated by adjacent rotifers could add together, so that fewer algae are pushed away and each rotifer catches more algae? Go team! Seems some research has been done on this, I need to read more.

    I am also curious how much control the rotifer has over its cilia. Seems it can turn them on and off. I wonder if it can adjust the rate, or even adjust the rate of each side independently, like differential drive? The comment by @Manu that the jerky motions of the algae can reveal beat rate – wicked!

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